Irset T3 Fundamental of Electronics

T3 FUNDAMENTAL OF ELECTRONICS

Issued in October 2008

INDIAN RAILWAYS INSTITUTE OF SIGNAL ENGINEERING & TELECOMMUNICATIONS

SECUNDERABAD - 500 017

T3 FUNDAMENTAL OF ELECTRONICS

Contents

1. Theory of semiconductors: ......................................................... 01 2. Types of Semiconductor Materials: ............................................ 07 3. PN Junction ................................................................................ 13 4. Transistor Operation: .................................................................. 19 5. Transistor Current Configurations: .............................................. 23 6. The Common Emitter: ................................................................ 26 7. The Common Base: ................................................................... 30 8. The Common Collector: ............................................................. 32 9. Comparison of Transistor Configurations: .................................. 34 10. Characteristic Curves: ................................................................ 37 11. Operation Limit of Transistors: ................................................... 41 12. Specifications of Transistors: ...................................................... 43 13. Field Effect Transistor: ............................................................... 51 14. The Zener Diode: ....................................................................... 58 15. Silicon Controlled Rectifiers: ....................................................... 64 16. Uni Junction Transistors: ............................................................ 68 17. Special Devices: ......................................................................... 71 18. Semiconductor Microwave Devices: ........................................... 76

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Prepared by S.S. Muralidharan IMP-1 Checked by S.N. Pal, Asst. Professor-Tele Approved by S.K.Biswas, Sr. Prof. Tele (Nov. 2005) DTP and Drawings K.Srinivas, JE II(D) Date of Issue Nov. 2005 Edition No 01 First Reprint October 2008 No. of Pages 85 No.of Sheets 45

IRISET This is the Intellectual property for exclusive use of Indian Railways. No part of this publication may be stored in a retrieval system, transmitted or reproduced in any way, including but not limited to photo copy, photograph, magnetic, optical or other record without the prior agreement and written permission of IRISET, Secunderabad, India

THEORY OF SEMICONDUCTORS

IRISET 1 FUNDAMENTAL OF ELECTRONICS

CHAPTER-1 THEORY OF SEMICONDUCTORS

1.1 Matter is anything that has weight. Atomic theory describes all matter, whether it is solid, liquid or gas as being composed of atoms. The atom contains a central nucleus in which exist neutrons and protons. Protons are positively charged particles and neutrons are neutral particles both being approximately 1840 times as heavy as an electron. Electrons which are negatively charged particles are arranged in ' orbits around the nucleus in a manner similar to the arrangement of planets around the sun. Thus the electrons in atoms are frequently called planetary electrons. Representative drawing of an atom in shown in figure. 1.1.

1.2. Atoms of different elements are found to have a different number of protons, neutrons and electrons. In order to classify or identify the various atoms a number which indicates the no. of protons in the nucleus of a given atom, has been assigned to the atoms of each known element. This number is called the atomic number of the element. The atomic numbers for some of the elements associated with the study of semiconductors are as shown below:

Element Symbol Atomic No. Germanium Ge 32 Silicon Si 14 Antimony Sb 51 Arsenic As 33 Indium In 49 Gallium Ga 31 Boron B 5

The normal atom has an equal no. of protons and planetary electrons to maintain its net charge at zero. In the case of Germanium 32 protons and 32 planetary electrons comprise the atom.



1.3 The orbits of planetary electrons are grouped around the nucleus in rings or shells with specific number of electrons permitted in each ring. A discrete energy may then be associated with each ring or shell. The rings or shells are numbered starting with the first ring nearest to the nucleus as No. 1 the second ring from nucleus as No. 2 etc. The maximum number of electrons permitted in each ring is as follows:

Ring No.1 2 electrons Ring No.2 8 electrons Ring No.3 18 electrons Ring No.4 32 electrons

It may be noted that the permissible no. of 1 electrons for each ring or shell is equal to 2N2 where N is the ring or shell no subject to the following conditions.

a) No. of electrons should 'not exceed 8 in the outermost shell of any element and

b) it should not exceed 18 in the last but one, shell.

The atomic structures of silicon or germanium atoms are illustrated in Fig.1.2. Silicon having an atomic number of the 14, has 14 planetary electrons whereas Germanium having an atomic number of 32, has 32 planetary electrons. It may be noted that the outer rings, the 3rd, ring in silicon and 4th ring in germanium, are incomplete, each of these rings having only four electrons. The fact that the outer rings are incomplete is important to the nature of semiconductor devices.

1.4 The atomic weight of an atom designates the mass of the nucleus and is referred to oxygen having an atomic weight of 16. as a standard. The atomic weight may be other than an integral multiple or sub-multiple of the atomic weight of oxygen.

SILICON( 14) GERMANIUM( 32)

FIG. 1.2 SILICON AND GERMANIUM ATOMIC STRUCTURE

28

14



In addition to protons, the nucleus of an atom may also contain neutrons. A neutron is a neutral particle, whose mass is slightly greater- than the mass of proton.

The mass of an electron is 9.1083 x 10-31 Kg.

The mass of a proton is 1.6724 x 10-27 Kg.

The mass of a Neutron is 1.6747 x 10 -27 kg.

An example of an element with neutrons present in the nucleus is carbon shown in Fig.1.3. The net charge of the nucleus is +6, but the additional mass of six neutrons makes the atomic weight of carbon very nearly 12.

1.5 Valence is defined as the chemical combining ability of an element referenced to hydrogen and is a function of the number of planetary electrons in the outer electronic orbit of an atom. Valence is the capacity of the atom to combine with other atoms in order to form a molecule. For example, the element Helium, shown in part A of Fig.1.4 (i) has a valence of zero. This means that the outer orbit is complete i.e. the maximum no of electrons is present. Thus helium does not chemically combine and said to be inert. On the other hand, hydrogen, shown in part B of the figure, has a valence of 1 and will go into chemical combination readily. The reason for this is that hydrogen, has an incomplete outer ring which requires one more electron in order to be complete. One atom of oxygen (Part C) has a combining power of valence of 2 since there are two vacancies in the outer orbit of the atom. Where oxygen and hydrogen combine. to form water, two hydrogen atoms contribute their electrons to the outer ring, of the oxygen atom in such a way as to complete the outer rings of both atoms as shown in part A of Fig.1.4(ii). All atoms continually strike to complete their outer ring of electrons and when this is accomplished a stable state exists. The type of bond produced, when hydrogen and oxygen combine to form water, is called an ionic valence bond or an

FIG. 1.3 THE CARBON ATOM

FIRST RING COMPLETE

SECOND RING COMPLETE

ATOMIC No. 6ATOMIC WEIGHT 12



electro valence bond. The tendency of an atom to complete its outer ring is illustrated by the atomic form of hydrogen. In this form hydrogen has only one electron in the outer ring. The atom, is therefore unstable since its outer ring required two electrons to be complete. As a result one hydrogen atom will combine with another hydrogen atom to produce the molecular form of hydrogen, consisting of two atoms bonded together each sharing the others electron to form a stable molecule. This type of bond is called covalent bond. An example of covalent bond is shown in part 6 of fig.1.4 (ii).

1.6 Referring again to the silicon and Germanium, atoms it appears that in order for the silicon atom to chemically combine with another atom 14 additional electrons are required to complete its outer. Ring similarly it appears that 28 additional electrons are required to complete the outer ring of the Germanium atom. However, for atoms having 3 or more rings, it is found that if the outermost ring contains eight electrons it can be considered to be complete and the atom can be considered to the stable. Therefore, the outer ring of silicon or germanium require only four electrons to become stable hence the valence of silicon or geranium is 4.

1.7 In some elements, the electrons are bound closer to the nucleus than in others and the effect of external forces such as due to gravity and magnetism on the highly bound electrons is much smaller than the effect of forces within. the atom where the electrons are tightly bound, they are difficult to remove. It is the case of difficulty encountered in dislodging the outer electrons that determine whether the element is a conductor, insulator or semiconductor.



1.8 It is established that a definite amount of energy must be supplied in order to affect an electron held at an atom. That is in order to move an electron from one energy level to higher energy level, a given amount of energy is required. If less than the required amount of energy is supplied to the electron, it will remain at its original level. If more than the required amount of energy is supplied to the electron, it will leave its orbit and move to the next higher level. The excess energy will be of no use unless it is sufficient to cause the electron to move to a higher energy level. These definite amounts of energy are called "quanta" and they can be supplied to the electrons only in whole numbers such as .1,2,3,4 etc. It is possible for electrons to lose energy as well as receive it. When an electron in an atom loses energy, it moves to, a lower energy level or closer to the nucleus. The energy that is lost in this process may appear in the form of heat as in a conductor passing current or visible light as in gaseous tubes. The different elements have different energy levels; hence the amount of energy released or absorbed by the electrons of different atoms varies.

1.9 A fact which is brought out by quantum theory explains more precisely the difference between conductors and insulators. The reasoning that the difference between conductors and insulators is due to the no. of electrons in the outer ring of an atom still holds true. However in a solid crystal the energy levels are considered as bands instead of rings or orbits.

An example of the band structure of an insulating material is shown in part A of Fig.1.5. Since such a diagram illustrates the electron energy bands of a material it is often reported to as energy band diagram or simply energy diagram.

CONDUCTION BAND

VALANCE BAND

FORBIDDEN REGIONOR ENERGY GAP

(A) INSULATORS

ENER

GY

IN EL

ECTR

ON

VO

LTE

Fig. 1.5 ENERGY DIAGRAM FOR INSULATORS & CONDUCTORS

ENER

GY

IN EL

ECTR

ON

VO

LTE

(B) CONDUCTORS

VALANCE BAND

CONDUCTION BAND



The lower portion of the diagram, called the valence band, represents the energy levels closest to the nucleus of the atom. in the normal atom the energy levels in the valance band contain the correct number of electron necessary to valance the positive charge of the nucleus. Thus band is called the filled band. The electrons in this band are tightly bound to the nucleus with the electrons being more lightly bound in each succeeding energy level, toward the nucleus. It is more difficult to disturb electrons in the energy levels, closer to tile nucleus since their movement involves greater energies. The top or outermost band in the diagram is called the conduction band. An electron in an energy level which lies within this band is relatively free to move about the, crystal and hence conduct ails electric current.

Between the bands is a range of energy values across which electrons may pass but the values of which they actually may not have. That is although electrons can jump across this region from the valence band to the conduction band, they never have energy values ill this range. Hence this region is appropriately called the forbidden region or energy gap. Note that the forbidden region of an insulator is relatively wide. When compared to the valence band and the conduction band. The wider the energy gap in a material the greater the amount of energy required to cause an electron from the valence band to jump the gap ad appear in the conduction band where it can be used as a current carrier. It is apparent that due to the wide energy gap, a large amount of energy is required to produce a small amount of current through an insulating material.

Part B of figure shows the band structure of a conducting material. Notice that the valence band and conduction band touch each other and that there is no forbidden region. Whenever these two band touch, only an extremely small amount of energy to move electrons into the conduction band and an electrical current is readily passed by the material.

1.10 The measure used in the diagram is the electron volt, one electron volt being equal to the energy acquired by an electron in passing through a difference of potential of 1 volt. By applying this method of measuring energy to an insulator the width of energy gap is generally 1 electron volt or more. For conductors, the energy gap is less than 0.05 electron volt from the valence band to the conduction band.

TYPES OF SEMICONDUCTOR MATERIALS


CHAPTER -2 TYPES OF SEMICONDUCTOR MATERIALS

2.1 The conductivity of a semiconductor is midway between that of a conductor and that of an insulator. Germanium or silicon which is considered to be semiconductor, in pure form actually is insulators. However in the manufacture of these elements for electronic use, impurities are added to them so that they become semiconductors.

2.2 The fact that the movement of electrons through a conducting material produce a current has been used as the basis for explaining both alternating and direct current. This is called the electron theory.

Although the movement of electrons in a semiconductor material causes a current to pass, a current also results in such materials from the movement of positive charges or holes, through the material. A hole is nothing more than the space left by the electron. Since this space has an attraction for any negatively charged electron, the hole is considered to have a positive charge.

2.3 Holes are considered to be capable of motion around in the covalent bonds. Hole movement is somewhat different from electron movement. Electrons in motion out of an orbit of an atom are considered to be free. Holes however make only when electrons leave their positions or orbits.

An analogy to hole motion can be drawn from the arrangements of bearing in a tube or cylinder as shown in Fig.2.1. By removing No.1 bearing a hole or space is left which is then filled by the No.2 bearing. The No.3 bearing then moves into the No.2 space. This action continues until all the bearings have moved one space to the left at which time there is a space, left by the No. 7 bearing, at the right end of the tube. Therefore whether this process is looked upon as a motion of the bearings to the left or a motion of the space (absence of a bearing) to the right the end result is the same. This motion is similar to that of a hole in the covalent bond structure of a semiconductor material with the hole movement being governed by the shifting of electrons. In the covalent bonds, the same electrical effect is obtained whether electrons move in one direction (electron current) or holes move in the opposite direction (hole current). This is an important concept and is fundamental to the study of transistors, since both types of current occur in transistors. Usually electrons move through the conduction band and holes move through the valence band.



2.4(a) Germanium has four electrons in its outermost shell, in bonds atoms are shown with their outer electrons only since these electrons which a maximum of 32 electrons is permitted. The germanium atoms will share valence electrons in a covalent bond. This is shown in Fig.2.2. The, germanium are the ones associated with the covalent The crystalline form of germanium called the crystal lattices structure has the atoms arranged in this manner. The electrons in such an arrangement are in very stable condition and thus are less apt to be associated with conductors. Germanium in a pure form is an insulating material and is called an intrinsic semiconductor.

2.4(b) Silicon is also used in the manufacture of semiconductor devices. Silicon also has four electrons in its outermost shell. The atomic structure of silicon and germanium are shown in the sketch. The crystal lattice structure of silicon is that similar to the Germanium.

2.5 Pure form of germanium is of no use as a semiconductor device. By the addition of impurities however a desired amount of conductivity can be obtained. In order to do this, the quality of added impurity must be carefully controlled. The added impurities will create either an excess or a deficiency of electrons depending on the type of impurity added.

1 2 43 6 75

5 763 42

2 3 54 76

1

BEARING IN A TUBE

SPACE LEFT BY No. 1 BEARING

No. 1 BEARING MOVED

SPACE LEFT BY No. 7 BEARING

FIG. 2.1 ANALOGY TO HOLE MOTION

FIG. 2.2 ( b) SILICON AND GERMANIUM( 32)SILICON( 14)

LATTICE STRUCTURE OF PURE GERMANIUMFIG. 2.2. COVALENT

GE GE GE GE GE

GEGEGEGEGE

GE GE GE GE GE

GEGEGEGEGE

GERMANIUM ATOMIC STRUCTURE



2.6 Primary importance in semiconductors are these impurities that align themselves in the regular germanium lattice structure despite the fact that they have one valance electron too many or one too few. The first type easily loses its extra electron, and in so doing it increases the conductivity of the material by contributing a free electron. This type of impurity has five valence electrons and is called as pentavalent impurity. Arsenic, Antimony, Bismuth and Phosphorous are pentavalent impurities. Since these impurities give up or donate, one electron to the material, they are referred to as donor impurities. The second type of impurity tends to make up its deficiency of one electron by acquiring an. electron from its neighbour. Impurities of this type in the lattice structure have three valence electrons and are, therefore, called trivalent impurities. Examples of trivalent impurities are aluminium, gallium, Indium and boron. Since these impurities accept one electron from the material, these are referred to as acceptor, impurities. Semiconductors produced by adding either acceptor or donor impurities are called extrinsic semiconductors.

2.7 When a pentavalent impurity such as Arsenic is added to germanium, it will form covalent bonds with the germanium atoms. Fig.2.3 illustrates the presence of an arsenic atom (As) within the germanium lattice structure. Only A of the five electrons of arsenic in the outer ring is used to form covalent bonds leaving one electron relatively free in the crystal structure. Since this semiconductor material conducts by electron movement it is termed a negative carrier type of N- type semiconductor. Pure Germanium may be turned into an N-type semiconductor by doping it with an element containing five electrons in. its outer ring. The amount of impurity added is ordinarily in the neighbourhood of one atom of impurity material per ten million atoms of germanium.

GE GE GE GE GE

GEGEGEGEGE

GE GE GE GE GE

GEGEGEGEGE

DONAR IMPURITY

EXCESS ELECTRON DUE TO IMPURITY ELEMENT

FIG. 2.3. GERMANIUM LATTICE WITH A DONAR IMPURITY ADDED(N-TYPE GERMANIUM)



2.8 Application of an electric field to an N-type semiconductor causes a current conducted by negative (electron) carriers. Fig. 2.4 illustrates one N-type semiconductor with an electric field applied. Electric field causes the loosely bound electron to be released from the impurity atom and move toward the positive potential point. The conduction is similar to that in a copper conductor. But, certain difference exists between a semiconductor and the familiar copper conductor. For example, the semiconductor resistance decreases with increasing temperature because more carriers are made available at higher temperatures, while the resistance of copper increases with temperature.

2.9 A Trivalent impurity element can be added to pure germanium to dope the material. In this case the valence electrons of the trivalent element will also enter into covalent bonds with the germanium atoms. However, tile trivalent impurity will borrow a fourth electron from a Germanium atom to complete the covalent bond structure. This removal of an electron from the covalent bonds of the Germanium by the trivalent impurity creates a hole or space.

2.10 In Fig.2.5, the germanium lattice structure is shown with the addition of an indium atom (In). The indium atom takes a hole in the structure. Other, elements which display the same characteristic are gallium and boron. The holes are present only if a trivalent impurity is used. Since such a semiconductor material conducts by the movement of holes which are positively charged, it is termed positive carrier type or P-type semiconductor.

Fig. 2.4 ELECTRON MOVEMENT IN A N-TYPE SEMICONDUCTOR

DIRECTION OF ELECTRON MOVEMENT

DIRECTION OF ELECTRIC FIELD

+ -



2.11 Application of an electric field to a P-type semiconductor causes a current conducted by positive carriers (holes). In order for the hole to move, an electron from a near by site must shift to the position where the hole originally existed. Hence, the holes illustrated in Fig.2.6 move from the positive terminal to negative terminal. Electrons from the negative terminal cancel holes at the vicinity of the terminal while at the positive terminal; electrons are being removed from the covalent bends, thus creating new holes. The new holes then move towards the negative terminal and are cancelled by more electrons emitted from the negative terminal.

2.12 It should be realised that in either N-type of P-type germanium both electrons and holes are present and can act as current carriers. In N-type germanium, electrons greatly out number the holes and thus are said to be the major current carriers while the holes are referred to as minor current carriers. On the other hand, P-type

GE GE GE GE GE

GEGEGEGEGE

GE GE GE GE GE

GEGEGEGEGE

FIG. 2.5. GERMANIUMLATTICE WITH AN ACCEPTOR IMPURITY ADDED (P-TYPE GERMANIUM

ACCEPTORIMPURITY

HOLE CAUSED BY IMPURITY ELEMENT

-+

HOLE MOVEMENT

ELECTRON MOVEMENT

Fig. 2.6 HOLE MOVEMENT IN P-TYPE SEMICONDUCTOR

+ -

+

+



germanium contain a greater number of holes than electrons and thus in this material holes are the major current carriers while electrons are considered to be minor current carriers. In an intrinsic semiconductor there is thermal break up of covalent bond producing an electron hole pair i.e. the no of electrons = no. of holes. When pentavalent impurity is added the no. of electrons increases without corresponding increases in holes and when a trivalent impurity is added, the no. of holes increases without a corresponding increases the no. of electrons. However, the product of electron and hole concentration remain the same whether impurity is added or not. Thus, if the hole concentration is TIP and electron is e

Then p X e = n2 where is the no. of electrons = no. of holes in the intrinsic semiconductors.

2.13 Mobility of the charge carriers is defined as the speed at which they drift in, unit electric field. The intrinsic Conductivity 1 is given by the formula

i = e(1+2) where, i is conductivity in semiconductor e is charge of electron (or hole) is intrinsic concentration of carriers per cc. i = nxp where n & p are the concentration of electrons and holes is mobility of free electron, em/sec. per volt/cm. p is mobility of free hole cm/sec per volt/cm.

The mobility of electron and holes in silicon is 1250cm2/V.sec and 480 cm2/V. whereas in germanium it is 3900 cm2 /V.sec and 1900 cm2/V.sec.

This applies to lightly doped silicon and germanium. These values will decrease for higher doping levels. The ratio of / p however remains relatively constant. This ratio is about 2 to 6 in silicon and 2 in germanium.

PN JUNCTION


CHAPTER - 3 PN JUNCTION

3.1 Representative diagrams of both P-type and N-type semiconductor materials are illustrated in Fig.3.1. A P-type semiconductor is shown in part A with the symbol representing the accepter atoms of the added impurity and the plus sign without the circle representing hole carriers. A N-type semiconductor is represented in part B, the plus sign within the circle + representing the donor atoms and the minus sign without the circle representing free electrons.

3.2 If a piece of P-type semiconductor material and a piece of N-type semiconductor material are joined together, the result is known as PN junction (Fig.3.2). A PN junction is formed during the process of manufacturing the semiconductor crystal. Several methods are employed, one of which is to add the desired impurities as the crystal is being made so that one section of the crystal is N-type and the other P-type. This impurity in the process of taking electrons from covalent bonds of the N-type crystal, creates an area of P-type crystal. It should be noted that the addition of impurity atoms to a semiconductor does not create a change or potential difference in the semiconductor. Such impurity atom added is electrically neutral. When it enters into a covalent bond it allows the carrier to be free through the semiconductor under the influence of an electric field.

(A) P - TYPE MATERIAL (A) N - TYPE MATERIAL

+

- - - -

- - - -

+ + +

+ +

+++----

----

+

+ + + +

HOLES ELECTRONSACCEPTOR ATOMSDONOR ATOMS

Fig. 3.1 P-TYPE & N-TYPE SEMICONDUCTOR MATERIAL

FIG. 3.2. THE P-N JUNCTIONP N

ACCEPTOR ATOMSDONAR ATOMS

+ + +

+ +

+++

+ +

+ + +

- - -

- -

- - -

- -

- - -

-

+

+

+

+

+

+

+

+

+

+

+

+

+

+

-

-

-

-

-

-

-

-

-

-

-

-

-

PN JUNCTION


3.3 The addition of arsenic to a germanium crystal supplies electron carriers that are not bound by the covalent forces. Similarly the addition of Indium supplies hole carriers that are not bound by the covalent forces. The atomic state remains neutral in charge as long as the carrier is present. When the electron or hole carriers move off under the influence of an electric field, the atomic state may temporarily be positive or negative, respectively but the net charge of the material is still zero.

3.4 In the absence of external forces, there is a process of carrier movement called diffusion occurring across the PN junction. This is caused by the holes attempting to move to the N-type material and the electrons attempting to move to the P-type material. However, only a few electrons and holes actually cross the junction. As soon as a crossing takes place, a few atomic states near the junction lose their compensating carrier and become uncompensated and are no long neutral as shown in Fig.3.3. The donor atomic sites become positive, having lost a neutralising electron, the acceptor sites becomes negative having gained an electron. The carrier movement reaches an equilibrium condition at which the net current between P-type and N-type materials is zero and a potential difference exists between the materials.

FIG.3.3 CONDITION OF EQUILIBRIUM ACROSS A PN - JUNCTION

3.5 The potential difference existing between the P-type and N-type materials is called the "barrier region" or "potential hill" and can be represented by a battery. This does not mean that a potential may be measured from one end of the material to the other. The overall piece of material is neutral even though a charge is displaced within the semiconductor to create the barrier.

3.6 In order to produce a current across a PN junction, the potential hill existing at the junction must be neutralized. The potential hill can reduce or neutralized by the addition of a bias battery across the two crystal section. For the reduction of the potential hill, the polarity of the bias battery must be opposite to the polarity of the

-

-

-

-

-

-

-

-

-

-

-

-

-

+

+

+

+

+

+

+

+

+

+

+

+

+

---

--

---

--

---

+++

++

+ + +

++

+++

UNCOMENSATED DONAR ATOMS

UNCOMPENSATED ACCEPTOR ATOMS

NPDIFFUSSION OF CARRIER ACROSS JUNCTION

PN JUNCTION


potential hill battery. In this case the junction is said to be in the forward bias direction. When the polarity of the bias battery has the same polarity as the potential hill battery, little or no current will cross the junction. In this case, the junction is said to be reverse biased. Since the PN junction has the same directional characteristic, as a vacuum tube diode. It is called a PN junction diode or simply a junction diode.

3.7 Fig.3.4 illustrates die junction diode in the forward bias connection. Where only the electron carriers and hole carriers are shown. The negative terminal of the battery is connected to the N-section and the positive terminal to the P-section which is just opposite of the potential hill battery. The N-section electrons are driven toward the junction by the negative battery terminal while holes in the P-section are forced towards the junction by the positive terminal. A number of electrons and holes depending on the battery potential cross the barrier region or junction and combine. Along with the combination of holes and electrons in the barrier region, there are two simultaneous actions that take place. Near the positive terminal in the P-section the covalent bonds of the atoms are broken and electrons are freed. The free electrons enter the positive terminal. This creates a new hole which is attracted toward the N-section. At the same time an electron enters the negative terminal of the N-section and move toward the positive terminal of P-section.

3.8 Although the voltage of the potential hill is in the order of tenths of a volt, the applied voltage must overcome 1 the, internal resistance of the diode. Since the germanium used is a semiconductor, its internal resistance is many times that of a conductor.

+ - POTENTIAL HILL

BIAS BATTERY

FIG. 3.4. FORWARD BIAS CONNECTION OF A JUNCTION DIODE

BARRIER LIMITS WITHOUT BIAS BATTERY

P-REGION N-REGIONBARRIER AREA HOLES & ELECTRONS COMBINE

NEW HOLES DUE TO ELECTRON REMOVAL

NEW ELECTRONS ENTERING TO REPLACE THOSE REMOVED

P N

PN JUNCTION


Hence the voltage across P-N type circuit sections is large leaving small voltage a to reduce the potential hill. If the actual size of junction is considered, the external voltage source need only be of the order of 1 or 2 volts to neutralize the potential hill. As the battery potential' increases, causes a rise in current, the resistance of the barrier decreases if the battery potential is allowed to reach a level at which, the potential hill is completely neutralized, heavy current will pass and junction may be damaged by the resultant heat. For this, reason the potential of the bias battery should not be too large.

3.9 Fig.3.5 illustrates the reverse bias connection of an external battery to a junction diode. Positive terminal of the battery is connected to the N region and Negative terminal to the P region. These connections are the same as these of the potential hill battery. Consequently the bias battery potential now aids the potential hill battery and very little or no current passes across the junction. In the P region the negative battery terminals draws the holes away from the barrier area. The electrons in the N region are similarly affected by the positive battery terminal. The potential hill is reinforced by the bias battery. This condition is shown in the upper portion of the fig.3.5.

JUNCTIONINCREASES

WITH REVERSE BIAS

BARRIER LIMITS WITHOUT BIAS BATTERY

P-REGION N-REGION

HOLES DRAWN AWAY FROM BARRIER AREA BY NEGATIVE OF BATTERY

ELECTRONS DRAWN AWAY FROM BARRIER AREA BY POSITIVE OF BATTERY BARRIER AREA

P N

BIAS BATTERY

POTENTIAL HILL BATTERY

FIG. 3.5 REVERSE BIAS CONNECTION OF A JUNCTION DIODE

+-

PN JUNCTION


3.10 Actually, there is a very small current due to the minority carriers present in each side of the crystal with their action being similar to the action of the majority carriers during the forward bias condition. The reverse current remains small with the increase in battery potential until a certain voltage is reached. At this voltage the covalent bond structure begins to break down and a sharp rise in reverse current occurs. This action is called "avalanche breakdown" and is due to the acceleration of the few electrons and holes that comprise the reverse current to such a point that they have violent collisions with the germanium crystal atoms. The maximum reverse voltage of the semiconductor diode corresponds to the peak inverse voltage of the vacuum tube. When a PN Junction is reverse biased all the applied voltage appears across the depletion region, since it has no free charge carriers hence 5 an infinite resistance. If the impurity content is about 1 part in 10 the relatively small dimensions of the depletion layer result in very high field strength in this region. This causes a rapid increase of current at low voltages due to the breaking of the covalent bonds and is known as the Zener effect. Zener breakdown is a field emission phenomenon, the strong electric field in the junction region pulling carriers from their atoms.

Zener diodes using this mode of operation are usually made of silicon with a zener breakdown voltage lying between 3V and 6V and a negative temperature coefficient. Higher voltage stabilising diodes utilise the avalanche effect and have much lower impurity content. They have a positive temperature co-efficient. It is difficult to separate the two effects in a practical zener diode.

3.11 The fig.3.6 illustrates the static characteristic of a junction diode. There are different current scales for forward bias and reverse bias operations. The forward portion of the curve indicates that the diode conducts easily when the P region is made positive and the N region negative. The diode conducts poorly in the high resistance direction i.e. when the P region is made negative and the N region is made positive. Now the holes and electrons are drawn away from the junction, causing the barrier hill to increase. This condition is indicated by the reverse current portion of the curve. The dotted section of the curve indicates the ideal curve which would result if it were not for avalanche breakdown.

PN JUNCTION


It is therefore seen that the forward resistance is low, the reverse resistance is very high.

The current voltage relationship of a diode is given by

l=lR(e QV/KT_ 1)

Where Q is the' charge on an electron (1.602 X 10 - 19 coulomb) V is the potential difference in volts. K is the Boltzmann constant (I.38 X10-23 Joules/Kelvin) T is the absolute temperature in degree Kelvin. IR is the reverse saturation current.

FORWARD CURRENTIN mA

REVERSE BIAS IN VOLTS

FORWARD BIAS IN VOLTS

REVERSE CURRENT IN microA

FIG. 3.6. TYPICAL STATIC CHARACTERISTIC CURVE FOR A SEMICONDUCTOR DIODE

IDEAL CURVE

TRANSISTOR OPERATION


CHAPTER 4 TRANSISTOR OPERATION

4.1 A junction of two junction diodes with either P-type or section being, common to, both, the resultant transistor is either an NPN or PNP type Junction transistor. In either case, 'the middle or section is very narrow compared to other sections. The junction transistor is produced in several different ways but the end result is the formation of PN, junctions. The junction may be formed in the process of growing the crystal. The thickness of the germanium of silicon crystal is important because of the possibility of its shorting out if it is too thin. On the hand the crystal is too thick; the operation of the transistor will be poor.

4.2 The description given below for a germanium transistor applies equally to a silicon transistor. The NPN consists of a, very thin layer of P-type germanium between two sections of N-type germanium as shown, in, Fig. 4.1. The potential hills of the two junctions are positive for the N-section and negative for the P-section. The emitter (or input NP section) is biased in the forward direction. The collector (or output PN section) is biased in the reverse direction that is the collector is positive with respect to base.

4.3 With the aid of negative potential applied to it, the free electrons in the emitter N-section will be pushed towards the first junction. The potential hill of this junction is essentially reduced by the polarity of the emitter bias battery. A number of electrons will pass through the junction and enter the middle or P-section where some of them combine with holes while other pass through. The electrons that pass through the P-section do so because of the thickness of the section and the effect of the potential hill of the second or PN junction. Actually, the potential hill, at the second junction accelerates the electrons into the collector N-section. In the collector area, the free electrons are attracted by the applied positive collector base voltage. It is important to note that the movement of electrons and holes is not in one for one process. A small

N P N


EMITTER COLLECTOR

BASE

_ + _ +FIG. 4.1 THE NPN JUNCTION TRANSISTOR



percentage (about to 5%) of the electrons entering the P-section (base region) form the emitter N-section combine with the P-section holes. However the majority of the electrons from the emitter do pass through the P region. Thus most of the electron flow is between emitter and collector. The electrons leaving the emitter are controlled by the bias potential between the emitter and base. (The similarity to a vacuum tube triode, where the bias is between the control rid and the cathode controls the electron flow to the plate which receives most of the, electrons should now be obvious).

4.4 The PNP transistor consists of a very thin layer of N-type germanium between two sections of P-type germanium as shown in Fig.4.2.In this type, the potential hills of the two junctions are positive for the PN-section and negative for the NP-section. In the PNP transistor the connection of emitter bias battery must be positive to the emitter and negative to the base in order to forward bias the emitter. The collector bias battery must have its positive terminal connected to the base to reverse bias the collector. The potential hill of the emitter junction is reduced by the forward bias.

FIG.4.2 THE PNP JUNCTION TRANSISTOR 4.5 In the operation of the PNP junction transistor holes are forced from the emitter

P-section into the base N-region by the positive potential of the emitter which is also creating more holes by electron removal. In the base region a small number of holes (about 1% to 5%) combine with electron from the base. Because the base region is very narrow most of the holes move on into the collector P region before they can combine with base electrons. In the collector P region the holes are attracted to the collector negative terminal and combine with electrons from the collector. Thus the major hole current is from emitter to collector, while emitter base current is very small. It is important to note that the major current carriers in the PNP transistor are holes while in the NPN transistor electrons are the major current carriers.

4.6 The NPN and PNP transistors are identified on schematics by the symbols shown in fig.4.6. The three regions comprising the transistor are called the collector, base and emitter. Emitter-base junction is always forward biased while collector-base junction is reverse biased. The emitter region is so called because it emits majority carriers into

BASE

COLLECTOREMITTER


PNP

+ _+ _



the base region. The collector gets its name because it collects the majority carriers from the base region. The base region is so called, because it is a support or base for emitter and collector materials.

4.7 The direction of electron flow in the wires connected to the transistor is shown in fig. For the NPN transistor where, electrons are the majority carriers, the electron flow shown is continuation of the internal flow. For the PNP, the majority current carriers are holes and the internal conduction is due to hole current. However, hole conduction takes place only within the semiconductor crystal itself. This internal hole conduction leads to electron flow in the external wires connected to the semiconductor material. The direction of the electron flow is opposite to the internal hole conduction and it is electron direction that is indicated for the PNP transistor.

4.8 Fig. 4.3 shows the basic current paths for the NPN and PNP transistors. The battery labelled VBB provides the forward bias for the base-emitter junction. Forward biasing causes current from one terminal of the battery through the junction and resistor and back to other battery terminal. This is called base current. The resistor is included in this path to indicate that some means of controlling this current is necessary. Recall that most of the majority carriers that are injected, into the base region from the emitter do not continue in the base emitter path. They are attracted toward the larger potential applied to the collector region. This potential is supplied by the battery marked Vcc. This current that is attracted to Vcc battery is called collector current. Since both the base current and collector current come from the emitter region, a simple relationship exists between the currents:

Ib + IC = le

In words the emitter current separates in the transistor into the base current and also the collector current.

Fig. 4.3 Basic Current in a Transistor 4.9 The amount of collector current depends on the amount of base current. More the

base current, the more the majority carriers that are injected into the base region and



the collector current is, therefore, larger. The base current converts the current supplied by Vcc into a controlled current, namely the collector current. The amount of collector current is related to the base current by the following simple but important relationship:

lc = 1b

The Greek letter (Beta) represents the current gain of the transistor. It is important to understand the twin loop concept depicted in the preceding illustration. One loop is the base current path (the input circuit) and the other loop is the collector current path (the output circuit). As will be seen later, the signal to be amplified is added to the base bias current and the output signal is derived from the collector current. The idea of base current regulating or controlling collector current is the basic operation of the transistor amplifier.

4.10 The most important thermal consideration is the increase in base to collector reverse current that occurs as temperature increases. The reverse biased base to collector junction has very small current through it due to minority carriers. The situation is shown for the NPN transistor in figure. This current is referred to as ICE0 (an abbreviation for collector cut off current) This is the collector current that would flow if the base lead were left disconnected. ICE0 has a particular value at room temperature, but it increases as the temperature increases. This results in a situation where there is a certain amount of collector current which is not controlled by the base current, leading to unpredictable results. Precautions have, therefore, to be taken to minimize ICEO, and related effects due to change in ICEO with temperature.

TRANSISTOR CURRENT CONFIGURATIONS


CHAPTER 5 TRANSISTOR CURRENT CONFIGURATIONS

There are three basic configurations for transistor circuits. The three configurations are called the common emitter, the common base, and the common collector circuit.

The input signal to a transistor is applied between two elements. The output signal is taken between two elements. ~ Since there are only three elements in a transistor, one of the elements has to be part of both the input and output circuits. The type of configuration derives its name from the element that is common to both input and output.

The most widely used transistor circuit is the common emitter. It is called thus because the emitter is common to both the input and output circuits. This is shown in figure. Figure A shows the input circuit and figure B shows the output circuit. An important point should be mentioned concerning the illustration is Fig. 5.1. The emitter is shown as grounded. Ground is the reference point in the circuit from which voltages are measured.

Fig.5.1 Common Emitter Input and Output Circuit

It will be noticed in Figure that both the input and output signals are measured with reference to ground. This reference point is called ground because quite often it is connected to the actual earth or ground. -Because the common element, the emitter in this case, is grounded this circuit is sometimes referred to as a grounded emitter circuit. Common emitter or grounded emitter refers to the same type of circuit. The drawing shows a common emitter stage. Figure does not show all the components usually needed for a working circuit, but is intended to show that emitter is common to both the input and output.

Fig.5.2 Common Emitter Stage

EMITTER

GROUND

BASECOLLECTOR

( A) INPUT CIRCUIT ( B) OUTPUT CIRCUIT

OUTPUT SIGNAL

INPUT SIGNAL

TRANSISTOR CURRENT CONFIGURATIONS


The example shown above is for the NPN type transistor. Every thing would still be valid for the PNP type, except that the power supply polarity would be reversed.

Figure shows the common base configuration. The input signal is applied to the emitter base circuit. Thus the base of the transistor is the common element. As was the case for the common emitter, figure is only intended to show' why this circuit is called the common base and does not represent a complete working circuit. In PNP transistor except that the polarity of the power supply would be reversed and naturally, the arrow on the emitter lead would point in the opposite direction.

Fig.5.3 Common Base Stage The third and final type of configuration is called is common collector and is illustrated in figure A and B. The input signal is applied between the base and collector, and the output signal is taken between the emitter and collector. Figure A shows the circuit as is normally drawn, but it does not clearly illustrate why it is called a common collector. The identical circuit is redrawn in figure B. The transistor has been turned around and this shows clearly that the collector is common to both the input and output signals.

Fig.5.4 Common Collector Stage

As in the case of the other two configurations, another name for the common collector is the grounded collector. The most popular name for this circuit is the emitter follower.

OUTPUT SIGNAL

INPUT SIGNAL

THE COMMON EMMITTER


CHAPTER - 6 THE COMMON EMITTER

6.1 The common emitter circuit is the most popular and versatile of the three types. The best way to arrive at a clear understanding of the performance of the common emitter is to start with the basic concepts and add ideas bit by bit until a working circuit is attained.

6.2 For a transistor to conduct, the base to emitter junction is forward biased and the base to collector junction is reverse biased. This will cause a base current, which in turn results in a collector - current (see. Fig.6.1). They are related by the expression lc = Ib, where is the current gain.

Fig.6.1 Base Collector Current

6.3 The first step then is to establish a forward biased base to emitter junction and produce some base current. This is done by connecting the base to a power supply through a resistor to establish the desired current. This is shown in figure A. To provide a reverse bias to collector junction and a path for collector current, the collector is connected to a power supply through a resistor (figure B). The two currents together are shown in figure C. The same power supply is used for both currents. The process of establishing a base current and a collector voltage is called biasing the transistor. So far only biasing current has been established and no mention has been made of the signal to be amplified.

Fig.6.2 Common Emitter Biasing

IbIe

IcB

C

E

B

C Ic

Ib

E Ie

P

N

N

Ic = Ib

Ib

( A) BASE CURRENT

Ic Ib

Ic

20V

( B) COLLECTOR CURRENT( C) BASE & COLLECTOR CURRENT

THE COMMON EMMITTER


6.4 The signal to be amplified is superimposed or added to the base bias current. It is introduced into the circuit by means of a capacitor, as shown in figure. This capacitor is called a coupling capacitor because it couples or joins the input signal to the circuit. A capacitor is used because it blocks the base biased direct current from flowing into the source of the signal and yet, lets the signal flow into the circuit.

FIG.6.3 INTRODUCTION OF SIGNAL

6.5 The collector current is P times the base current. P can be looked up in the specifications for the particular type transistor used. Whatever the base current is, P times that current will flow in the collector circuit (assuming linear operation to be explained later). A simple numerical example will help to illustrate these ideas.

Assume we are given the following data:

= 50 from transistor specifications. RL= 5000 ohms, determined by load requirements.

It is desirable that when no signal current is present, the collector is midway between its minimum and maximum possible operating excursion. In this case, that would be +10 volts since the power supply voltage is 20 volts. This means that with no signal current we want a 10 volt drop across the 5000 ohm resistor. The desired collector current is then:

IC = 10 volts = .002 ampere = 2 mA 5000 ohms.

INPUT SIGNAL CURRENT

OUTPUT SIGNAL CURRENT

LOAD RESISTANCE

BASE BIAS CURRENT

COUPLING CAPACITOR 20V

THE COMMON EMMITTER


If we want 2 mA of collector current, then the required 'base current can be calculated:

lc Ib

Ib lc = 2mA = .04mA = 40A.

50

To achieve this desired base current, a resistor of appropriate value is connected to the base from the power supply. Assuming that there is a negligible voltage drop from base to emitter, the value of base resistor in use to achieve the desired 40 A base current can be calculated:

RB = E or RB 20 volts = 50,000 ohms Ib 40 A

This common emitter sage is now biased at 40 A base current, 2mA collector current, and 10 volts collector voltage.

6.6 An input signal current is now superimposed on the base bias current. Assume it is an alternating sine wave that first flow 10 A in one direction and then 10 A in the other. This is shown in Fig.6.4(A). Such a signal might come from an antenna, a microphone, or another circuit. This current will add to or subtract from the base-bias current depending on which direction it is flowing at that instant of time. This is shown in Fig.6.4(B). The total base current is no longer just 40 A but it now vanes between 30 A and 50A. The total collector current will vary accordingly. When the base current is 30 A. the collector current will be 50 times that or 1.5 mA; and. when the

base current is 50 A the collector current is 2.5 mA. Thus, we have an amplifier. The base current is varied by 10 A in each direction and the collector current is varied by 500 A. This is shown in figure. The varying collector current through the load resistor develops the output voltage.

It will be noted at this time that the output current (collector current) is actually supplied by the power supply, and the transistor regulates or governs this current under the influence of the base current.

THE COMMON EMMITTER


Fig.6.4 Input Signal Current

6.7 The common emitter circuit just analysed is not a very practical circuit. The biasing technique described may work well at a room temperatures. Also varies from one transistor to another (even of the same type) and a more practical biasing arrangement can be used to compensate for this.

6.8 The common emitter circuit is said to provide a phase reversal because the output voltage is 180 degrees, out of phase with the input voltage. Referring to ignore it will be noticed that as lc increases, the voltage drop across RL increases.

Since at any one time, the voltage drop across RL and the transistor (from collector to emitter) must be equal to 20 volts, the voltage from collector emitter must be decreasing. Likewise, when the voltage drop across the RL is decreasing due to a decrease in lc, the voltage across the transistor is increasing. Thus, the actual output voltage which is taken from the collector to emitter is 180 degrees out of phase with the collector current. Since the collector current is in phase With the base voltage, the output voltage is also 180 degrees out of phase with the input base voltage. This is what is termed a phase reversal. The various relationships are illustrated in the curves of figure B. The common emitter circuit is the only one of the three configurations to give a phase reversal.

10

A

-10

A( A) SIGNAL ONLY ( B) SIGNAL & BASE BIAS

50

A40

A

30

A

THE COMMON COLLECTOR


CHAPTER - 7 THE COMMON BASE

7.1 In the common or grounded base configuration the input signal is applied between the emitter and base and the output signal is extracted between the collector and base. The internal workings of the transistor remain the same as in the common emitter.

7.2 Just as in the common emitter circuit, in the common base circuit the emitter current divides inside the transistor with a large percentage (about 95%) going to the collector and a very small amount (about 5%) becoming base current. This is shown in figure. For the common emitter, the important equation lc=.Ib relates the input current (Ib) to the output current (lc). An equally important equation arises in the common base configuration.

Ic = lE

Where , the Greek letter alpha, is referred to as the common base current gain. Since about 95% of the emitter current develops into collector current, generally a has values in the .95 region. Since the output current (Ic) is less than the input current (le), it will not immediately be apparent as to how this circuit is used as an amplifier. It is true that it cannot be used as a current amplifier, but by selecting the proper values for the input and load resistor, a voltage gain may be attained.

Fig.7.1 Common Base Current

7.3 Fig.7.2 shows the emitter current, collector current, and the two currents together, just as in the common emitter circuit. A particular bias condition is achieved by proper choice of biasing resistors. The current paths are shown in the illustration. The signal currents will be superimposed on these steady d.c. , currents.



Fig.7.2 Emitter and Collector Current

7.4 The resistor values have to be calculated to give the desired bias currents. The procedure would be similar to that used in the common emitter example. However, for the common base amplifier, the input signal is superimposed or added on to the emitter current rather than on the base current, as it was in the common emitter circuit. This is shown in Fig.7.3 where the signal to be amplified is coupled by a capacitor to the circuit.

FIG.7.3 SIGNAL ADDED TO BIAS CURRENT

7.5 The drawing of the wave forms (see figure) shows the phase relationships among the voltage and current at the input and the voltage and current at the output. As the input signal voltage increases, the emitter current will decrease since the collector current is related to the emitter current by lc= le, it also will decrease. The voltage across RL will increase, as lc decreases, so that output signal increases. The reverse occurs as the input voltage decreases. It will be noted from figures A and D that tile input voltage and output voltage are in phase, showing that there is no phase reversal in the common base configuration.

IC( A) EMITTER ( B) COLLECTOR ( C) EMITTER & COLLECTOR

IC IC IC IC



CHAPTER-8 THE COMMON COLLECTOR

8.1 The common collector configuration are shown in figure. These are the same as those shown for the common emitter. However the collector, instead of the emitter, is common to input and output, and the load resistor is in the emitter circuit. This makes a difference in the circuit operation of the two configurations. Since the load resistor is in the emitter lead, both the input and output currents pass through this resistor. This is the only configuration where this situation occurs.

Fig.8.1 Common Collector Currents

8.2 Figures shows the biasing currents. Just as in the common emitter, base current is established by connecting the base to a power supply through a resistor. However, in this case, the base current also goes through the resistor in the emitter lead. Collector current appears in this resistor and establishes a voltage drop across this resistors in such a directions as to oppose base current. Thus, the base current upon which the input signal will be superimposed encounter or "sees a large resistance from the base, through the transistor and load resistor to ground. The fact that the common collector has a high input resistance and low output resistance is one of its most important characteristics.

FIG. 8.2 COMMON COLLECTOR BIASING

( C) BASE & COLLECTOR CURRENT(

B) COLLECTOR CURRENT

20V

Ic

IbIc

( A) BASE CURRENT

Ib



8.3 The signal to be amplified is capacitively coupled to the base and is extracted between the emitter and collector. This is illustrated in figure. Since the base to emitter is forward biased, there is very little voltage drop across this junction, and the output voltage at the emitter is almost the same amplitude as the input base signal. For this circuit

8.4 Figure shows the phase relationship between input and output signals, it will be noted that the input voltage and output voltage are in phase.

COMPARISION OF TRANSISTOR CONFIGURATIONS


CHAPTER -9

COMPARISON OF TRANSISTOR CONFIGURATIONS

9.1 In general, current gain is the ratio of the output signal current to the input signal current. The output current for the common base is less than the input current (since loc = I.e.), so this configuration actually givers a current loss. However, voltage amplification is possible. Voltage amplification occurs when the output voltage is larger than the input voltage. In the common base configuration the input current is fed through a small resistor. Thus, the collector current, although smaller than the emitter current, passes through a larger resistance and thus develops a larger voltage. A simple example will explain this feature. Assume an input signal current of 10 A through an emitter resistance of 500 ohms. The emitter voltage can be calculated:

Vet=loc Re =10 A X 500 ohms 0.005 volts.

Since approximately, 0.98 of the emitter current is in the collector circuit, the collector signal current will be 9.8 RA, Assuming a collector resistance of 2,50,000 ohms, the collector voltage will be:

VC = IC RC = 9.8 A X 2,50,000 ohms = 2.45 volts.

Thus, the voltage gain is 2.45/.005 = 490. Voltage gain has been attained even-though there is a current loss. 9.2 The common emitter circuit gives both a current gain and a voltage gain. For this reason, it is by far the most versatile and widely used configuration. Recall the relationship Ice= .IBM, where is the common emitter current gain and generally has values between 30 and 300. This current gain comes about by the very nature of the internal working of the transistor. A voltage gain is realized because the collector current (output) is, fed through larger resistor than the base current (input). For example, if 10 A of base signal current goes through a 1000 ohm input resistance, the input voltage is .01 volt. Assuming a of 50, the collector current is 500 A. A typical collector resistor might be 10,000 ohms, so the output voltage is 500 A X 10,000 ohms, or 5 volts. The voltage gain is 510.01 = 500.



9.3 The common collector is different from the common base that it can produce a current gain, but it has a voltage gain less than one. The current gain for the common collector is almost the same as that for the common emitter. This is because the input current to both circuits is base current, while the output current for the common collector is emitter current and for the common emitter it is collector current. These two currents are almost equal, making the current gain almost the same. . As mentioned before the voltage gain is less than one. This is because the output is taken off at emitter, which is at a slightly lower voltage than the base. Thus the common collector is used as a current amplifier, only.

9.4 In general, the input resistance of a transistor circuit can be considered to be the resistance that signal current encounters between the input element and the -common element, this resistance is usually rather low since the only resistance encountered is the base to emitter forward biased junction. The input signal in the common collector circuit encounters not only the forward biased junction but the load resistor in the emitter lead as well. As mentioned before, the output current also goes through this resistor and establishes a voltage drop across it that opposes the input current. For this reason, the input resistance is quite high for the common collector.

9.5 The output resistance can be thought of, as being the resistance from the output element Looking back into the transistor to the common element. The output resistance for the common base is the reverse biased collector to base junction. This is the highest output resistance of the three configurations. The common emitter output resistance is lower than the common base, because it includes the reverse based collector base junction and provides an internal feedback path that lowers the resistance of the reverse biased collector base junction and therefore decreases the whole output resistance. The output resistance of the common collector circuit is the lowest of the three configurations. Because of its high input resistance and low output resistance, the common collector circuit is often used to match a high resistance device. Such as a crystal phonograph pickup to a low resistance load, the table below shows some typical values (for comparison purposes) of the more important characteristics of the three configurations.



9.4 A comparison of characteristics of three configurations is given below: Characteristics Common Common Common

Base emitter collector Current Gain Less than 1 30 to 300 30 to 300 (.95 to.99 ) (about.95 ) Voltage Gain 500 to 800 300 to 600 Less than 1

Input resistance 50 to 200 500 to 1000 20k-100k

Output resistance 300,000 50,000 500

Input-Output phase In phase 1800 out of phase in Phase

CHARECTERISTIC CURVES


CHAPTER -10 CHARACTERISTIC CURVES

10.1 A clear picture of how a transistor amplifies can be derived from the characteristic curves. These curves are plots of various voltages and currents as other voltages or currents are varied. Two sets of curves are particularly useful. They are the output characteristic curves for the common emitter and common base circuits.

10.2 The most widely used family of curves supplied by the transistor manufactures is the common emitter output characteristics curves. Figure shows a circuit that can be used to develop the family of curves. The three items of interest are the base current, the collector current, and the collector voltage. Meters are inserted in the circuit to measure these parameters. The curves are plotted by selecting a particular base current (by adjusting Rb) and increasing the collector voltage while monitoring the collector current.

FIG. 10.1 CIRCUIT USED FOR CONSTRUCTION OF CURVES

10.3 Figure shows a portion of the family of curves that results from measurements taken for different values of lb. Start with lb = 0 A. Set the collector voltage to 5 volts and measure lc. This is point 1 on the curve. Then set the collector voltage to 10, 15 and 20 volts, measuring lc, each time. This results in points 2, 3 and 4 on the curve. The result of joining these points together is the single curve marked lb = 0 A. By increasing the base current to say 20 A (by adjusting Rb) and again making measurements of Ic as collector voltage is varied will lead to another curve marked lb = 20 A. A whole family of curves can be generated this way by selecting convenient values of base current.



FIG. 10.2 OUTPUT CURVES (PARTIAL) 10.4 For previous section described how the output curves are generated. This was done by keeping lb constant and varying the collector power supply. However, this is done only to construct the curves. The actual use of the curves is quite different. For practical working, load lines have to be drawn.

10.5 An important line, called the load line, is determined and drawn on the curves. Assume, from the amplifier specifications, that a load resistor RL of 4000 ohms is required. a straight line, called the 4000 ohm line is shown in the figure and it can be constructed on the family of curves in the following way. The example uses a 20 volt power supply. The two points used to determine the location of the load line are, 1. When Vce=0 and the full 20 volts are across the 4000 ohm resistor this makes Ic = 4 mA and is marked A (on the curves), and 2, when Vce = 20 volts and there is no current in the collector (this is marker B on the curves). The line joining these two points is the 4000 ohm load line.

Note that point B is determined solely by the power supply voltage. If RL were 8000 ohms, point B would be the same as with a 4000 ohm load. However when Vce = 0, Ic would be 2.5 mA and it is shown as point C on the curves. The 8000 ohm load line would be the line joining the points B and C.

Once the load line has been drawn On the family of curves, a suitable operating point can be determined. Suppose it is decided to operate somewhere in the middle of the load line at point Q, this is specified by a particular base current (40 A) and collector voltage (12 volts). The whole purpose of bias current then is to establish an operating point with no signal in the circuit. This point is called the quiescent point because it is the quiet, still, or no signal operating point. It is, determined solely by the bias conditions.

4

3

2

1

0

5

5 10 15 20

Ib = 100

A

Ib = 20

A

Ib = 0

A

VCE ( VOLTS)

I C (

mA )



This same set of curves can be used for the common collector circuit since Ic and le are almost the same (Ic = le). The only changed needed would be to use le instead of lc in the curves shown in the illustration.

FIG. 10.3 COMMON EMITTER OUTPUT CURVES

10.6 To construct the common base output characteristics curves, a circuit such as the one shown in figure is used. Meters are placed in the circuit to measure the parameters of interest. For the plotting of the common base curves, le is held constant and the collector current is monitored while the collector supply is varied. For each value of le a single curve is determined. When all these curves for different values of IC are plotted together, a family of curves shown in figure results.

They look very similar to the common emitter curves except for one very important point. The so called "running" parameter in the common base curves is the emitter current, while in the common emitter it was the base current. These are the input currents to their respective circuits.

FIG. 10.4 CIRCUIT FOR COMMON BASE CURVE CONSTRUCTION

012345

0 5 10 15 20

Ib = 100

AIb = 80

A

Ib = 60

AIb = 40

A

Ib = 20

A

VCE ( VOLTS)

400

LOAD LINEA

C

B

800

LOAD LINE Q

20V

VCE

RLIb



10.7 The load line for the common base is determined in a similar manner as that used for the common emitter. An example is shown in figure for a 4000 ohm load resistor. Just as for the common emitter, the operating point with no signal is called the quiescent point and is labelled Q in the figure. Note that the le curves have the same value as the value of lc along vertical axis. This is as expected, since le & lc are approximately equal (lc =. (le)

FIG. 10.5 COMMON BASE OUTPUT CIRCUIT CURVES

OPERATING LIMITS OF TRANSISTORS


CHAPTER - 11 OPERATING LIMITS OF TRANSISTORS

11.1 Several precautions must be considered in selecting a particular type, of transistor for reliable operation in a circuit. The most important limitations are maximum collector voltage and current, maximum power dissipation and cut off frequency.

11.2 Just as for other electronic devices, it is reasonable to assume that the voltage and current applied to a transistor cannot be increased indefinitely without damage occurring. The maximum voltage that can be applied between the collector and emitter is limited by the "breakdown voltage". The output curves previously shown for the common emitter did not extend in the We direction far enough to show the breakdown voltage. Fig. shows that if we is extended far enough, a sudden increase in collector current occurs even though the base current has been kept constant. The voltage at which the curves break sharply upward is called "Breakdown voltage". Most transistors have a voltage breakdown rating of at least 30 volts. Since the transistor does not function normally and is subject to destruction in this region, the quiescent operating point has to be selected, so that this area is avoided.

Fig.11.1 Breakdown Voltage

11.3 Breakdown voltage establishes the maximum allowable voltage. The maximum current, however, is not as well defined. The maximum current allowed really depends on the operating voltage, and the combinations of the two establish maximum power dissipation. Except in special cases, such as pulse - switching applications a large collector current is not desirable. The, higher the collector current, the lower is the current gain. Generally speaking, the particular requirements for establishing the quiescent point will' establish the collector current.

OPERATING LIMITS OF TRANSISTORS


11.4 There is a maximum power that the transistor itself can dissipate. If more than this power is dissipated, the transistor will be destroyed. Since power is current times voltage, the maximum allowable power can be plotted on the collector voltage versus collector current output curves, as shown in figure. If the collector voltage is multiplied by the collector current a curve such as that labelled "maximum allowable power" results. The shaded area is where the power dispassion exceeds the manufacture's rating. The curve in the illustration is for an assumed maximum rating of .5 watt. The load line and quiescent point should be selected so that no time does the collector voltage and current result in operating in the shaded area.

Fig.11.2 Maximum Power Dissipation

11.5 The current gain,, is usually specified for a low-frequency signal such as 1000 hertz (cycle per second). However, as frequency is increased, gain begins to diminish; this is due to transit time of the internal carriers and various capacitances in the transistor. The fall of with frequency is shown in figure. The frequency at which (x falls to 0.7 of its value at low frequency is called the cut-off frequency. The example in the figure shows a cut off frequency of approximately 10 MHz. The transistor still operates beyond this frequency, but the gain will be lower.

FIG. 11.3 FREQUENCY CUT-OFF

SPECIFICATION OF TRANSISTORS


CHAPTER - 12 SPECIFICATIONS OF TRANSISTORS

12.1 Transistors are often classified as either general - purpose, high frequency or high power types. These groups, however, unlike the groups for vacuum tubes, are further sub-divided according to the material from which the transistor is made such as germanium or silicon. The method of basic construction provides further separation into type groups. Junction transistors, either PNP or NPN are usually listed apart from point contact transistors. Surface barrier (SBT) transistors form still another group, since their possible applications differ from those of the above mentioned groups. For convenience, PNP and NPN types are often listed separately.

12.2 Power transistors are generally listed separately from the physically smaller general purpose transistor. The same holds true for the special high frequency transistors, tetrode transistors, photo and other special purpose types.

12.3 Semiconductor like, diodes, transistors, are classified according to the basic materials from which they are made, their basic construction, and their use. Thus silicon diodes are usually listed separately from germanium diodes. Point contact diodes may be listed apart from junction diodes and so forth. Again, usage forms a basis for further separation. General purpose diodes comprise most of the germanium types. High frequency types used for radar or microwave applications are usually grouped separately since An physical shape and construction, they differ appreciably from the general purpose types. The same holds true for the high power types used in power handling circuits and power supplies. This last type may be grouped or listed with metallic rectifiers, such as selenium or copper oxide, since they serve a similar purpose.

12.4 Semiconductors, like vacuum tubes, are available in a large variety of different types, each with its own unique characteristics. The characteristics of each of these devices are usually presented in specification sheets, or they may be included in tube or transistor, manuals.

12.5 The lead paragraph of a semiconductor' specification is a general description of the device, and usually contains three specific pieces of information, the kind of transistor (or diode), a few major application areas, and general sales features, including physical size and packaging.



12.6 The absolute maximum ratings should not be exceeded under any circumstance. Exceeding them may cause semiconductor failure.

12.7 The power dissipation of a transistor depends on its junction temperature. The higher the temperature of the air surrounding the transistor (ambient temperature), the less power the device can dissipate. A factor telling how much the transistor must be derated for each degree of increase in ambient temperature in degrees centigrade is usually given. All of the remaining ratings define the capabilities of the device under specified test conditions. These characteristics are used by the design engineer to design matching networks and to calculate exact circuit performance.

12.8 Small signal characteristics are usually expressed in minimum, nominal and maximum values. Included in these are: the current transfer ratio which is another name for input impedance; output admittance; power gain; and noise figure. When is expressed as an a-c characteristic, the symbol life may be used. Many specification sheets also list the dc using the symbol hFE. Since it is somewhat dependent on frequency some specification sheets list P for more than one frequency. The noise figure is a measurement derived to evaluate the amount of noise produced in a circuit by a transistor.

12.9 High frequency characteristics, usually listed separately include the frequency cut off of a transistor, which is, defined as that frequency at which the grounded base current gain drops to 0.707 of the 1 KHz value. It gives a rough indication of the useful, frequency range of the device. The collector to base capacity and power gain at specified frequencies may also be included in high frequency characteristics.

12.10 The d-c characteristics usually include the collector breakdown voltage, collector cut-off current, and collector saturation resistance. The collector cut-off current is the leakage current from collector to base when no emitter current is being applied. This leakage current varies with temperature changes, and must be taken on to account whenever any semiconductor, devices is designed into equipment to be used over a wide range of ambient temperature.

12.11 Switching characteristics, also listed separately, show how the device responds to an input pulse under specified driving conditions. The response time given is very much dependent on the circuit. The terms used are explained in the curves shown in figure. IB1 and IB2 are base current values of an applied input pulse, and lc is resultant collector current. The delay time (td) is the time between the start of the input pulse



and the point at which the output pulse reaches ten percent of its maximum amplitude. The rise time (tr) is that time required for the output pulse to reach ninety per cent amplitude from the ten percent level. The stoppage time (ts) is the time between the removal of the input pulse and the point at which the amplitude of the output pulse has decreased to ninety percent of' its millimetre. The fall time (tf) is the time it takes the output pulse to reach ten percent of its amplitude from the ninety percent level. Such values are usually expressed in microseconds (s.).

Fig, 12.1 Switching Characteristics Of Semi-Conductor Devices

12.12 Probably the oldest standard numbering system in current common use is the American JEDEC". In this, the Electronic Industries Association (E.I.A.) in the United States registers devices from specifications put up by manufactures. It uses a numbering system in which the first numeral shows how many diode junctions the device has, with a 1 for a diode, a '2' for a triode transistor and a "3 for a tetrode. After this initial numeral comes an 'N' and then the number in serial order under which the device was registered with the authority. As an example, the N914" is the 914th triode transistor registered. By the end of 1967 both IN (diode) and 2N (triode) numbers registered has passed the 5,000 marks.

Any manufacturer, provided he meets the specification as registered by the original manufacturer with E.I.A. can supply devices to JEDEC numbers. The full details of any individual registered device can be obtained from E.I.A. 200 Eye st,N.W., Washington, D.C.20006. Unfortunately, they do not publish an easily available comprehensive authoritative list of "JEDEC" devices and their characteristics.

INPUT PULSE

OUTPUT PULSE

IB1IB2

t & tr tr

90%10%

IC

0

tr



12.13 Although the "JEDEC" standard numbering has come into fairly common use in Europe, there is a European standard system, known as "PRO ELECTRON" which is also widely used in parallel with "JEDEC". The organising authority is the Association International "PRO ELECTRON", of 10, Avenue Namoir, Brussels.

As with the "JEDEC" system, the manufacturer registers- with PRO ELECTRON" a device he has developed. Any other manufacturer can then supply devices marked with the same registered number, provided his version also meets the electrical and mechanical specification registered with PRO ELECTRON".

The "PRO ELECTRON" system has one high advantage over "JEDEC". All you can tell from a 'JEDEC" numbers is whether the device is a diode, triode, etc., and same indications of the time or registration, since low numbers, mean the device was registered years age. With "PRO ELECTION, the letters and numbers used are much more significant.

The "PRO ELECTRON" type number always has five places: either two letters and three numerals (as in BC 107) or three letters and two numerals (as in BCY72). The first letter indicates the bulk semiconductor material used:

A: Germanium; B: Silicon;

The second letter indicates the circuit type of the device. A: Signal diode, non-power; B: Variable capacitance diode; C: Transistor I.f. non-power; D: Transistor I.f. power; E: Tunnel diode; F: Transistor h.f. non-power; G: Multiple device; H: Field probe; K: Hall generator; L: Transistor h.f. power; M: Hall modulator or multiplier; P: Radiation sensitive device (photo- diode. Photo-transistor or photo conductive

device);



Q: Radiation generating device; R: Specialised break down device., S: Transistor, switching non-power; T: Controlling and switching device with break down characteristics; power (S.C.R. or thyristor etc.); U: Transistor switching power; X: Multiple diodes; Y: Rectifier, power; Z: Zener diode (voltage reference or regulator);

The final three places of the PRO ELECTRON five place registration numbers give an indication of the general area of use and serial number. Where three numerals are used (BC 107) this indicates a device for "entertainment or "Consumer" use, i.e. for ratio etc. The three numbers run from 100 to 999. Where a letter indicates a device for use in industrial and professional equipment.. The letters (which bear no significance) in this case start from Z back through Y,X etc. The accompanying serial numbers run from 10 to 99 only. Sub-classifications are permitted in certain devices such as Zener diodes, and thyristors (SCRs) in the "PRO ELECTRON" system. These are indicated by further coding added after a hyphen at the end of the five place basic number according to a significant system.

From Zeners, the code addition gives information on the nominal voltage and its tolerance. The tolerance appears first as a single letter:

A = 1%; B = 2%; C = 5%; D = 10% and E = 15%.

The nominal voltage follows as a numeral plus the letter V in the position of the decimal point where necessary. Thus BZY88-09V1 represents a silicon zener for industrial use with registration number Y88 tolerance 5% and nominal voltage 9.1V.

For rectifiers and thyristers, the additional "PRO ELECTRON" code numbers signify the repetitive peak reverse voltage in volts. Thus BYX36-100 indicates a silicon rectifier for industrial use with registration number X36 and a 100-Y rating while the BTY 99-100 represents a silicon thyrister for industrial use with registration number Y 99 and 100 - V rating. With power rectifiers and thyristors, the cathode is normally connected to the stud mounting. Where the anode is connected to the stud ("reverse polarity"), a final letter R is added. By this, a BTY99 - 100R signifies a reverse polarity BTY99-100.



Recently supplementary coding have arisen for ordinary transistors too, viz. the well known BC108 in versions coded BC 108A,B and C. The final letter suffix in this case denotes narrow spread limits of the Basic BC108 device.

12.14 The "PRO ELECTRON" system has become widely accepted in EUROPE during the 1960s and is often referred to as the "new" European system. It has replaced the old European system under which semiconductors were indicated by an initial "0" (standing for zero hector volts in the then existing valve coding) After the initial 0 'came a letter in the cod

Irset T3 Fundamental of Electronics

Documents

transistor operation

atomic theory

neutral particles

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field effect transistor

uni junction transistors

operation limit of transistors

specifications of transistors