Semiconductor physics

Semiconductor

Physics

Introduction

• Semiconductors are materials whose electronic properties are intermediate between those of Metals and Insulators.

• They have conductivities in the range of 10 -4

to 10 +4S/m.

• The interesting feature about semiconductors is that they are bipolar and current is transported by two charge carriers of opposite sign.

• These intermediate properties are determined by

1.Crystal Structure bonding Characteristics. 2.Electronic Energy bands.

• Silicon and Germanium are elemental semiconductors and they have four valence electrons which are distributed among the outermost S and p orbital's.

• These outer most S and p orbital's of Semiconductors involve in Sp3 hybridanisation.

• These Sp3 orbital's form four covalent bonds of equal angular separation leading to a tetrahedral arrangement of atoms in space results tetrahedron shape, resulting crystal structure is known as Diamond cubic crystal structure

Semiconductors are mainly two types

1. Intrinsic (Pure) Semiconductors 2. Extrinsic (Impure)

Semiconductors

Intrinsic Semiconductor

• A Semiconductor which does not have any kind of impurities, behaves as an Insulator at 0k and behaves as a Conductor at higher temperature is known as Intrinsic Semiconductor or Pure Semiconductors.

• Germanium and Silicon (4th group elements) are the best examples of intrinsic semiconductors and they possess diamond cubic crystalline structure.

Si

Si

SiSiSi

Valence Cell

Covalent bonds

Intrinsic Semiconductor

When any pentavalent element such as Phosphorous, Arsenic or Antimony is added to the intrinsic Semiconductor , four electrons are involved in covalent bonding with four neighboring pure Semiconductor atoms.

The fifth electron is weakly bound to the parent atom. And even for lesser thermal energy it is released Leaving the parent atom positively ionized.

N - type Semiconductors

N-type Semiconductor

Si

Si

SiPSi

Free electron

Impure atom (Donor)

P-type semiconductors

• When a trivalent elements such as Al, Ga or Indium have three electrons in their outer most orbits , added to the intrinsic semiconductor all the three electrons of Indium are engaged in covalent bonding with the three neighboring Si atoms.

• Indium needs one more electron to complete its bond. this electron maybe supplied by Silicon , there by creating a vacant electron site or hole on the semiconductor atom.

• Indium accepts one extra electron, the energy level of this impurity atom is called acceptor level and this acceptor level lies just above the valence band.

• These type of trivalent impurities are called acceptor impurities and the semiconductors doped the acceptor impurities are called P-type semiconductors.

Si

Si

SiInSi

HoleCo-Valent bonds

Impure atom (acceptor)

• Even at relatively low temperatures, these acceptor atoms get ionized taking electrons from valence band and thus giving rise to holes in valence band for conduction.

• Due to ionization of acceptor atoms only holes

and no electrons are created.

• Thus holes are more in number than electrons and hence holes are majority carriers and electros are minority carriers in P-type semiconductors.

Intrinsic or Pure Semiconductor:

C.B

V.B

Eg 0.74 eV

Heat Energy

+

+

+

Ge Ge

Ge Ge

Ge Ge

Ge Ge

Ge Ge

Ge Ge

Ge Ge

Ge Ge

Broken Covalent Bond

Free electron ( - )

Valence electrons

Covalent Bond

Hole ( + )

Intrinsic Semiconductor is a pure semiconductor. The energy gap in Si is 1.1 eV and in Ge is 0.74 eV.

Ge: 1s2, 2s2, 2p6,3s2, 3p6, 3d10, 4s2, 4p2. (Atomic No. is 32)Si: 1s2, 2s2, 2p6,3s2, 3p2. (Atomic No. is 14)

In intrinsic semiconductor, the number of thermally generated electrons always equals the number of holes. So, if ni and pi are the concentration of electrons and holes respectively, then ni = pi. The quantity ni or pi is referred to as the ‘intrinsic carrier concentration’.

Doping a Semiconductor:Doping is the process of deliberate addition of a very small amount of impurity into an intrinsic semiconductor.

The impurity atoms are called ‘dopants’.

The semiconductor containing impurity is known as ‘impure or extrinsic semiconductor’.

Methods of doping:i) Heating the crystal in the presence of dopant atoms.

ii) Adding impurity atoms in the molten state of semiconductor.

iii) Bombarding semiconductor by ions of impurity atoms.

Extrinsic or Impure Semiconductor:N - Type Semiconductors:

Ge Ge Ge

Ge

Ge

Ge

Ge Ge+

+As

0.045 eVEg = 0.74 eV

C.B

V.B

Donor level

-

When a semiconductor of Group IV (tetra valent) such as Si or Ge is doped with a penta valent impurity (Group V elements such as P, As or Sb), N – type semiconductor is formed.When germanium (Ge) is doped with arsenic (As), the four valence electrons of As form covalent bonds with four Ge atoms and the fifth electron of As atom is loosely bound.

The energy required to detach the fifth loosely bound electron is only of the order of 0.045 eV for germanium.

A small amount of energy provided due to thermal agitation is sufficient to detach this electron and it is ready to conduct current.

The force of attraction between this mobile electron and the positively charged (+ 5) impurity ion is weakened by the dielectric constant of the medium.

So, such electrons from impurity atoms will have energies slightly less than the energies of the electrons in the conduction band.

Therefore, the energy state corresponding to the fifth electron is in the forbidden gap and slightly below the lower level of the conduction band.

This energy level is called ‘donor level’.

The impurity atom is called ‘donor’.

N – type semiconductor is called ‘donor – type semiconductor’.

P - Type Semiconductors:

When a semiconductor of Group IV (tetra valent) such as Si or Ge is doped with a tri valent impurity (Group III elements such as In, B or Ga), P – type semiconductor is formed.When germanium (Ge) is doped with indium (In), the three valence electrons of In form three covalent bonds with three Ge atoms. The vacancy that exists with the fourth covalent bond with fourth Ge atom constitutes a hole.

Ge Ge Ge

Ge

Ge

Ge

Ge Ge+

+In 0.05 eV

Eg = 0.74 eV

C.B

V.B

Acceptor level

Distinction between Intrinsic and Extrinsic Semiconductor:

S. No. Intrinsic SC Extrinsic SC1 Pure Group IV elements. Group III or Group V elements

are introduced in Group IV elements.

2 Conductivity is only slight. Conductivity is greatly increased.

3 Conductivity increases with rise in temperature.

Conductivity depends on the amount of impurity added.

4 The number of holes is always equal to the number of free electrons.

In N-type, the no. of electrons is greater than that of the holes and in P-type, the no. holes is greater than that of the electrons.

Distinction between Semiconductor and Metal:

S. No. Semiconductor Metal1 Semiconductor behaves like an

insulator at 0 K. Its conductivity increases with rise in temperature.

Conductivity decreases with rise in temperature.

2 Conductivity increases with rise in potential difference applied.

Conductivity is an intrinsic property of a metal and is independent of applied potential difference.

3 Does not obey Ohm’s law or only partially obeys.

Obeys Ohm’s law.

4 Doping the semiconductors with impurities vastly increases the conductivity.

Making alloy with another metal decreases the conductivity.

SEMICONDUCTORSSemiconductors have resistivity between good conductors and insulators.

The resistivity of semiconductor lies approximately in between 10 -2

ohm m and 10 4 ohm m at room temperature..05/01/23 weyes57 19

ENERGY BANDS IN SOLIDSThere are discrete energy levels in the case of an isolated atom.

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Arrangement of electrons in an isolated Silicon atom

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Insulators

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Valence Band

Conduction Band

FORBIDDEN GAP

Ener

gy

In an insulator, the forbidden gap is very large and in general is more than 3eV.

No electron is available for conduction.

Large amount of energy is needed to move electron from valance band to conduction band.

Filled Band

Semiconductors

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Valence Band

Conduction Band

FORBIDDEN GAPAround 0.7eV (Ge) and 1.1 eV (Si)

Ener

gyIn the case of semiconductors the forbidden gap is very small.

At 0K the conduction band is empty and the valence band is completely filled.

When a small amount of energy is supplied, the electrons can easily jump the forbidden gap.

The conductivity of a semiconductor is of the order of 10 2mho m-1

Filled Band

Conductors

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Valence Band

Conduction Band

Ener

gy

In conductors there is no forbidden gap.

The valence band and the conduction band overlap.

The electrons from valence band freely enter into the conduction band due to overlapping of bands.

Therefore very low potential difference can cause continuous flow of current.

No forbidden gap

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Electrons and holes in semiconductors

At absolute 0 temperature, in a pure semiconductor the valence band is completely filled and the conduction band is vacant.

At 0 K

Conduction Band

Valence BandElectron

Ener

gy

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Electrons and holes in semiconductors

At room temperature some of the electrons get energy to break the covalent bond and moves in to the conduction band.

At Room temperature

Conduction Band

Valence Band

Hole

Electron

Ener

gy

27

Presentation Topic:

Half Wave RectifierFull Wave Rectifier

Crystal Diode Rectifiers

• Rectifier: Rectifier is that circuit, that converts ac to dc.

• The following two types of rectifier circuit can be used:

I. Half wave rectifierII.Full wave rectifier

28

Half wave Rectifier• The process of removing one-half the input signal to

establish a dc level is called half-wave rectification.• In Half wave rectification, the rectifier conducts current

during positive half cycle of input ac signal only.• Negative half cycle is suppressed.

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Half wave Rectifier AC voltage across secondary terminals AB changes its polarity after each half cycle.

During negative half cycle terminal A is negative so diode is reversed biased and conducts no current.So, current flows through diode during positive half cycle

only.

In this way current flows through load RL in one direction only

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Half wave Rectifier

• Disadvantage of Half wave rectifier:

The pulsating current in output contains ac components whose frequency is equal to supply frequency so filtering is needed.

The ac supply delivers power during half cycle only so output is low.

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Half wave Rectifier

• Output frequency of HWR: Output frequency of HWR is equal to input frequency.

This means when input ac completes one cycle, rectified wave also completes one cycle.

inout ff

32

Full-Wave Rectifier

• In Full wave rectification current flow through the load in same direction for both half cycle of input ac.

• This can be achieved with two diodes working alternatively.

• For one half cycle one diode supplies current to load and for next half cycle another diode works.

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p – type semiconductorThe addition of trivalent impurities such as

boron, aluminum or gallium to an intrinsic semiconductor creates deficiencies of valence electrons, called "holes". It is typical to use B2H6 diborane gas to diffuse boron into the silicon material.

6

n – type semiconductorThe addition of pentavalent impurities such

as antimony, arsenic or phosphorous contributes free electrons, greatly increasing the conductivity of the intrinsic semiconductor. Phosphorous may be added by diffusion of phosphine gas (PH3).

7

Depletion region is the region having no free carriers.

Further movement of electrons and holes across the junction stops due to formation of depletion region.

9

Depletion region acts as barrier opposing further diffusion of charge carriers. So diffusion stops within no time.

Current through the diode under no-bias condition is zero.

Reverse bias

Positive of battery connected to n-type material (cathode).

Negative of battery connected to p-type material (anode).

10

Reverse bias….. Free electrons in n-region are drawn towards

positive of battery, Holes in p-region are drawn towards negative of battery.

Depletion region widens, barrier increases for the flow of majority carriers.

Majority charge carrier flow reduces to zero. Minority charge carriers generated thermally can

cross the junction – results in a current called “reverse saturation current” Is , Is is in micro or nano amperes or less. Is does not increase “significantly” with increase in the reverse bias voltage

Forward bias Positive of battery connected to p-type

(anode)

Negative of battery connected to n-type (cathode)

11

Forward bias… Electrons in n-type are forced to recombine

with positive ions near the boundary, similarly holes in p-type are forced to recombine with negative ions.

Depletion region width reduces. An electron in n-region “sees” a reduced

barrier at the junction and strong attraction for positive potential.

As forward bias is increased, depletion region narrows down and finally disappears – leads to exponential rise in current.

Forward current is measured in milli amperes

12

Zener Diode

A diode which is heavily doped and which operates in the reverse breakdown region with a sharp breakdown voltage is called a Zener diode.

This is similar to the normal diode except that the line (bar) representing the cathode is bent at both side ends like the letter Z for Zener diode.

13

When the electrical field across the junction is high due to the applied voltage, the Zener breakdown occurs because of breaking of covalent bonds and produces a large number of electrons and holes which constitute a steep rise in the reverse saturation current (Zener current IZ). This effect is called as Zener effect.

Zener current IZ is independent of the applied voltage and depends only on the external resistance.

I-V characteristic of a Zener diodeThe forward characteristic is simply

that of an ordinary forward biased junction diode. Under the reverse bias condition, the breakdown of a junction occurs.

Its depends upon amount of doping. It can be seen from above figure as the reverse voltage is increased the reverse current remains negligibly small up to the knee point (K) of the curve.

Zener diode Applications:I. Zener diodes are used as a voltage

regulator.II. They are used in shaping circuits as peak

limiters or clippers.III. They are used as a fixed reference voltage

in transistor biasing and for comparison purpose.

IV. They are used for meter protection against damage from accidental application of excessive voltage.

HOW IT WORKS..?• Photovoltaic cells are made of special

materials called semiconductors such as silicon. An atom of sili con has 14 electrons, arranged in three different shells. The outer shell has 4 electrons. Therefore a silicon atom will always look for ways to fill up its last shell, and to do this, it will share electrons with four nearby atoms. Now we use phosphorus(with 5 electrons in its outer shell). Therefore when it combines with silicon, one electron remains free. 47

48

• When energy is added to pure silicon it can cause a few electrons to break free of their bonds and leave their atoms. These are called free carriers, which move randomly around the crystalline lattice looking for holes to fall into and carrying an electrical current. However, there are so few, that they aren't very useful. But our impure silicon with phosphorous atoms takes a lot less energy to knock loose one of our "extra“ electrons because they aren't tied up in a bond with any neighboring atoms. As a result, we have a lot more free carriers than we would have in pure silicon to become N-type silicon.

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• The other part of a solar cell is doped with the element boron(with 3 electrons in its outer shell)to become P-type silicon.

• Now, when this two type of silicon interact, an electric field forms at the junction which prevents more electrons to move to P-side.

• When photon hits solar cell, its energy breaks apart electron-hole pairs. Each photon with enough energy will normally free exactly one electron, resulting in a free hole as well. If this happens close enough to the electric field, this causes disruption of electrical neutrality, and if we provide an external current path, electrons will flow through the P side to unite with holes that the electric field sent there, doing work for us along the way. The electron flow provides the current, and the cell's electric field causes a voltage.

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• Now to protect the solar cell, we use antireflective coating to reduce the losses and then a glass plate to protect the cell from elements.

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TYPES OF SOLAR CELL..• There are basically 3 types of solar cell

technology: Discrete Cell technology. Integrated Thin Film

technology. Multi crystalline Silicon

technology.

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Transistors• They are unidirectional current carrying

devices like diodes with capability to control the current flowing through them

• Bipolar Junction Transistors (BJT) control current by current

• Field Effect Transistors (FET) control current by voltage

• They can be used either as switches or as amplifiers

• A transistor allows you to control the current, not just block it in one direction.

• A good analogy for a transistor is a pipe with an adjustable gate.

• A transistor has three terminals. • The main path for current is between the collector and

emitter. • The base controls how much current flows, just like the gate

controls the flow of water in the pipe.

BIPOLAR JUNCTION TRANSISTOR• Two back to back P-N junctions• Emitter

– Heavily doped– Main function is to supply majority carriers to base

• Base– Lightly doped as compared to emitter– Thickness 10-6 m

• Collector– Collect majority carriers from emitter through base– Physically larger than the emitter region

EE

BB

NN PP NN CC EE

BB

PP NN PP CC

The BJT – Bipolar Junction TransistorThe BJT – Bipolar Junction Transistor

The Two Types of BJT TransistorsThe Two Types of BJT Transistorsnpnnpn pnppnp

nn pp nnEE

BB

CC pp nn ppEE

BB

CC

Cross SectionCross Section Cross SectionCross Section

BB

CC

EE

Schematic Schematic SymbolSymbol

BB

CC

EE

Schematic Schematic SymbolSymbol

LP4 57

NPN Bipolar Junction Transistor

LP4 58

PNP Bipolar Junction Transistor

• The collector surrounds the emitter region, making it almost impossible for the electrons injected into the base region to escape being collected, thus making the resulting value of α very close to unity, and so, giving the transistor a large β

STRUCTURE

Common-Emitter Common-Emitter Circuit DiagramCircuit Diagram

++__VVCCCC

IICCVVCECE

IIBB

Collector-Current CurvesCollector-Current Curves

VVCECE

IICC

Active Active RegionRegion

IIBB

Saturation RegionSaturation RegionCutoff RegionCutoff RegionIIBB = 0 = 0

Region of Operation

Description

Active Small base current controls a large collector current

Saturation VCE(sat) ~ 0.2V, VCE increases with IC

Cutoff Achieved by reducing IB to 0, Ideally, IC will also be equal to 0.

BJT’s have three regions of operation:1) Active - BJT acts like an amplifier (most common use)2) Saturation - BJT acts like a short circuit3) Cutoff - BJT acts like an open circuit

BJT is used as a switch by switchingbetween these two regions.

rsat

Vo

_ +

C

B

E Saturat ion Region Model

Vo

_ +

C

B

E

Active Region Model #1

dc IB

IB

Ro

Vo

_ +

C

B

E

Active Region Model #2

dc IB ICEO

RBB

VCE (V)

IC(mA)

IB = 50 A

IB = 0

30

5 10 15 20 0

0

IB = 100 A

IB = 150 A

IB = 200 A

22.5

15

7.5

Saturation Region

Active Region

Cutoff Region

C

E

B

When analyzing a DC BJT circuit, the BJT is replaced by one of the DC circuit models shown below.

DC Models for a BJT:

DC DC and DC and DC

= Common-emitter current gain= Common-emitter current gain = Common-base current gain= Common-base current gain = I= ICC = I = ICC

IIBB I IEE

The relationships between the two parameters are:The relationships between the two parameters are: = = = = + 1+ 1 1 - 1 -

Note: Note: and and are sometimes referred to as are sometimes referred to as dcdc and and dcdc because the relationships being dealt with in the BJT because the relationships being dealt with in the BJT are DC.are DC.

Common Emitter Configuration• The input signal is applied between the base and emitter

and the output signal is taken out from the collector and the emitter

• Ratio of collector current to base current is called dc beta (dc) of a transistor

C

E

+B

-

BC

B

C

IIOR

II

Relation between and

E

C

II

B

C

II

and

B

E

II

CEB III usingB

C

II

then becomes

CE

C

III

or

1//

/

ECEE

EC

IIIIII

1 or 1 or 1/

Common Collector Configuration• The input signal is applied between the base and collector

and the output signal is taken out from the emitter-collector circuit

• Ratio of emitter current to base current is

11/

.B

C

C

E

B

E

II

II

II

From the figure C

E

+B- BBBCBE IIIIII 1

Output current=(1+) x Input current

Relation between transistor currents

CBE III ::We know

EBC III and

EEEEC

B IIIIII

11

1/

1

1and

because

111We get

Static Characteristics• Common Emitter Static characteristics

– Input characteristics. IB varies with VBE when voltage VCE is held constant

• VCE is adjusted with the help of R1

• VBE is increased and corresponding values of IB are noted• The plot gives input characteristics• Procedure is repeated for different (constant) values of VCE

• This characteristics is used to find the input resistance of the transistor. Its value is given by the reciprocal of its slope

Rin= VBE / IB

VVBEBE

IIBB

2 mA2 mA

4 mA4 mA

6 mA6 mA

8 mA8 mA

0.7 V0.7 V

Static Characteristics• Common Emitter Static characteristics

– Output characteristics. IC varies with VCE when IB is held constant

• IB is held constant

• VCE is increased and corresponding values of IC are noted

• The plot gives output characteristics• Then IB is increased to a value little higher and whole

process is repeated• The output resistance in this case is very less as

compared to CB circuit and is given by Rout= VCE / IC

As VAs VCECE increases from zero, I increases from zero, ICC rapidly increases to saturation level for a fixed rapidly increases to saturation level for a fixed value of Ivalue of IBB

IICC flows even when I flows even when IBB=0 (Collector leakage current or reverse saturation =0 (Collector leakage current or reverse saturation current Icurrent ICEOCEO), the transistor is said to be cutoff), the transistor is said to be cutoffWhen VCB is permitted to increase beyond a certain value, IC increases rapidly due to avalanche breakdown This characteristics may be used to find ac ac =IC/ IB

VVCECE

IICC

Active Active RegionRegion

IIBB

Saturation RegionSaturation RegionCutoff RegionCutoff RegionIIBB = 0 = 0

Region of Operation

Description

Active Small base current controls a large collector current

Saturation VCE(sat) ~ 0.2V, VCE increases with IC

Cutoff Achieved by reducing IB to 0, Ideally, IC will also equal 0.

Figure: Common-emitter characteristics displaying exaggerated secondary effects.

Figure: Common-emitter characteristics displaying exaggerated secondary effects.

Various Regions (Modes) of Operation of BJT Various Regions (Modes) of Operation of BJT

• Most important mode of operationMost important mode of operation• Central to amplifier operationCentral to amplifier operation• The region where current curves are practically flatThe region where current curves are practically flat

Active:Active:

Saturation:Saturation: • Barrier potential of the junctions cancel each other out Barrier potential of the junctions cancel each other out causing a virtual short (behaves as on state Switch)causing a virtual short (behaves as on state Switch)

Cutoff:Cutoff: • Current reduced to zeroCurrent reduced to zero• Ideal transistor behaves like an open switchIdeal transistor behaves like an open switch

* Note: There is also a mode of operation called * Note: There is also a mode of operation called inverse active mode, but it is rarely used.inverse active mode, but it is rarely used.

BJT Trans-conductance CurveBJT Trans-conductance CurveFor Typical NPN Transistor For Typical NPN Transistor 11

VVBEBE

IICC

2 mA2 mA

4 mA4 mA

6 mA6 mA

8 mA8 mA

0.7 V0.7 V

Collector Current:Collector Current:

IICC = = I IESES e eVVBEBE//VVTT Transconductance: Transconductance: (slope of the curve)(slope of the curve)

ggmm = I = ICC // V VBEBE

IIESES = The reverse saturation current = The reverse saturation current of the B-E Junction.of the B-E Junction.VVTT = = kT/qkT/q = 26 mV (@ T=300 = 26 mV (@ T=300ooK)K) = the emission coefficient and is = the emission coefficient and is usually ~1usually ~1

Three Possible Configurations of BJTThree Possible Configurations of BJT

Biasing the transistor refers to applying voltages to the transistor Biasing the transistor refers to applying voltages to the transistor to achieve certain operating conditions.to achieve certain operating conditions.1. 1. Common-Base Configuration (CB)Common-Base Configuration (CB) : : input input = V= VEBEB & I & IEE

output = Voutput = VCBCB & I & ICC

2. 2. Common-Emitter Configuration (CE):Common-Emitter Configuration (CE): input = V input = VBEBE & I & IBB

output= Voutput= VCECE & I & ICC

3. 3. Common-Collector Configuration (CC)Common-Collector Configuration (CC) :input = V :input = VBCBC & I & IBB

(Also known as Emitter follower)(Also known as Emitter follower) output = V output = VECEC & I & IEE

Introduction• The basic of electronic system nowadays is

semiconductor device. • The famous and commonly use of this device

is BJTs(Bipolar Junction Transistors).

• It can be use as amplifier and logic switches.• BJT consists of three terminal:

collector : C base : Bemitter : E

• Two types of BJT : pnp and npn

Transistor Construction• 3 layer semiconductor device consisting:

– 2 n- and 1 p-type layers of material npn transistor– 2 p- and 1 n-type layers of material pnp transistor

• The term bipolar reflects the fact that holes and electrons participate in the injection process into the oppositely polarized material

• A single pn junction has two different types of bias:– forward bias– reverse bias

• Thus, a two-pn-junction device has four types of bias.

Position of the terminals and symbol of BJT.

• Base is located at the middle and more thin from the level of collector and emitter• The emitter and collector terminals are made of the same type of semiconductor material, while the base of the other type of material

Transistor currents -The arrow is always drawn on the emitter

-The arrow always point toward the n-type

-The arrow indicates the direction of the emitter current:

pnp:E Bnpn: B EIC=the collector current

IB= the base currentIE= the emitter current

• By imaging the analogy of diode, transistor can be construct like two diodes that connetecd together.

• It can be conclude that the work of transistor is base on work of diode.

Transistor Operation• The basic operation will be described using the pnp

transistor. The operation of the pnp transistor is exactly the same if the roles played by the electron and hole are interchanged.

• One p-n junction of a transistor is reverse-biased, whereas the other is forward-biased.

Forward-biased junction of a pnp transistor

Reverse-biased junction of a pnp transistor