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    A semiconductor has electrical conductivity intermediate in magnitude between

    that of a conductor and an insulator. This means conductivity roughly in the range

    of 10-2 to 104siemens per centimeter (Scm-1). Semiconductors are the foundation

    of modern electronics, including radio, computers, and telephones. Semiconductor-

    based electronic components include transistors, solar cells, many kinds

    ofdiodes including the light-emitting diode (LED), the silicon controlled rectifier,

    photo-diodes, and digital and analog integrated circuits. Semiconductor solar

    photovoltaic panels directly convert light energy into electricity. In a metallic

    conductor, current is carried by the flow ofelectrons.

    The energy band structure of the semiconductors is similar to the insulators but in

    their case, the size of the forbidden energy gap is much smaller than that of the

    insulator. In this class of crystals, the forbidden gap is of the order of about 1ev,

    and the two energy bands are distinctly separate with no overlapping. At absolute

    0K, no electron has any energy even to jump the forbidden gap and reach the

    conduction band. Therefore the substance is an insulator. But when we heat the

    crystal and thus provide some energy to the atoms and their electrons, it becomes

    an easy matter for some electrons to jump the small ( 1 ev) energy gap and go to

    conduction band. Thus at higher temperatures, the crystal becomes a conductors.

    This is the specific property of the crystal which is known as a semiconductor .
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    Valence band and the conduction band lies in the energy gap.

    According to the Bohr's theory, free electrons in an isolated atom have certain

    definite discrete amount of energy. If large number of atoms is brought close to

    one another to form a crystal, they begin to influence each other. The valence

    electrons are attracted by the nucleus of the other atoms. This brings about a

    considerable modification in the case of energy levels of the electrons in the outer

    shells. The process of splitting of energy levels can be understood as follows:

    a) If intratomic spacing of atoms is very large i.e., r = d>>a, there is no intratomic

    separation. Each atom in the crystal behaves as free atom. Take for example silicon

    whose electronic configuration is 1s2




    3p2. If N atoms were to be

    considered in silicon crystal, then there will be 2N electrons filling 2N possible

    energy levels in 3s, 6N possible levels in 3p of which only 2N is completely filled.

    b) When the spacing is progressively decreased i.e., c

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    c) When r = c, the 3s and 3p electrons of neighbouring silicon atoms becomes

    appreciable. The energy of electrons of each atom starts changing, whereas the

    energies of electrons in the inner shell do not change.

    d) When r lies between b and c, the energy levels get slightly changed and instead

    of a single 3s or 3p levels, we get a large number of closely packed levels. This

    collection of closely spaced energy levels is called an energy band.

    e) When r = b>a, the gap between 3s and 3p completely disappear and the 8N

    energy levels are (2N of 3s and 6N of 3p sub shells) continuously distributed. In

    this stage 4N levels are filled and 4N levels are empty.

    f) When r = a i.e., actual spacing in the crystal the 4N filled energy levels are

    separated from 4N unfilled energy levels. This gap or separation is called the

    forbidden gap. E.g., the lower completely filled band is called valence band and

    upper unfilled band is called conduction band


    Pure semiconductors are called intrinsic semi-conductors. In a pure

    semiconductor, each atom behaves as if there are 8 electrons in its valence shell

    and therefore the entire material behaves as an insulator at low temperatures.

    A semiconductor atom needs energy of the order of 1.1ev to shake off the valence

    electron. This energy becomes available to it even at room temperature. Due tothermal agitation of crystal structure, electrons from a few covalent bonds come

    out. The bond from which electron is freed, a vacancy is created there. The

    vacancy in the covalent bond is called a hole.

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    This hole can be filled by some other electron in a covalent bond. As an electron

    from covalent bond moves to fill the hole, the hole is created in the covalent bond

    from which the electron has moved. Since the direction of movement of the hole is

    opposite to that of the negative electron, a hole behaves as a positive charge

    carrier. Thus, at room temperature, a pure semiconductor will have electrons and

    holes wandering in random directions. These electrons and holes are called

    intrinsic carriers.

    As the crystal is neutral, the number of free electrons will be equal to the number

    of holes. In an intrinsic semiconductor, if ne denotes the

    electron number density in conduction band, nh the hole number density in valence

    band and ni the number density or concentration of charge carriers, then

    ne = nh = ni

    DopingThe property of semiconductors that makes them most useful for constructing

    electronic devices is that their conductivity may easily be modified by introducing

    impurities into their crystal lattice. The process of adding controlled impurities to a

    semiconductor is known as doping. The amount of impurity, or dopant, added to an
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    intrinsic (pure) semiconductor varies its level of conductivity. Doped

    semiconductors are often referred to as extrinsic. By adding impurity to pure

    semiconductors, the electrical conductivity may be varied not only by the number

    of impurity atoms but also, by the type of impurity atom and the changes may be

    thousand folds and million folds. For example, 1 cm3

    of a metal or semiconductor

    specimen has a number of atoms on the order of 1022

    . Since every atom in metal

    donates at least one free electron for conduction in metal, 1 cm3

    of metal contains

    free electrons on the order of 1022

    . At the temperature close to 20 C , 1 cm3


    pure germanium contains about 4.21022

    atoms and 2.51013

    free electrons and


    holes (empty spaces in crystal lattice having positive charge) The addition

    of 0.001% of arsenic (an impurity) donates an extra 1017

    free electrons in the same

    volume and the electrical conductivity increases about 10,000 times."

    Extrinsic semiconductors

    As the conductivity of intrinsic semi-conductors is poor, so intrinsic semi-conductors are of little practical importance. The conductivity of pure semi-

    conductor can, however be enormously increased by addition of some pentavalent

    or a trivalent impurity in a very small amount (about 1 to 10 6 parts of the semi-

    conductor). The process of adding an impurity to a pure semiconductor so as to

    improve its conductivity is called doping. Such semi-conductors are called

    extrinsic semi-conductors. Extrinsic semiconductors are of two types :

    i) n-type semiconductor

    ii) p-type semiconductor

    n-type semiconductor
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    When an impurity atom belonging to group V of the periodic table like Arsenic is

    added to the pure semi-conductor, then four of the five impurity electrons form

    covalent bonds by sharing one electron with each of the four nearest silicon atoms,

    and fifth electron from each impurity atom is almost free to conduct electricity. As

    the pentavalent impurity increases the number of free electrons, it is called donor

    impurity. The electrons so set free in the silicon crystal are called extrinsic carriers

    and the n-type Si-crystal is called n-type extrinsic semiconductor. Therefore n-

    type Si-crystal will have a large number of free electrons (majority carriers) and

    have a small number of holes (minority carriers).

    In terms of valence and conduction band one can think that all such electrons

    create a donor energy level just below the conduction band as shown in figure. As

    the energy gap between donor energy level and the conduction band is very small,

    the electrons can easily raise themselves to conduction band even at room

    temperature. Hence, the conductivity of n-type extrinsic semiconductor is

    markedly increased.

    In a doped or extrinsic semiconductor, the number density of the conduction band

    (ne) and the number density of holes in the valence band (n h) differ from that in a

    pure semiconductor. If (ni) is the number density of electrons is conduction band,

    then it is proved that

    ne nh = ni2

    IfNd represents number density of donor atom then,

    ne Nd > > nh

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    p-type semiconductor

    If a trivalent impurity like indium is added in pure semi-conductor, the impurity

    atom can provide only three valence electrons for covalent bond formation. Thus a

    gap is left in one of the covalent bonds. The gap acts as a hole that tends to accept

    electrons. As the trivalent impurity atoms accept electrons from the silicon crystal,

    it is called acceptor impurity. The holes so created are extrinsic carriers and the p-

    type Si-crystal so obtained is called p-type extrinsic semiconductor. Again, as the

    pure Si-crystal also possesses a few electrons and holes, therefore, the p-type si-

    crystal will have a large number of holes (majority carriers) and a small number

    of electrons (minority carriers).

    It terms of valence and conduction band one can think that all such holes create an

    accepter energy level just above the top of the valance band as shown in figure.

    The electrons from valence band can raise themselves to the accepter energy level

    by absorbing thermal energy at room temperature and in turn create holes in the

    valence band.

    Number density of valence band holes (nh) in p-type semiconductor is

    approximately equal to that of the acceptor atoms (Na) and is very large as

    compared to the number density of conduction band electrons (ne). Thus,

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    nh Na > > ne


    A p-n junction is formed by combining P-type and N-type semiconductors together

    in very close contact. Normally they are manufactured from a single crystal with

    different dopant concentrations diffused across it. Creating a semiconductor from

    two separate pieces of material introduces a grain boundary between them which

    would severely inhibit its utility by scattering the electrons and holes. The term

    junction refers to the region where the two regions of the semiconductor meet.

    The p-n junction possesses some interesting properties which have useful

    applications in modern electronics. A p-doped semiconductor is relatively

    conductive. The same is true of an n-doped semiconductor, but the junction

    between them is a nonconductor. This no conducting layer, called the depletion

    zone, occurs because the electrical charge carriers in doped n-type and p-type

    silicon (electrons and holes, respectively) attract and eliminate each other in a

    process called recombination. By manipulating this nonconductive layer, p-njunctions are commonly used as diodes: electrical switches that allow a flow of

    electricity in one direction but not in the other (opposite) direction. This property is

    explained in terms of the forward-bias and reverse-bias effects, where the term bias

    refers to an application of electric voltage to the p-n junction.
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    In a p-n junction, without an external applied voltage, an equilibrium condition is

    reached in which a potential difference is formed across the junction. This potential

    difference is called built-in potential Vbi.

    In an equilibrium PN junction, electrons near the PN interface tend to diffuse into

    the p region. As electrons diffuse, they leave positively charged ions (donors) on

    the n region. Similarly holes near the PN interface begin to diffuse in the n-type

    region leaving fixed ions (acceptors) with negative charge. The regions nearby the

    PN interfaces lose their neutrality and become charged, forming the space charge

    region or depletion layer.

    Biasing of the P-N junction
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    Forward biasing

    A p-n junction is said to be forward biased, if the positive terminal of the external

    battery B is connected to p-side and the negative terminal to the n-side of the p-n

    junction. Here the forward bias opposes the potential barrier VB and so the

    depletion layer becomes thin. The majority charge carriers in the P type and N

    types are repelled by their respective terminals due to battery B and hence cross the

    junction. On crossing the junction, recombination process takes place. For every

    electron hole combination, a covalent bond near the +ve terminal of the battery B

    is broken and this liberates an electron which enters the +ve terminal of B through

    connecting wires. This in turn creates more holes in P-region. At the other end, the

    electrons from -ve terminal of B enter n-region to replace electron lost due to

    recombination process. Thus a large current will flow to migration of majority

    carriers across the p-n junction which is called forward current.

    Reverse biasing

    A p-n junction is said to be reverse biased if the positive terminal of the battery B

    is connected to N-side and the negative terminal to p-side of the p-n junction. The

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    majority carriers are pulled away from the junction and the depletion region

    becomes thick. The resistance becomes high when reverse biased and so there is no

    conduction across the junction due to majority carriers. The minority carriers

    however cross the junction and they constitute a current that flows in the opposite

    direction. This is the reverse current.



    Although in our daily life we use A.C. current devices. But rectifier

    is a Electronic device which converts A.C. power into D.C. power.

    The study of the junction diode characteristics reveals that the

    junction diode offers a low resistance path, when forward biased, and a high

    resistance path, when reverse biased. This feature of the junction diode enables it

    to be used as a rectifier.The alternating signals provides opposite kind of biased voltage at

    the junction after each half-cycle. If the junction is forward biased in the first half-

    cycle, its gets reverse biased in the second half. It results in the flow of forward

    current in one direction only and thus the signal gets rectified.

    In other words, we can say, when an alternating e.m.f. signal is

    applied across a junction diode, it will conduct only during those alternate half

    cycles, which biased it in forward direction.

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    RECTIFIERWhen a single diode is used as a rectifier, the rectification of only one-half

    of the A.C. wave form takes place. Such a rectification is called half-wave

    rectification. The circuit diagram for a half-wave rectifier is shown in Fig.

    Principle :

    It is based upon the principle that junction diode offers low resistance path

    when forward biased, and high resistance when reverse biased.

    Arrangement :-

    The A.C. supply is applied across the primary coil(P) of a step down

    transformer. The secondary coil(S) of the transformer is connected to the junction

    diode and a load resistance RL. The out put D.C. voltage is obtained across the

    load resistance(RL)

    Theory :

    Suppose that during the first half of the input cycle, the junction diode gets

    forward biased the conventional current will flow in the direction of the arrow-

    heads. The upper end of RL will be at positive potential w.r.t. the lower end.

    During the negative half cycle of the input a.c. voltage, the diode is reverse biased.

    No current flows in the circuit, and therefore, no voltage is developed across (RL).

    Since only the positive half cycle of the input appears across the load, the a.c. input

    is converted into pulsating direct current (d.c.).

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    Disadvantage of Half-Wave-Rectifier :

    1. Half wave rectification involves a lot of wastage of energy and

    hence it is not preferred.

    2. A small current flows during reverse bias due to minority charge

    carriers. As the output across (RL) is negligible.

    3. The resulting d.c. voltage is not steady enough for some purpose.

    The following device is used when a very steady d.c. voltage is required.

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    WAVE RECTIFIERA rectifier which rectifies both waves of the a.c. input is called a full wave


    Principle :- It is based upon the principle that a junction diode offers low

    resistance during forward biased and high resistance, when reverse biased.

    Difference from half-wave-rectifier :- The main difference is that in

    full wave rectifier we use two diodes. For this when we apply a.c. current to the

    rectifier then the first half wave get forward biased due to first diode. And when

    the second half wave comes. Then at that time the second diode comes in action

    and gets forward biased. Thus output obtained during both the half cycles of the

    a.c. input

    Arrangement :- The a.c. supply is applied across the primary coil(P) of a step

    down transformer. The two diodes of the secondary coil(S) of the transformer are

    connected to the P-sections of the junction diodes (D1) and (D2). A load

    resistance (RL) is connected across the n-sections of the two diodes and at centre

    of the secondary coil. The d.c. output will be obtained across the load resistance


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    Theory :-

    Suppose that during first half of the input cycle, upper end of (S) coil is at

    positive potential. And lower end is at negative potential. The junction diode (D1)

    gets forward biased, while the diode. (D2) get reverse biased. When the second

    half of the input cycle comes, the situation will be exactly reverse. Now the

    junction diode (D2) will conduct. Since the current during both the half cycles

    flows from right to left through the load resistance (RL) the output during both the

    half cycles will be of same nature.

    Thus, in a full wave rectifier, the output is continuous but pulsating in

    nature. However it can be made smooth by using a filter circuit.


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    Zener Diode

    A Zener diode is a type ofdiode that permits current not only in the forward

    direction like a normal diode, but also in the reverse direction if the voltage is

    larger than the breakdown voltage known as "Zener knee voltage" or "Zener


    It contains a heavily doped p-n junction allowing electrons to tunnel from the

    valence band of the p-type material to the conduction band of the n-type material.

    In the atomic scale, this tunneling corresponds to the transport of valence band

    electrons into the empty conduction band states; as a result of the reduced barrier

    between these bands and high electric fields that are induced due to the relatively

    high levels of doping on both sides.

    A reverse-biased Zener diode will exhibit a controlled breakdown and allow the

    current to keep the voltage across the Zener diode at the Zener voltage. However,

    the current is not unlimited, so the Zener diode is typically used to generate areference voltage for an amplifier stage, or as a voltage stabilizer for low-current

    applications. The breakdown voltage can be controlled quite accurately in the

    doping process. While tolerances within 0.05% are available, the most widely used

    tolerances are 5% and 10%. Breakdown voltage for commonly available zener

    diodes can vary widely from 1.2 volts to 200 volts.

    Zener diodes are widely used as voltage references and as shunt regulators toregulate the voltage across small circuits. When connected in parallel with a

    variable voltage source so that it is reverse biased, a Zener diode conducts when

    the voltage reaches the diode's reverse breakdown voltage. From that point on, the

    relatively low impedance of the diode keeps the voltage across the diode at that

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    Light-emitting diode (LED)

    A light-emitting diode (LED) is a semiconductor light source. LEDs are used as

    indicator lamps in many devices, and are increasingly used for lighting. Introduced

    as a practical electronic component in 1962, early LEDs emitted low-intensity red

    light, but modern versions are available across

    the visible, ultraviolet and infrared wavelengths, with very high brightness. It is

    based on the semiconductor diode. When a diode is forward biased (switched

    on), electrons are able to recombine with holes within the device, releasing energy

    in the form ofphotons. This effect is called electroluminescence and the color of

    the light (corresponding to the energy of the photon) is determined by the energy

    gap of the semiconductor.

    Like a normal diode, the LED consists of a chip of semiconducting material

    impregnated, or doped, with impurities to create a p-n junction. As in other diodes,

    current flows easily from the p-side, or anode, to the n-side, or cathode, but not in

    the reverse direction. Charge-carrierselectrons and holesflow into the junction

    from electrodes with different voltages. When an electron meets a hole, it falls into

    a lower energy level, and releases energy in the form of a photon.

    The wavelength of the light emitted, and therefore its color, depends on the band

    gap energy of the materials forming the p-n junction.

    In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition which produces no optical emission, because these are indirect

    band gap materials. The materials used for the LED have a direct band gap with

    energies corresponding to near-infrared, visible or near-ultraviolet light. LED

    development began with infrared and red devices made with gallium arsenide.

    Advances in materials science have made possible the production of devices with
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    ever-shorter wavelengths, producing light in a variety of colors. LEDs are usually

    built on an n-type substrate, with an electrode attached to the p-type layer

    deposited on its surface. P-type substrates, while less common, occur as well. Most

    materials used for LED production have very high refractive indices. This means

    that much light will be reflected back in to the material at the material/air surface

    interface. Therefore Light extraction in LEDs is an important aspect of LED


    Typical indicator LEDs are designed to operate with no more than 30-

    60 milliwatts [mW] of electrical power.

    One of the key advantages of LED-based lighting is its high efficiency, as

    measured by its light output per unit power input. White LEDs quickly matched

    and overtook the efficiency of standard incandescent lighting systems.


    A photodiode is a type ofphoto detector capable of converting light into either

    current or voltage, depending upon the mode of operation. Photodiodes are similar

    to regular semiconductors diodes except that they may be either exposed (to detect

    vacuum UV or X-rays) or packaged with a window or optical fiber connection to

    allow light to reach the sensitive part of the device.

    Photodiodes are often used for accurate measurement of light intensity in scienceand industry (e.g. consumer electronics devices) such as compact

    disc players, smoke detectors, and the receivers for remote controls

    in VCRs and televisions.
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    Under forward bias, conventional current will pass from the anode to the cathode,

    following the arrow in the symbol. Photocurrent flows in the opposite direction.

    A photodiode is a PN junction or PIN structure. When a photon of sufficient

    energy strikes the diode, it excites an electron, thereby creating a mobile electron

    and a positively charged electron hole. If the absorption occurs in the junction's

    depletion region, or one diffusion length away from it, these carriers are swept

    from the junction by the built-in field of the depletion region. Thus holes move

    toward the anode, and electrons toward the cathode, and a photocurrent is


    When used in zero bias or photovoltaic mode, the flow of photocurrent out of the

    device is restricted and a voltage builds up. The diode becomes forward biased and

    "dark current" begins to flow across the junction in the direction opposite to the

    photocurrent. This mode is responsible for the photovoltaic effect, which is the

    basis for solar cells.

    In photoconductive mode the diode is often reverse biased, dramatically reducing

    the response time at the expense of increased noise. This increases the width of the

    depletion layer, which decreases the junction's capacitance resulting in fasterresponse times. The reverse bias induces only a small amount of current (known as

    saturation or back current) along its direction while the photocurrent remains

    virtually the same


    A transistor is a semiconductor device used to amplify and switch electronic

    signals. It is made of a solid piece of semiconductor material, with at least three

    terminals for connection to an external circuit. A voltage or current applied to one
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    pair of the transistor's terminals changes the current flowing through another pair

    of terminals. Because the controlled (output) power can be much more than the

    controlling (input) power, the transistor provides amplification of a signal.

    The essential usefulness of a transistor comes from its ability to use a small signal

    applied between one pair of its terminals to control a much larger signal at another

    pair of terminals. This property is called gain. A transistor can control its output in

    proportion to the input signal, that is, can act as an amplifier. Or, the transistor can

    be used to turn current on or off in a circuit as an electrically controlled switch,

    where the amount of current is determined by other circuit elements.

    The three regions are called emitter (E), base (b), collector (c). The direction of

    arrows indicate the conventional current in the above symbol.

    Emitter (E) is a heavily doped region of the device and is a supplier of majority

    charge carriers to the base.

    Base (B) is made thin and is lightly doped. This is done to reduce the

    recombination process.

    Collector (c) is moderately doped and collects majority carriers through base.
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    (a) p-n-p Transistor:

    The emitter base junction is forward biased. It means the positive pole of emitter

    base battery VEE is connected to emitter, and its negative pole to the base. Collector

    base junction is reversed biased i.e. the negative pole to the base battery Vcc isconnected to collector and its positive pole to the base.

    The resistance of emitter base junction is very low. So the voltage of V EE is quite

    small. The resistance of collector base junction is very high. So the voltage ofVcc

    is quite large.

    Holes which are majority carriers in emitter (p-type semiconductor) are repelled

    towards base by positive potential on emitter due to battery V BB, resulting emitter

    current IE. The base being thin and lightly doped (n-type semiconductor) has

    number density of electrons. When holes enter the base region, then only a few

    holes (say 5%) get neutralized by the electron-hole combination, resulting base

    current IB (=5% IE=0.05Ie). The remaining 95% holes pass over the collector on

    account of high negative potential of collector due to battery V CC resulting collector

    current IC (=95%Ie=0.95Ie).

    As one hole reaches the collector, it is neutralized by the flow of one electron from

    the negative terminal of battery VCC to collector through connecting wire. At the

    same time a covalent bond is broken in the emitter, the electron goes to the positive

    terminal of the battery VEE through connecting wires and hole produced begins to

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    move towards base. Then one electron flows from negative terminal of battery VEE

    to positive terminal of battery Vcc. When the hole coming from emitter combines

    with the electron in base, the deficiency of electron in base is compensated by the

    flow of electron from negative terminal of battery VEE to the base throughconnecting wire. Thus the current in p-n-p transistor is carried by holes and at the

    same time, their concentration is maintained. However, in the external circuit

    current is due to the flow of electrons. The direction of conventional current (of

    holes currents) in the various arms of the circuit has been shown.

    IE = IB + IC

    In the base IE and IC flow in opposite direction.

    ( b ) n-p-n Transistor :

    In this case also, the emitter base junction is forward biased i.e. the positive pole of

    the emitter base battery VBB is connected to base and its negative pole to emitter.

    The resistance of emitter base junction is very low. So the voltage of V BB (i.e. VcB)

    is quite small (=1.5 V).

    The collector base junction is reverse biased i.e. the positive pole of the collector

    base battery Vcc is connected to the collector and negative pole to base. The

    resistance of this junction is very high. So the voltage of Vcc (i.e. VcB) is quite large

    (=45 V).

    Electrons that are majority carriers in emitter (n-type semiconductor) are repelled

    toward base by negative potential of VEE on emitter resulting emitter current Ie. The

    base being thin and lightly doped (p-type semiconductor) has low number density

    of holes. When electron enter the base region, then only a few holes (say 5%) get

    neutralized by the electron-hole combination resulting base current Ib

    (=5%Ie=0.05Ie). The remaining 95% electrons pass over to the collector, onaccount of high positive potential of collector due to battery Vcc, resulting

    collector current Ic (=95%Ie=0.95Ie).

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    As one electron reaches the collector, it flows to the positive terminal of the battery

    Vcc through connecting wire. At the same time one electron flows from negative

    terminal of Vcc to positive terminal of VEE and one electron flows from negative of

    VEE to emitter. When the electron coming from emitter combines with the hole on

    base, the deficiency of hole in base is compensated by the breakage of covalent

    bond there. The electron so released flows to positive terminal of battery VEE ,

    through connecting wire. Thus, in n-p-n transistor, the current is carried inside

    transistor as well as in external circuit by the electrons. The direction of

    conventional current (of holes currents) in the various arms of the circuit has been


    IE = IB + IC

    In the base IE and IC flow in opposite direction.

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