Semiconductors 1

7/31/2019 Semiconductors 1

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SEMICONDUCTOR

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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ENERGY BAND IN SOLIDS

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

2s2

2p6

3s2

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

INTRINSIC SEMICONDUCTORS

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

of

pure germanium contains about 4.21022

atoms and 2.51013

free electrons and

2.51013

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

P-N JUNCTION

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.

JUNCTION DIODE AS RECTIFIER

INTRODUCTION

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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JUNCTION DIODE AS HALF WAVE

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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JUNCTION DIODE AS A FULL

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

rectifier.

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

(RL).


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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.

TYPES OF JUNCTION DIODE


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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

voltage".

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

value.
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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

production.

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.

Photodiode

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

produced.

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

TRANSISTOR

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
http://en.wikipedia.org/wiki/Conventional_currenthttp://en.wikipedia.org/wiki/PN_junctionhttp://en.wikipedia.org/wiki/PIN_diodehttp://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/Bias_(electrical_engineering)http://en.wikipedia.org/wiki/Dark_current_(physics)http://en.wikipedia.org/wiki/Photovoltaic_effecthttp://en.wikipedia.org/wiki/Solar_cellhttp://en.wikipedia.org/wiki/P-n_junction#Reverse_biashttp://en.wikipedia.org/wiki/Capacitancehttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Semiconductor_devicehttp://en.wikipedia.org/wiki/Electronic_amplifierhttp://en.wikipedia.org/wiki/Electronicshttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Electronicshttp://en.wikipedia.org/wiki/Electronic_amplifierhttp://en.wikipedia.org/wiki/Semiconductor_devicehttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Capacitancehttp://en.wikipedia.org/wiki/P-n_junction#Reverse_biashttp://en.wikipedia.org/wiki/Solar_cellhttp://en.wikipedia.org/wiki/Photovoltaic_effecthttp://en.wikipedia.org/wiki/Dark_current_(physics)http://en.wikipedia.org/wiki/Bias_(electrical_engineering)http://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/PIN_diodehttp://en.wikipedia.org/wiki/PN_junctionhttp://en.wikipedia.org/wiki/Conventional_current


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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

shown.

IE = IB + IC

In the base IE and IC flow in opposite direction.

Semiconductors 1

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