Amlan Nag (Btech, Mtech University of Alberta, Canada)

Semiconductor Amlan Nag (Btech, Mtech University of Alberta, Canada)

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On the basis of electrical conductivity, the materials can be divided into three categories :

(1) Conductors (e.g., Cu, Al, Fe, etc.),

(2) Insulators (e.g., wood, diamond, mica, etc.), and

(3) Semiconductors (e.g., Ge, Si, GaAs, … etc.)

At room temperature (≈ 300 K), the conductivity of conductors is in the range 106 - 108 S/m, and

that of insulators, 10–8 to 104 S/m. The conductivity of semiconductors lies in between that of

conductors and insulators.

Electronic devices (such as diodes, transistors, ICs) are made from silicon (Si) and germanium

(Ge). In the early days (around 1950), mostly germanium was used, because it was comparatively

easier to purify germanium. However, it was found that the devices made from silicon are more

stable and their functioning is less dependent on temperature variations. Now, we have much

improved metallurgical processes to purify silicon. So, now-a-days mostly the devices are of Si.

An integrated circuit (IC) is made from a silicon chip or wafer.

Energy Bands in Solids

The electrons in an isolated atom have discrete energy levels. However, in a crystal an atom is

surrounded by a large number of other atoms. Due to interatomic interactions, the energy levels are

modified. This modification is more prominent for the electrons in the outermost shell. (The

electrons in the inner shells are shielded by the electrons in outer shells and are not much affected

by the electric fields of the neighbouring atoms.) Due to this modification, each energy level splits

into a very large number of levels (~ 1023) lying close to one another. We can regard a bunch of

these energy levels as a continuous energy distribution, and call it energy band.

The energy bands, which are completely filled at 0 K are called valence bands (VB). The bands

with higher energies are called conduction bands (CB). We are generally concerned with the

highest valance band and the lowest conduction band.

Note that a conduction band is either completely empty or partially filled.

The difference between the highest energy in a valence band and the lower energy in the next higher

conduction band is called forbidden energy gap (Eg).

As an example, consider a specimen of sodium (Na) containing N atoms. Its atomic number is 11

(1s2, 2s2, 2p6, 3s1). The details of its energy bands are shown in the table.

As temperature is raised, the electrons may collide with each other and with ions to exchange

energy. The order of energy exchanged is kT. At room temperature (300 K), kT is 0.026 eV.

Ordinarily, the energy gaps are much larger than kT. An electron in a completely filled band (i.e., a

valence band) does not find an empty state with a slightly higher or lower energy. Hence, it cannot

accept or donate any energy of the order of kT. However, the outermost electrons, which are in the

highest occupied energy band, may take up this energy

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≈ kT provided some empty states are available in the same band.

Similar thing happens when a piece of sodium is connected to a battery. The electric field can

supply only a small amount of energy to the electrons. Only the electrons in the highest occupied

band can accept this energy and then move according to the field. This gives rise to an electric

current. The electrons in the inner (valence) bands cannot accept this small amount of energy and

hence cannot take part in electric conduction.

Why Materials Have Different Conductivities

There can be four broad types of energy band structures, as shown.

(A) The highest occupied energy band is only partially filled at 0 K. (Such is the case with sodium,

copper, etc.) When electric field is applied, the electrons in the partially filled band can accept

energy from the field and can drift accordingly. Hence, such materials are good conductors of

electricity.

(B) The highest occupied energy band (VB) is completely filled at 0 K and next higher band is

completely empty (CB). But the two are overlapping. (Zinc has such energy band structure.)

Therefore, there are empty energy stats close to the occupied states. Hence, such solids are also

good conductors.

(C) The highest occupied energy band (VB) is completely filled and the next higher band is

completely empty (CB). There is a large gap (Eg > 5 eV) between these two bands. (Diamond is of

this type.) The electrons in VB refuse to accept any energy from electric field, because there is no

empty state nearby. Only when the energy supplied (either by applied field or by raising the

temperature) is more that Eg (~ 5 eV), an electron from VB can jump to CB, after which it can take

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part in electrical conduction. Therefore, at ordinary temperatures, these materials behave as

insulators.

(D) The VB and CB are separated by small gap at 0 K (Eg ~ 1 eV). [For Ge, Eg = 0.72 eV ; and for

Si, Eg = 1.12 eV.] At 0 K, an ordinary battery cannot supply even this much energy. Hence

electrical conduction cannot take place. That is, at 0 K, these materials behave as perfect

insulators. However, at room temperature, thermal energy pushes some of these electrons in VB to

CB. Thus, small conduction becomes possible. Such solids are therefore called semiconductors.

Valance Energy

Band

Forbidden Energy

Band

Conduction Energy Band

In this band there are valence

electrons.

No electrons are found in this

band

In this band the electrons are

rarely found

This band may be partially or

completely filled with

electrons.

This band is completely

empty.

This band is either empty or

partially filled with electrons.

In this band the electrons are

not capable of gaining energy

from external electric field.

In this band the electrons can

gain energy from electric field.

The electrons in this band do

not contribute to electric

current.

Electrons in this band

contribute in this band

contribute to electric current.

In this band there are

electrons of outermost orbit of

atom which contribute in band

formation.

In this band there are

electrons which are obtained

on breaking the covalent

bands.

This is the band of maximum

energy in which the electrons

are always present.

This is the band of minimum

energy which is empty.

This band can never be

empty.

This band can be empty.

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• Difference between Conductors, Semi-conductors and Insulators:-

S.N

o.

Property Conductors Semi-conductors Insulators

1 Electrical conductivity and

its value

Very high

10–7 mho/m

Between those of

conductors and

insulators

i.e. 10–7 mho/m to

10–13mho/m

Negligible

10–13mho/m

2 Resistivity and its value Negligible Less than

10–5 W-m

Between those of

conductors and

insulators i.e. 10–

5 W-m to 105 W-m

Very high

more than

105 W-m

3 Band structure

4 Energy gap and its value Zero or very small More that in con-

ductors but less

than that in insu-

lators e.g. in Ge, ?

Eg =0.72 eV is Si, ?

Eg =1.1 eV in Ga As

?Eg =1.3 eV

Very large

e.g. in

diamond ?

Eg = 7 eV

5 Current carriers and

current flow

Due to free electrons

and very high

Due to free

electrons and holes

more than that in

insulators

Due to free

electrons

but

negligible.

6 Number of current carriers

(electrons or holes) at

ordinary temperature

Very high very low negligible

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7 Condition of valence band

and conduction band at

ordinary temperature

The valence and

conduction bands are

completely filled or

conduction band is

some what empty

(e.g. in Na)

Valence band in

somewhat empty

and conduction

band is somewhat

filled

Valence

band is

completely

filled and

conduction

band is

completely

empty.

8 Behaviour at 0 K Behaves like a

superconductor.

Behaves like an

insulator

Behaves

like an

insulator

9 Temperature coefficient of

resistance (a)

Positive Negative Negative

10 Effects of temperature on

conductivity

Conductivity

decreases

Conductivity

increases

Conductivity

increases

11 On increasing temperature

the number of current

carriers

Decreases Increases Increases

12 On mixing impurities their

resistance

Increases Decreases Remains

unchanged

13 Current flow in these takes

place

Easily Very slow Does not

take place

14 Examples Cu, Ag, Au, Na, Pt, Hg

etc.

Ge, Si, Ga, As etc. Wood,

plastic,

mica,

diamond,

glass etc

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INTRINSIC SEMICONDUCTORS

To make a diode or a transistor, the first step is to obtain a sample of semiconductor in its purest

form. This is called intrinsic semiconductor. The impurity content is less than one part impurity in

100 million parts of semiconductor.

Crystal Structure of Semiconductors

Each atom of an intrinsic semiconductor (Ge or Si) has four valence electrons. These four electrons

of each atom form covalent bonds with the four neighbouring atoms. Thus, the semiconductor has

tetrahedral lattice structure. A simplified two-dimensional representation of this crystalline

structure is shown in figure. The core represents the nucleus and all the orbiting electrons except

the four valence electrons. Therefore the core has +4 charge. A covalent bond consists of two

electrons, one from each adjacent atom. At 0 K, all the valence electrons are tightly bound to the

parent atoms. No free electrons are available for electrical conduction. Hence, the semiconductor

behaves as a perfect insulator at 0 K.

Charge Carriers in Intrinsic Semiconductors

At room temperature, thermal energy is sufficient to make a valence electron jump to the

conduction band. It starts orbiting the nucleus at a larger radius, and frequently it jumps from one

nucleus to the other. It has become free electron.

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When an electron breaks a covalent bond and moves away, a vacancy is created in the bond. A

positive charge is associated with this vacancy. This vacancy is called a hole. Free electrons and

holes are always generated in pairs. At any time, the concentration of free electrons is same as the

concentration of holes in an intrinsic semiconductor, ni=pi

Just as free electrons move randomly in the crystal, so do the holes. An electron from the

neighbouring bond can make a jump to fill the vacancy, thereby shifting the vacancy (or the holes)

to new location. Much energy is not needed to induce such a transfer, as all the electrons in the

valence band have roughly the same energy. A free electron carries negative charge (–1.6 x 10–19

C) with it. A hole carries a positive charge (+1.6 x 10–19 C) with it.

A metal (such as Cu, Al, etc.) has only one type of charge carriers, namely, free electrons. But a

semiconductor has two types of charge carriers free electrons and holes.

When an electric field is applied, the free electrons (in the conduction band) drift opposite to the

field and the holes drift along the field. Thus, both types of carriers contribute to electric

conduction.

In a semiconductor, not only the thermal generation of electron-hole pairs takes place, but also there

is simultaneous pair recombination. When a free electron encounters a hole, during their random

motion, the electron occupies the vacancy, re-establishing the covalent bond. The individual

identity of both is lost. In this recombination process, same amount of energy is given out as was

taken to generate electron-hole pair. At equilibrium the rate of pair recombination is equal to the

rate of pair generation. If temperature increases, more bonds are broken, that is, the rate of

generation increases. This increases the concentration of free electrons and holes, which in turn

increases the rate of recombination. Equilibrium is again established.

Note that Eg is more in silicon (Eg = 1.12 eV) than in germanium (Eg = 0.72 eV). Therefore, at a

given temperature, less number of electron-hole pairs will be generated in silicon than in

germanium. Hence, the conductivity of silicon is less than that of germanium.

Conduction in Intrinsic Semiconductor

When a battery is connected across a semiconductor, the free electrons drift towards +ve terminal

and holes drift towards –ve terminal. The total current I is summation of the current due to electron

flow In and the current due to hole flow. The current in the connecting wire is due to electron flow.

Effect of Temperature on Conductivity of a Semiconductor

When temperature is raised, more electron-hole pairs are generated. The higher the temperature, the

higher is the concentration of charge carriers. Because of this, the conductivity increases with

temperature. In other words, the resistivity (ρ=1/σ) decreases with rise in temperature. That is, the

semiconductors have negative temperature coefficient of resistance.

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EXTRINSIC SEMICONDUCTORS

Intrinsic (pure) semiconductors are of little use. For making a semiconductor device, we

deliberately add a tiny and controlled amount of desired impurity to the highly purified

semiconductor. This process is called doping. A doped semiconductor is called extrinsic

semiconductor. The proportion of impurity added is about 1 in 106.

N-type, or Donor Type

Pentavalent impurity (from group V, such as phosphorous, arsenic, or antimony) is added so that an

impurity atom substitutes for a silicon atom in the crystalline structure. Four of its valence

electrons make four covalent bonds with neighbouring atoms. The fifth electron remains unpaired,

and is quite loosely bound to the nucleus. It needs very little energy (0.01 eV in Ge, 0.05 eV in Si)

to free itself from the attractive force of the nucleus. At room temperature, the thermal energy is

enough to do this job for all the impurity atoms added. Since each impurity atom donates one

electron to the conduction band, this type of impurity is called donor type.

After donating an electron, the impurity atom becomes +ve ion. However, this +ve charge is

immobile as the ion is held in its place by covalent bonds. In addition to the free electrons donated

by impurity atoms, there are some more due to breaking of covalent bonds. Thus, a few holes are

also produced. If ND is the concentration of donor (impurity) atoms, n that of free electrons and p

that of hole, we have

ND + p = n

(+ve ions) (holes) (electrons)

As a whole, the N-type semiconductor is neutral. It has electrons in majority and holes in minority

(n>>p).

P- type, or Acceptor Type

Here, trivalent impurity (from group III, such as boron, aluminium, gallium and indium) is added.

The three valence electrons make only three covalent bonds with neighbours. The fourth bond

remains incomplete. There exists a vacancy of an electron in this bond.

Note that this vacancy is not a hole, as no charge is associated with it. However, the single electron

in the incomplete bond has a great tendency to snatch an electron from neighbouring bonds. Only

a little (about 0.01 eV) additional energy is needed by the electron in adjacent bond to jump and

occupy the vacancy around the impurity atom. When this happens, a hole is now formed in the

adjacent bond. This hole goes on moving around randomly in the crystal carrying +ve charge with

it.

When impurity atom accepts an electron to complete fourth bond, it becomes –ve ion (immobile).

At room temperature, all impurity atoms (concentration NA) convert into –ve ions. In addition to

the holes created due acceptor impurity atoms, there will be some covalent bonds broken due to

thermal energy.

NA + n = p

(–ve ions) (electrons) (holes)

The holes are in majority (p>>n).

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Doping Level

If the doping level (i.e., the impurity concentration) is increased in an extrinsic semiconductor, the

concentration of majority carriers increases. As a result, the chances of their recombining with

minority carriers increase; and hence the concentration of minority carriers decreases. In fact, in an

extrinsic semiconductor, we have

np=ni2

where ni is the intrinsic concentration of free electrons (or of holes).

Effect of Temperature on Extrinsic Semiconductor

The number of charge carriers in an extrinsic semiconductor is much larger than that in intrinsic

semiconductor. Hence, its conductivity is also many times that of an intrinsic semiconductor.

Consider an N-type semiconductor. All the donors have already donated the electrons to the crystal

at room temperature. If the temperature is raised further, more covalent bonds are broken. As a

result, the concentration of minority carriers increases. Eventually a temperature is reached when

the concentration of minority carriers becomes almost same as that of majority carriers. It will then

behave like an intrinsic semiconductors (with higher conductivity). Any device made of P- and N-

type semiconductor will fail at such a temperature. This critical temperature is about 85 °C for Ge

and 200 °C for Si.

P-N JUNCTION DIODE

A P-N junction diode is formed by growing a single crystal of Si (or of Ge), half of which is P-type

and the other half is N-type. The term junction refers to the boundary or the region of transition

between P-type and N-type in the crystal.

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P-N Junction with No External Voltage Applied

As soon as the P-N junction is formed, following processes are initiated :

(1) Holes from P-region diffuse into N-region, and then combine with the free electrons.

(2) Similarly, free electrons from N-region diffuse into P-region, and then combine with holes.

(3) This diffusion takes place because the charge carriers move haphazardly due to thermal energy,

and there exists a sharp difference in carrier-concentration on the two sides of the junction.

(4) After a few recombinations of holes and electrons in the immediate neighbourhood of the

junction, a restraining force is automatically developed. This force, called barrier, checks further

diffusion of the majority carriers across the junction.

(5) As shown in the figure, a few layers of immobile ions on the two sides of the junction become

depleted of their corresponding mobile (oppositively charged) carriers. The region containing the

uncompensated acceptor ions (in the P-region) and donor ions (in the N-region) is called depletion

region or layer. Since this region contains fixed ions with electrical charges, it is also called space-

charge region. As there are no charge carriers (holes and free electrons) in this region, it has

extremely high resistance compared to the remaining portions of P- and N-regions. The width of

this layer depends on the impurity concentration. High doping level of P- and N-regions favours

thin width of depletion layer. It is of the order of 10–6 m.

(6) The electric field between the acceptor and donor ions is called a barrier. The barrier

potential for an unbiased junction is called contact potential or diffusion potential (VD). This

potential is about 0.7 V for Si, and about 0.3 V for Ge P-N junctions

(7) The barrier discourages the diffusion of majority carriers across the junction. But, the same

barrier helps the minority carriers to drift across the junction. Therefore, there should be constant

drift current due to these minority carriers crossing the junction. However, no current can flow as

no closed circuit is connected to the P-N junction. This drift current is actually counterbalanced by

the diffusion current due to the crossing of some majority carriers having high kinetic energy,

Idrift+Idiffusion =0

The net current across the junction is zero, as it should be.

P-N Junction with Forward Bias

When +ve terminal of the battery is connected to the P-side and –ve to the N-side, the P-N junction

diode is said to be forward-biased. As a consequences of forward biasing, following things

happen :

(1) The applied electric field opposes the contact electric field.

(2) The barrier potential (VB) is reduced.

(3) The depletion layer becomes thin.

(4) Even those majority carriers which have less kinetic energy are now able to cross the reduced

barrier. So, the diffusion current increases.

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(5) The drift current (due to crossing of minority carriers) is slightly reduced.

(6) The forward biasing current is the difference of the diffusion current and drift current.

If =Idiffusion − Idrift

(7) As the external applied voltage Vf is increased, the current If increases sharply.

P-N Junction with Reverse Bias

When a P-N junction is reverse biased, following things happen :

(1) The applied electric field strengthens the contact electric field.

(2) The barrier potential (VB) is increased.

(3) The depletion layer becomes thick.

(4) The diffusion current due to the flow of majority carriers reduces almost to zero.

(5) The drift current due to the flow of minority carriers slightly increases.

(6) As the external applied voltage (Vr) is increased, the current I0 almost remains constant (it

increases slightly) until breakdown occurs.

V-I Characteristics of a P-N Junction Diode

The circuit symbol of a P-N junction diode is shown in the figure. The direction of the arrow

reminds us that the conventional current flows easily from P-region (called anode, A) to N-region

(called cathode, K). The stopper at the end of arrow reminds us that the diode stops the current

entering cathode.



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Typical V-I characteristics of a silicon diode are shown in figure. Note that we have taken different

scales of voltages and currents in the forward bias (FB) and in reverse bias (RB) regions. This is

done to show details of variation of current with applied voltage.

Forward Bias Characteristics

The external applied voltage opposes the contact potential. The potential barrier decreases.

However, the diode current is very small for the first few tengths of a volt. The diode does not

conduct well until the external voltage overcomes the contact potential (≈0.7 V for Si diode).

Beyond this voltage, even a small increase in the applied voltage produces a sharp rise in current.

The voltage at which the current starts to increase rapidly is called cut-in voltage or knee voltage

(Vo).

Vo=0.7 for Si

Vo=0.3V for Ge

At knee voltage, the barrier practically disappears. The resistance to the diode current is due to the

P- and N-regions, which may be deemed constant. The characteristic curve becomes almost linear.

Reverse Bias Characteristics

The external applied voltage adds up to the contact potential so as to increase the barrier voltage.

Even the high kinetic energy majority carriers are not able to cross the junction. This brings about

sudden fall in the diffusion current. The total current, Ir = Idrift – Idiffusion, abruptly increases. Any

further increase in reverse voltage Vr, does not produce any significant change in current Ir. This

current is now entirely due to the drift of minority carriers. Since, the concentration of minority

carriers remains constant at a given temperature, the reverse current remains constant at Io,

irrespective of how large is the applied voltage. That is why Io is called reverse saturation

current. For Si diodes, Io is of the order of 100 nA; for Ge diodes it is of the order of 10 µA.

Diode Equation

The diode current I for an applied voltage V is given by

I=I0 [exp(V/VT)−1]

where Io is the reverse saturation current, and VT is voltage equivalent of temperature and is given

by VT=kT/e

where k = Boltzmann constant

= 1.38 x 10–23 J/K



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T = absolute temperature

e = charge on an electron

= 1.6 x 10–19 C

The value of VT for room temperature (T = 300 K) is 0.026 V. In the above equation,

V = external applied voltage (+ve for FB, –ve for RB)

Reverse Breakdown

If the reverse bias is made too high, the current through P-N junction increases abruptly. This

phenomenon is called breakdown, and the voltage at which it occurs is called breakdown

voltage, Vz.

Normally, a diode is not subjected to such breakdown. Even if breakdown occurs, the crystalline

structure will return to normal when the excess reverse bias is removed, provided that the

overheating has not permanently damaged the crystal.

There are diodes specifically designed to operate under the

conditions of breakdown. These are called Zener diodes, the symbol of which is shown in the

figure. These are used in voltage regulator circuits (as the voltage across the diode remains

constant, whatever be the reverse current through it).

There are two processes causing breakdown

(1) Zener breakdown, and (2) Avalanche breakdown.

When reverse bias is increased, the electric field across the depletion region (which is normally

non-conducting) becomes so high (about 107 V/m) so as to suddenly break large number of covalent

bonds. This generates large number of charge carriers (electrons and holes). Hence, large current

flows. This mechanism is called Zener breakdown. It occurs in junctions having very high doping

level on the two sides.

In the second mechanism, the increased electric field causes high increase in the velocities of

minority carriers crossing the depletion layer. These high energy carriers strike the bond and

remove electrons from the lattice atoms. This is called impact ionization. These electrons in turn

again get accelerated by the excess field and strike other bonds to give more electrons. A sudden



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cumulative multiplication of charge carriers occurs, which causes the current to increase sharply.

This mechanism is called avalanche breakdown.

Effect of Temperature on Semiconductor Diodes

At higher temperatures, more bonds are broken generating more electrons-hole pairs. The

conductivity increases. The effect of increased temperature is not appreciable on the forward

current. In forward bias the current is mainly due to extrinsic conduction.

The reverse saturation current is due to the flow of minority carriers. The concentration of the

minority carriers is very much temperature dependent. For germanium diodes, Io nearly doubles for

every 10 °C rise in temperature. For Si diode, it becomes 2.5 times.

With increase in temperature, the breakdown voltage falls off.

DIODE RESISTANCE

A P-N junction diode, when connected in a circuit, offers some resistance to the flow of current.

There are two types of resistances.

Static Resistance

The resistance offered by the diode to a dc current is called static or dc resistance, R. It is simply

the ratio of the dc voltage across the diode to the dc current through it,R=VI

The figure shows forward-bias characteristics of a Si diode. The static resistance at the operating

point P is given as Rf=OA/AP=cot𝛂=0.75V/14mA=53Ω

Obviously, the static resistance of a reverse-biased diode will be very high (Rr≈106Ω).

Dynamic Resistance

Often a diode is connected in a circuit where some (small value) ac current is superimposed on the

dc current flowing through it. The resistance offered by the diode to the ac current is called

dynamic or ac resistance, r. It is given by the reciprocal of the slope of the characteristic at that

point, r =ΔV/ΔI = dV/dI =cotβ



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At the operating point P, the ac resistance is rf=ΔV/ΔI=(0.8−0.7)V/(18−10)mA=0.1V/8mA =12.5Ω

Note that the ac resistance is less than the dc resistance. In the reverse bias, the characteristics

curve is almost a straight line parallel to voltage axis. Hence, the ac resistance is very high (rr

∼107Ω).

Ideal Diode

A P-N junction diode has good conduction in forward bias and bad conduction in reverse bias.

Ideally we would like the diode to conduct fully in FB, and not to conduct at all in RB. Such an

ideal diode will have V–I characteristics as shown in Fig. (A). As shown in Fig. (B), it works like a

switch. It is ON or closed, when FB; OFF or open when RB. In other words, Rf = 0, and Rr =∞.

Circuit Model of a Diode

The analysis of a circuit containing a P-N junction diode becomes easy, if the actual diode is

replaced by its circuit model.

Normally a diode is not operated in its breakdown region. So, its V-I characteristics can be

approximated by two straight lines, as shown. Vo is the knee voltage.



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It may be assumed that the diode conduction does not start till the FB crosses Vo. Beyond Vo, it

conducts offering low resistance (∼20Ω). Hence, in forward bias it works as a closed switch with a

resistance rf and a constant voltage drop Vo, as shown in figure. In reverses bias, it is like an open

switch.

In case the P-N junction is connected in a circuit where the other series resistances are much larger

than rf, we can ignore rf without committing much error. It means we further approximate the

characteristics to the one shown in Fig. (A). The corresponding simplified model is shown in Fig.

(B). In forward-bias, the circuit model is simply a closed switch in series with a battery of voltage

V0. In reverse-bias, it is an open switch.

Often, the diode is connected in circuits where not only the other series resistances are high, but

other series voltages or voltage drops are much higher than Vo. In such cases, we can ignore Vo



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and the V-I characteristic can further be modified to that of an ideal diode. The actual diode is just

replaced by the ideal diode (a switch) to solve the circuit.

DIODE AS RECTIFIER

Diode conducts current in only one direction. This unidirectional conduction property of the

P-N junction diode is used in a rectifier circuit. A rectifier is a circuit that converts (or rectifies) ac

into dc.

A half-wave rectifier uses only one diode. Only one half cycle of the input appears across load RL.

A full-wave rectifier uses two diodes, and the secondary winding of transformer has a centre tap.

One diode conducts for first half and the other diode for the second half cycle of the input. Current

through RL flow for both half cycles, in the same direction. Obviously, full-wave rectifier provides

better dc

(A)

(B)

LED (Light Emitting Diode):

&

A light emitting diode is simply a forward biased p-n junction which emits spontaneous light

radiation. When forward bias is applied, the electron and holes at the junction recombine and energy

released is emitted in the form of light. For visible radiation phosphorus doped GaAs is commonly

used. The advantages of LEDs are:

(i) Low operational voltage and less power.

(ii) Fast action with no warm up time.

Vdc = Vav =1

2π

π

∫0

Vm sin ωt d(ωt) +

2π

∫π

0. d(ωt) =Vm

π

Vdc = Vav =1

2π

π

∫0

Vm sin ωt d(ωt) +

2π

∫π

(−Vm sin ωt) d(ωt) =2Vm

π



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(iii) Emitted light is nearly monochromatic radiation.

(iv) They have long life.

I-V characteristics of LED are similar to that of Si junction diode but the threshold voltages are

much higher and slightly different for each colour. The reverse breakdown voltages of LED’s are

very low, about 5V.

Photodiode

&

It is a reversed biased p-n junction, illuminated by radiation. When p-n junction is reversed biased

with no current, a very small reverse saturated current flows across the junction called the dark

current. When the junction is illuminated with light, electron-hole pairs are created at the junction,

due to which additional current begins to flow across the junction; the current is solely due to

minority charge carriers.

The characteristic curves of a photodiode for two different illuminations I1 and I2(I2 > I1) are

shown in fig. (c).

&

Solar Cell

A solar cell is a junction diode which converts light energy into electrical energy. A p-n junction

solar cell consists of a large junction with no external biasing. The surface layer of p-region is made

very thin so that the incident photons may easily penetrate to reach the junction which is the active

region. In an operation in the photovoltaic mode (i.e., generation of voltage due to bombardment of

optical photons); the materials suitable for photocells are silicon (Si), gallium arsenide (GaAs),

cadmium sulphide (CdS) and cadmium selenide (CdSe).



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&

Working : When photons of energy greater than band gap energy (hv - Eg) are made incident on

the junction, electron-hole pairs are created which move in opposite directions due to junction field.

These are collected at two sides of junction, thus producing photo-voltage; this gives rise to

photocurrent. The characteristic curve of solar cell is shown in fig. solar cells are used in satellites

to recharge their batteries.

Zener Diode

A zener diode is a specially designed heavily doped p-n junction, having a very thin depletion layer

and having a very sharp breakdown voltage. It is always operated in breakdown region. Its

breakdown voltage VZ is less than 6V.

Zener diode as a voltage Regulator :

The Zener diode makes its use as a voltage regulator due to the following property :

When a Zener diode is operated in the breakdown region, the voltage across it remains practically

constant for a large change in the current.

A simple circuit of a voltage regulator using a Zener diode is shown in the Fig. The Zener diode is

connected across load such that it is reverse biased.

The series resistance R absorbs the output voltage fluctuations so as to maintain constant voltage

across the load.

The operation of the circuit may be explained as follows :

&

Let Vin be the unregulated input voltage and V0 be the output voltage across RL to be regulated

and VZ be the Zener voltage of the diode. The value of the series resistance is so chosen that the

diode operates in the breakdown region under the Zener voltage VZ across it.

Let I be the current drawn from supply, IZ the current through Zener diode and IL the current

through load. Then obviously

I = IZ + IL or IZ = I - IL



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If RZ is Zener diode resistance, then

V0 = VZ = IZ . RZ = ILRL

Applying Kirchhoff’s law to the mesh containing resistance R, Zener diode and supply voltage Vin

we have

&

RI + VZ = Vin

i.e., VZ = Vin -RI ...(1)

When the input voltage Vin is lower than the Zener voltage of diode, there is no current conduction

i.e., IZ = 0

This implies V0 = Vin

As input voltage Vin is increased so that it becomes equal to VZ the breakdown point is reached

and the voltage across the diode VZ = (Vin - RI) becomes constant.

A further increase of input voltage Vin does not result in the corresponding increase in V0 or

VZ but merely increases the voltage drop across

Thus in breakdown region, we have

V0 = VZ - Vin - RI ...(2)

Fig. (b) represents the plot of output voltage VZ versus input voltage Vin It is clear from graph that

the output voltage remains constant when the diode is in Zener region.

It may be pointed out that for maintaining constant regulated output, the series resistance R for a

given range of input voltage be so chosen that

(i) the diode operates in Zener region and

(ii) current should not exceed a certain value to cause burn out of diode.

Transistor

(i)The current driven device, which is formed by three doped semiconductor regions, is known as

transistor.

(ii)That current driven device, in which the emitter current controls the collector current, is known

as transistor.

(iii)There are three semiconductor regions in a transistor viz Emitter (E), Base (B) and collector (C).

(iv)Function of emitter: To send electrons or cotters into the base

Function of base: To send electrons or cotters received from the emitter into the collector region.

Function of collector: To collect electrons or cotters from the base region.

(v) The distance between E and B in a transistor is less than that between B and C and the collector

is marked with a dot (.)



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(vi)Transistors are of two types:

(i) PNP transistor

(ii)NPN transistor

PNP-transistor:

(iv)Working of PNP transistor

(a) The emitter-base junction is forward biased while base-collector junction is reverse biased.

(b) A large number of holes enter from emitter to base and at the same time a very small number of

electrons enter from the base to the emitter.

(c) The electrons in the emitter region recombine with an equal number holes and neutralise them.

(d) The loss of total number of holes in the emitter is compensated by the flow of an equal number

of electrons from the emitter to the positive terminal of battery.

(e) These electrons are released by breaking of covalent bonds among the crystal atoms in the

emitter and an equal number holes is again created.

(f) Thus in PNP transistor emitter current is mainly due to the flow of holes, but in eternal circuit it

is due to flow of electron from emitter to the positive terminal of the battery.

(g)The base is very thin and is lightly doped. Therefore only a few holes (~ 1%) combine with

electrons in base. Hence the base current IB is very small.

(h)Nearly 99% of the holes coming from the emitter are collected by the collector.



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(i) For each hole reaching the collector, an electron is released from the negative terminal of

collector base battery to neutralise the hole.

(j) The relation between three currents is as under

(k) The input impedance is low and output impedance is high. The output voltage required to be

applied is more than the input voltage.

(l) The functions of E, B and C are to send cotters into base region, to send these cotters into

collector region and to collect the cotters received from base region respectively.

(i)Symbolic representation

(ii)In this conventional current flows from base towards emitter, hence the arrow head on emitter is

directed from B to E.

(iii) Sketch diagram

(iv) Working of NPN transistor

(a) The emitter-base junction is forward biased whereas the collector-base junction is reverse

biased.

(b) The majority electrons in the emitter are pushed into the base.

(c) The base is thin and is lightly doped. Therefore a very small fraction (say 1%) of incoming

electrons combine with the holes. Hence base current is very small.

(d) The majority of electrons are rushing towards the collector under the electrostatic influence of

C-B battery.

(e) The electrons collected by the collector move towards the positive terminal of C-B battery.

(f) The deficiency of these electron is compensated by the electrons released from the negative

terminal of E-B battery.

(g) Thus in NPN transistors current is carried by electron both in the external circuit as well as

inside the transistor.

(h) The relation between these current is given by



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The above figure shows the NPN transistor circuit with supply voltages and resistive loads. Here the

collector terminal always connected to the positive voltage, the emitter terminal connected to the

negative supply and the base terminal controls the ON/OFF states of transistor depending on the

voltage applied to it.

NPN Transistor Working

The working of NPN transistor is quite complex. In the above circuit connections we observed that

the supply voltage VB is applied to the base terminal through the load RB. The collector terminal

connected to the voltage VCC through the load RL. Here both the loads RB and RL can limit the

current flow through the corresponding terminals. Here the base terminal and collector terminals

always contain positive voltages with respect to emitter terminal.

If the base voltage is equal to the emitter voltage then the transistor is in OFF state. If the base

voltage increases over emitter voltage then the transistor becomes more switched until it is in fully

ON state. If the sufficient positive voltage is applied to the base terminal i.e. fully-ON state, then

electrons flow generated and the current (IC) flows from emitter to the collector. Here the base

terminal acts as input and the collector-emitter region acts as output.

To allow current flow between emitter and collector properly, it is necessary that the collector

voltage must be positive and also greater than the emitter voltage of transistor. Some amount of

voltage drop presented between base and emitter, such as 0.7V. So the base voltage must be greater

than the voltage drop 0.7V otherwise the transistor will not operate. The equation for base current of

a bipolar NPN transistor is given by,

IB = (VB-VBE)/RB

Where,

IB = Base current

VB = Base bias voltage

VBE = Input Base-emitter voltage = 0.7V

RB = Base resistance

The output collector current in common emitter NPN transistor can be calculated by applying

Kirchhoff’s Voltage Law (KVL).



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The equation for collector supply voltage is given as

VCC = ICRL + VCE ………… (1)

From the above equation the collector current for common emitter NPN transistor is given as

IC = (VCC-VCE)/RL

In a common emitter NPN transistor the relation between collector current and emitter current is

given as

IC = β IB

In active region the NPN transistor acts as a good amplifier. In common emitter NPN transistor total

current flow through the transistor is defined as the ratio of collector current to the base current IC/

IB. This ratio is also called as “DC current gain” and it doesn’t have any units. This ratio is

generally represented with β and the maximum value of β is about 200. In common base NPN

transistor the total current gain is expressed with the ratio of collector current to emitter current IC/

IE. This ratio is represented with α and this value is generally equal to unity.

α, β and γ Relationship in NPN Transistor

Now let us see the relationship between the two ratio parameters α and β.

α = DC current gain for common base circuit = Output current/Input current

In common base NPN transistor output current is collector current (IC) and input current is emitter

current (IE).

α = IC/IE ………..(2)

This current gain (α) value is very close to unity but less than the unity.

We know that the emitter current is the sum of small base current and large collector current.

IE = IC + IB

IB = IE – IC

from equation 2, the collector

IC = αIE

IB = IE – αIE

IB = IE (1-α)

β = DC current gain for common emitter circuit = Output current/Input current

Here output current is collector current and input current is base current.

β = IC/IB

β = IC/IE (1-α)

β = α/(1-α)

From the above equations the relationship between α and β can be expressed as

α = β (1-α) = β/(β+1)

β = α (1+β) = α/ (1-α)

The β value may vary from 20 to 1000 for low power transistors which operate with high

frequencies. But in general this β value can have the values in between the range of 50-200.

Now we will see the relationship between α, β and γ factors.

In common collector NPN transistor the current gain is defined as the ratio emitter current IE to

base current IB. This current gain is represented with γ.

γ = IE/IB

We know that emitter current

IE = IC + IB

γ = (IC + IB )/IB

γ = (IC/IB) + 1

γ = β +1



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Hence the relationships between α, β and γ are given as below

α = β / (β+1), β = α / (1-α), γ = β +1

PNP Transistor Circuit (detailed study) Introduction

PNP transistor is another type of Bipolar Junction Transistor (BJT). The structure of the PNP

transistor is completely different from the NPN transistor. The two PN-junction diodes in the PNP

transistor structure are reversed with respect to the NPN transistor, such as the two P-type doped

semiconductor materials are separated by a thin layer of N-type doped semiconductor material. In

PNP transistor the majority current carriers are holes and electrons are the minority current carriers.

All the supply voltage polarities applied to the PNP transistor are reversed. In PNP transistor the

current sinks in to the base terminal. The small base current in the PNP transistor has the ability to

control the large emitter-collector current because it is a current-controlled device.

The arrow for BJT transistors is always located on the emitter terminal and also it indicates the

direction of conventional current flow. In PNP transistor this arrow indicates as ‘pointing in’ and the

current direction in PNP is completely opposite to the NPN transistor. The structure of PNP

transistor is completely opposite to the NPN transistor. But the characteristics and operation of the

PNP transistor is almost same as NPN transistor with small differences. The symbol and structure

for PNP transistor is shown below.

The above figure shows the structure and symbol of PNP Transistor. This transistor mainly consists

of 3 terminals and they are Emitter (E), Collector (C) and Base (B). Here if you observe, the base

current flows out of the base unlike NPN transistor. The emitter voltage is much positive with

respect to base and collector.

PNP Transistor Working

The circuit connection of PNP transistor with supply voltages is given below. Here the base

terminal has negative bias with respect to emitter and the emitter terminal has positive bias voltage

with respect to both base and collector because of PNP transistor.



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The polarities and current directions are reversed here compared to NPN transistor. If the transistor

is connected to all the voltage sources as shown above then the base current flows through the

transistor but here the base voltage needs to be more negative with respect to the emitter to operate

transistor. Here the base- emitter junction acts as a diode. The small amount of current in the base

controls the flowing of large current through emitter to collector region. The base voltage is

generally 0.7V for Si and 0.3V for Germanium devices.

Here the base terminal acts as input and the emitter- collector region acts as output. The supply

voltage VCC is connected to the emitter terminal and a load resistor (RL) is connected to the

collector terminal. This load resistor (RL) is used to limits the maximum current flow through the

device. One more resistor (RB) is connected to the base terminal which is used to limit the

maximum current flow through the base terminal and also a negative voltage is applied to the base

terminal. Here the collector current is always equal to the subtraction of base current from emitter

current. Like NPN transistor, the PNP transistor also has the current gain value β. Now let us see the

relation between the currents and current gain β.

The collector current (IC) is given by,

IC = IE – IB

The DC current gain (β) for the PNP transistor is same as the NPN transistor.

DC current gain = β = Output current/Input current

Here output current is collector current and input current is base current.

β = IC/IB

From this equation we get,

IB = IC/β

IC = β IB

And also we define the current gain as,

Current gain = Collector current/ Emitter current (In common base transistor)

α = IC/IE

The relation between α and β is given by,

β = α / (1- α) and α = β/ (β+1)

The collector current in PNP transistor is given by,

IC = – α IE + ICBO where ICBO is the saturation current.

Since IE = -(IC + IB)



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IC = – α (-(IC + IB)) + ICBO

IC – α IC = α IB + ICBO

IC (1- α) = α IB + ICBO

IC = (α/ (1- α)) IB + ICBO/ (1- α)

Since β = α / (1- α)

Now we get the equation for collector current

IC = β IB + (1+ β) ICBO

The output characteristics of PNP transistor are same as NPN transistor characteristics. The small

difference is that the PNP transistor characteristic curve rotates 1800 to calculate the reverse

polarity voltages and current values. The dynamic load line also exists on the characteristic curve to

calculate the Q-point value. The PNP transistors are also used in switching and amplifying circuits

like NPN transistors.

Common Emitter Configuration In common emitter configuration the emitter is common to both input and output. For normal

operation the Base-Emitter junction is forward biased and base- collector junction is reveres

biased .The input characteristics are plotted between IB and VBE keeping the voltage VCE constant.

This characteristic is very similar to that of a forward biased diode. The input dynamic resistance is

calculated using the formula

ri = ∆VBE /∆IB at constant VCE

The output characteristics are plotted between IC and VCE keeping IB constant. These curves are

almost horizontal. The output dynamic resistance is given by,

ro = ∆VCE/ ∆IC at constant IB

At a given operating point, we define DC and AC current gains (beta) as follows

DC current gain βdc = IC / IB at constant VCE

AC current gain βac = ∆ IC/ ∆ IB at constant VCE.



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Common Base Configuration

Common Collector Configuration

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Transistor Characteristics (Detailed Study)

These are the plots which represent the relationships between the current and the voltages of a

transistor in a particular configuration. By considering the transistor configuration circuits to be

analogous to two-port networks, they can be analyzed using the characteristic-curves which can be

of the following types

1 Input Characteristics: These describe the changes in input current with the variation in the

values of input voltage keeping the output voltage constant.

2 Output Characteristics: This is a plot of output current versus output voltage with constant

input current.

3 Current Transfer Characteristics: This characteristic curve shows the variation of output

current in accordance with the input current, keeping output voltage constant.

Common Base (CB) Configuration of Transistor

In CB Configuration, the base terminal of the transistor will be common between the input and the

output terminals as shown by Figure 1. This configuration offers low input impedance, high output

impedance, high resistance gain and high voltage gain.

Input Characteristics for CB Configuration of Transistor

Figure 2 shows the input characteristics of a CB configuration circuit which describes the variation

of emitter current, IE with Base-Emitter voltage, VBE keeping Collector-Base voltage, VCB constant.

This leads to the expression for the input resistance as



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Output Characteristics for CB Configuration of Transistor

The output characteristics of CB configuration (Figure 3) show the variation of collector current, IC

with VCB when the emitter current, IE is held constant. From the graph shown, the output resistance

can be obtained as

Current Transfer Characteristics for CB Configuration of Transistor

Figure 4 shows the current transfer characteristics for CB configuration which illustrates the

variation of IC with the IE keeping VCB as a constant. The resulting current gain has a value less than

1 and can be mathematically expressed as

Common Collector (CC) Configuration of Transistor

This transistor configuration has the collector terminal of the transistor common between the input

and the output terminals (Figure 5) and is also referred to as emitter follower configuration. This

offers high input impedance, low output impedance, voltage gain less than one and a large current

gain.


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Input Characteristics for CC Configuration of Transistor

Figure 6 shows the input characteristics for CC configuration which describes the variation in IB in

accordance with VCB, for a constant value of Collector-Emitter voltage, VCE.

Output Characteristics for CC Configuration of Transistor

Figure 7 shows the output characteristics for the CC configuration which exhibit the variations in IE

against the changes in VCE for constant values of IB.

Current Transfer Characteristics for CC Configuration of Transistor

This characteristic of CC configuration (Figure 8) shows the variation of IE with IB keeping VCE as a

constant.



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Common Emitter (CE) Configuration of Transistor

In this configuration, the emitter terminal is common between the input and the output terminals as

shown by Figure 9. This configuration offers medium input impedance, medium output impedance,

medium current gain and voltage gain.

Input Characteristics for CE Configuration of Transistor

Figure 10 shows the input characteristics for the CE configuration of transistor which illustrates the

variation in IB in accordance with VBE when VCE is kept constant.

From the graph shown, the input resistance of the transistor can be obtained as

Output Characteristics for CE Configuration of Transistor



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The output characteristics of CE configuration (Figure 11) are also referred to as collector

characteristics. This plot shows the variation in IC with the changes in VCE when IB is held constant.

From the graph shown, the output resistance can be obtained as

Current Transfer Characteristics for CE Configuration of Transistor

This characteristic of CE configuration shows the variation of IC with IB keeping VCE as a constant.

This can be mathematically given by

This ratio is referred to as common-emitter current gain and is always greater than 1.

Lastly, it is to be noted that although the characteristic curves explained are for BJTs, similar

analysis holds good even in the case of FETs.



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Bipolar Transistor Configurations

with the generalised characteristics of the different transistor configurations given in the following

table:

Various regions of operations of transistors

When used as an AC signal amplifier, the transistors Base biasing voltage is applied in such a way

that it always operates within its “active” region, that is the linear part of the output characteristics

curves are used.

However, both the NPN & PNP type bipolar transistors can be made to operate as “ON/OFF” type

solid state switch by biasing the transistors Base terminal differently to that for a signal amplifier.

Solid state switches are one of the main applications for the use of transistor to switch a DC output

“ON” or “OFF”. Some output devices, such as LED’s only require a few milliamps at logic level

DC voltages and can therefore be driven directly by the output of a logic gate. However, high power

devices such as motors, solenoids or lamps, often require more power than that supplied by an

ordinary logic gate so transistor switches are used.

CharacteristicCommon

Base

Common

Emitter

Common

Collector

Input Impedance Low Medium High

Output Impedance Very High High Low

Phase relation b/n

i/p and o/p0o 180o 0o

Voltage Gain High Medium Low

Current Gain Low Medium High

Power Gain Low Very High Medium


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If the circuit uses the Bipolar Transistor as a Switch, then the biasing of the transistor, either NPN

or PNP is arranged to operate the transistor at both sides of the “ I-V ” characteristics curves we

have seen previously.

The areas of operation for a transistor switch are known as the Saturation Region and the Cut-off

Region. This means then that we can ignore the operating Q-point biasing and voltage divider

circuitry required for amplification, and use the transistor as a switch by driving it back and forth

between its “fully-OFF” (cut-off) and “fully-ON” (saturation) regions as shown below.

Operating Regions

The pink shaded area at the bottom of the curves represents the “Cut-off” region while the blue area

to the left represents the “Saturation” region of the transistor. Both these transistor regions are

defined as:

1. Cut-off Region

Here the operating conditions of the transistor are zero input base current ( IB ), zero output

collector current ( IC ) and maximum collector voltage ( VCE ) which results in a large depletion

layer and no current flowing through the device. Therefore the transistor is switched “Fully-OFF”.

Cut-off Characteristics



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Then we can define the “cut-off region” or “OFF mode” when using a bipolar transistor as a switch

as being, both junctions reverse biased, VB < 0.7v and IC = 0. For a PNP transistor, the Emitter

potential must be negative with respect to the Base.

2. Saturation Region

Here the transistor will be biased so that the maximum amount of base current is applied, resulting

in maximum collector current resulting in the minimum collector emitter voltage drop which results

in the depletion layer being as small as possible and maximum current flowing through the

transistor. Therefore the transistor is switched “Fully-ON”.

• • The input and Base are grounded

( 0v )

• • Base-Emitter voltage

VBE < 0.7v• • Base-Emitter junction is reverse

biased

• • Base-Collector junction is

reverse biased

• • Transistor is “fully-OFF” ( Cut-off

region )

• • No Collector current flows

( IC = 0 )

• • VOUT = VCE = VCC = ”1″• • Transistor operates as an “open

switch”

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Saturation Characteristics

Then we can define the “saturation region” or “ON mode” when using a bipolar transistor as a

switch as being, both junctions forward biased, VB > 0.7v and IC = Maximum. For a PNP

transistor, the Emitter potential must be positive with respect to the Base.

Then the transistor operates as a “single-pole single-throw” (SPST) solid state switch. With a zero

signal applied to the Base of the transistor it turns “OFF” acting like an open switch and zero

collector current flows. With a positive signal applied to the Base of the transistor it turns “ON”

acting like a closed switch and maximum circuit current flows through the device.

The simplest way to switch moderate to high amounts of power is to use the transistor with an

open-collector output and the transistors Emitter terminal connected directly to ground. When used

in this way, the transistors open collector output can thus “sink” an externally supplied voltage to

ground thereby controlling any connected load.

An example of an NPN Transistor as a switch being used to operate a relay is given below. With

inductive loads such as relays or solenoids a flywheel diode is placed across the load to dissipate the

back EMF generated by the inductive load when the transistor switches “OFF” and so protect the

transistor from damage. If the load is of a very high current or voltage nature, such as motors,

heaters etc, then the load current can be controlled via a suitable relay as shown.

• • The input and Base are connected to

VCC

• • Base-Emitter voltage VBE > 0.7v• • Base-Emitter junction is forward

biased

• • Base-Collector junction is forward

biased

• • Transistor is “fully-

ON” ( saturation region )

• • Max Collector current flows

( IC = Vcc/RL )

• • VCE = 0 ( ideal saturation )

• • VOUT = VCE = ”0″• • Transistor operates as a “closed

switch”

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Basic NPN Transistor Switching Circuit

The circuit resembles that of the Common Emitter circuit we looked at in the previous tutorials. The

difference this time is that to operate the transistor as a switch the transistor needs to be turned

either fully “OFF” (cut-off) or fully “ON” (saturated). An ideal transistor switch would have infinite

circuit resistance between the Collector and Emitter when turned “fully-OFF” resulting in zero

current flowing through it and zero resistance between the Collector and Emitter when turned

“fully-ON”, resulting in maximum current flow.

In practice when the transistor is turned “OFF”, small leakage currents flow through the transistor

and when fully “ON” the device has a low resistance value causing a small saturation voltage

( VCE ) across it. Even though the transistor is not a perfect switch, in both the cut-off and

saturation regions the power dissipated by the transistor is at its minimum.

In order for the Base current to flow, the Base input terminal must be made more positive than the

Emitter by increasing it above the 0.7 volts needed for a silicon device. By varying this Base-

Emitter voltage VBE, the Base current is also altered and which in turn controls the amount of

Collector current flowing through the transistor as previously discussed.

When maximum Collector current flows the transistor is said to be Saturated. The value of the

Base resistor determines how much input voltage is required and corresponding Base current to

switch the transistor fully “ON”.

Transistor as a Switch Example No1

Using the transistor values from the previous tutorials of: β = 200, Ic = 4mA and Ib = 20uA, find the

value of the Base resistor (Rb) required to switch the load fully “ON” when the input terminal

voltage exceeds 2.5v.

The next lowest preferred value is: 82kΩ, this guarantees the transistor switch is always saturated.



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Transistor as a Switch Example No2

Again using the same values, find the minimum Base current required to turn the transistor “fully-

ON” (saturated) for a load that requires 200mA of current when the input voltage is increased to

5.0V. Also calculate the new value of Rb.

Transistor Base current:

Transistor Base resistance:

Transistor switches are used for a wide variety of applications such as interfacing large current or

high voltage devices like motors, relays or lamps to low voltage digital IC’s or logic gates like AND

gates or OR gates. Here, the output from a digital logic gate is only +5v but the device to be

controlled may require a 12 or even 24 volts supply. Or the load such as a DC Motor may need to

have its speed controlled using a series of pulses (Pulse Width Modulation). transistor switches will

allow us to do this faster and more easily than with conventional mechanical switches.

Digital Logic Transistor Switch

The base resistor, Rb is required to limit the output current from the logic gate.



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Transistor as an Oscillator



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Amlan Nag (Btech, Mtech University of Alberta, Canada)

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Transcript of Amlan Nag (Btech, Mtech University of Alberta, Canada)