Junction Diode and Transistors

8/6/2019 Junction Diode and Transistors

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Junction Diode. Transistors andApplications

Energy Bands in SolidsAs we see later, the allowed energy levels in a single atom are discrete (separate) and spaced widely apart. In thesolid state, however, as in a crystal, large numbers of atoms are packed closely together, and the electrons areinfluenced strongly by the assembly of nuclei. The allowable energy levels then broaden into bands of energy,Fig. 39.1 (i). The bands contain allowable energy levels very close to each other, as at P. There may also beforbidden bands of energy, as at Q, which electrons cannot occupy. The lowest available energy band is calledthe valence band. The next available energy band is called the conduction band.

In an insulator, the valence band energy levels are completely filled by electrons. The conduction band isempty and the two bands a re separated by a wide energy gap much greater than k in magnitude where kis theBoltzmann constant (p. 223), called the 'forbidden' band, Fig. 39.1 (ii). The electrons in the valence band havethermal energy of the orderk but at room temperature they cannot gain sufficient energy from an applied p.d. tomove to higher unoccupied energy levels. So the material is an insulator.

Semiconductors are a class of materials with a narrow forbidden band between the valence and conductionbands; the energy gap is of the orderkT. At , all the energy levels in the valence band are occupied andthe material is then an insulator. At normal temperatures, however, the thermal energy of some valenceelectrons, of the order kT, is sufficient for them to reach the conduction band, where they may becomeconduction electrons. The gap left in the valence band of energies by the movement of an electron is called ahole. Fig. 39.1 (ii). In semiconductor theory, both holes and conduction electrons play an active part, as wesoon see.


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In metals, however, the valence and conduction bands can overlap, as shown diagrammatically in Fig. 39.2.The electrons in the overlapping region of energy are conduction electrons. Since there is a large number ofconduction electrons, metals are good conductors.

Semiconductors. Movement of Charge Carriers

Semiconductors are a class of solids with electrical resistivity between that of a conductor and an insulator.For example, the resistivity of a conductor is of the order 10 -8 Ohm m, that of an insulator is 104 Ohm m andhigher, and that of a semiconductor is 10-1 Ohm m. Silicon and germanium are examples of semiconductor

elements widely used in the electronics industry.Silicon and germanium atoms are tetravalent. They have four electrons in their outermost shell, calledvalence electrons. One valence electron is shared with each of four surrounding atoms in a tetrah^dralarrangement, forming 'covalent bonds' which maintain the crystalline solid structure (p. 133). Figure 39.3 (i) isa two-dimensional representation of the structure.

At , all the valence electrons are firmly bound to the nucleus of their particular atoms.

At room temperature, however, the thermal energy of a vale electron may become greater than the energybinding to its nucleus. The covalent bond is then broken. The electron leaves the atom, X say, and becomes afree electron. This leaves X with a vacancy or hole, Fig. 39.3 (ii). Since X now has a net positive charge, anelectron in a neighbouring atom may then be attracted. Thus the hole appears to move to Y.

The hole movement through a semiconductor is random. But if a battery is connected, the valence electronsare urged to move in one direction and to fill the holes. The holes then drift in the direction of the field. Thusthe holes move as if they were carriers with a positive charge +e, where e is the numerical value of thecharge on an electron, Fig. 39.3 (iii). The current in the semi-conductor is also carried by the free electrons

present. These are equal in number to the holes in a pure semiconductor and drift in the opposite directionsince they are negative charges. The mobility of an electron, its average velocity per unit electric fieldintensity, is usually much greater than that of a hole.

In electrolytes (p. 676), the current is also carried by moving negative and positive charges but the carriershere are ions. It should be noted that, in a pure semiconductor, there are equal numbers of electrons and holes,the charge carriers. Electron-hole pairs are said to be produced by the movement of an electron from bound

state in an atom to a higher energy level, where it becomes a free electron.


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Effect of Temperature Rise

In contrast to a semiconductor, the charge carriers in a metal such as copper are only free electrons. Further,as the temperature of the metal rises, the amplitude of vibration of the atoms increases and more 'collisions'with atoms are then made by drifting electrons. Thus, as stated on p. 668, the resistance of a pure metalincreases with temperature rise.

In the case of a semiconductor, however, the increase in thermal energy of the valence electrons due totemperature rise enables more of them to break the covalent bonds and become free electrons. Thus moreelectron-hole pairs are produced which can act as carriers of current. Hence, in contrast to a pure metal, theelectrical resistance of a pure semiconductor decreases with tem-perature rise. This is one way ofdistinguishing between a pure metal and a pure semiconductor. Note that the pure or intrinsic semiconductoralways has equal numbers of electrons and holes, whatever its temperature.

P- and N-type SemiconductorsA pure or intrinsic semiconductor has charge carriers which are thermally generated. These are relativelyfew in number. By 'doping' a semiconductor with a tiny amount of impurity such as one part in a million, thusforming a so-called extrinsic semiconductor, a considerable increase can be made to the number of chargecarriers.

Arsenic atoms, for example, have five electrons in their outermost or valence band. When an atom of arsenicis added to a germanium crystal, the atom settles in a lattice site with four of its electrons shared withneighbouring germanium atoms, Fig. 39.4 (i). The fifth electron may thus become free to wander through thecrystal. Since an impurity atom may provide one free electron, an enormous increase occurs in the number ofelectron carriers. The impure semiconductor is called an 'n-type semiconductor' orn-semiconductor, where 'n'represnts the negative charge on an electron. Thus the majority carriers in an n-semiconductor are electrons.Positive charges or holes are also present in the n-semiconductor.

These are thermally generated, as previously explained, and since they are relatively few they are called the

minority carriers. The impurity (arsenic) atoms are called donorsbecause they donate electrons as carriers.

P-semiconductors are made by adding foreign atoms which are trivalent to pure germanium or silicon.Examples are boron or indium. In this case the reverse happens to that previously described. Each trivalentatom at a lattice site attracts an electron from a neighbouring atom, thereby completing the four valence bondsand forming a hole in the neighbouring atom, Fig. 39.4 (ii). In this way an enormous increase occurs in thenumber of holes. Thus in a p-semiconductor, the majority carriers are holes or positive charges. Theminority carriers are electrons, negative charges, which are thermally generated. The impurity atoms are calledacceptors in this case because each 'accepts' an electron when the atom is introduced into the crystal.

Summarising: In a n-semiconductor, conduction is due mainly to negative charges or electrons, with positivecharges (holes) as minority carriers. In a p-semiconductor, conduction is due mainly to positive charges orholes, with negative charges (electrons) as minority carriers.

P-N Junction

By a special manufacturing process, p- and n-semiconductors can be melted so that a boundary orjunction isformed between them. This junction is extremely thin and of the order 10-3 mm. It is called a p-n junction, Fig.39.5 (i).

When a scent bottle is opened, the high concentration of scent molecules in the bottle causes the moleculesto diffuse into the air. In the same way, the high


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concentration of holes (positive charges) on one side of a p-n junction, and the high concentration of electronson the other side, causes the two carriers to diffuse respectively to the other side of the junction, as shown.The electrons which move to the p-semiconductor side recombine with holes there. These holes thereforedisappear, and an excess negative charge A appears on this side Fig. 39.5 (ii).

In a similar way, an excess positive charge builds up in the n-semiconductor when holes diffuse across thejunction. Together with the negative charge A on the p-side, an e.m.f. or p.d. is produced which opposesmore diffusion of charges across the junction. This is called a barrier p.d. and when the flow ceases it has amagnitude of a few tenths of a volt. The narrow region or layer at the p-n junction which contains thenegative and positive charges is called the depletion layer. The width of the depletion layer is of the order10-3 mm.

Junction Diode

When a battery B, with an e.m.f. greater than the barrier p.d., is joined with its positive pole to the p-semiconductor, P, and its negative pole to the n-semi-conductor, N, p-charges (holes) are urged across the p-n

junction from P to N and n-charges (electrons) from N to P, Fig. 39.6 (i). Thus an appreciable current isobtained. The p-n junction is now said to be forward-biased, and when the applied p.d. is increased, thecurrent increases.

When the poles of the battery are reversed, only a very small current flows, Fig. 39.6 (ii). In this case the p-n junction is said to be reverse-biased. This time only the minority carriers, negative charges in the p-semiconductor and positive charges in the n-semiconductor, are urged across the p-n junction by the battery.Since the minority carriers are thermally-generated, the magnitude of the reverse current depends only on thetemperature of the semiconductors. It may also be noted that the reverse-bias p.d. increases the width of thedepletion layer, since it urges electrons in the p-semiconductor and holes in the n-semiconductor further away

from the junction.It can now be seen that the p-n junction acts as a rectifier. It has a low resistance for one direction of p.d. anda high resist ance for the opposi te direct ion, as shown by the characteristic curve in Fig. 39.6 (iii). It istherefore called a junction diode. The junction diode has advantages over a diode valve; for example, itneeds only a low voltage battery to function; it does not need time to warm up; it is less bulky, and it ischeaper to manufacture in large numbers.

Full Wave Rectification. Filter Circuit

In a.c. mains transistor receivers, diodes are used to rectify the alternating mams voltage and to produce steady

or d.c. voltage for the circuit of the receiver.


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Figure 39.7 (i) shows how two diodes, D1 and D2, can producefull-wave rectification. The secondary PQ ofthe mains transformer is centre-tapped at T, so that a.c. voltages of opposite polarity are appliedsimultaneously to D1 and D2 on one half of the input cycle. Thus in Fig. 39.7 (i), D1 conducts well but D2does not. On the other half of the same cycle D 2 conducts well but D1 does not. The output voltage V

between A and T, with a resistor joined between them, would hence be that shown in Fig. 39.7 (ii). It is aunidirectional voltage. Further, compared with half-wave rectification discussed on p. 811, it has a smallerdegree of fluctuation and a greater average voltage.

The output voltage is equivalent to a steady voltage together with varying voltages. In order to filter offthe varying voltages, a filter circuit is used. One form of f ilter circuit consists of a high inductanceL = 50 H

say, in series with a large capacitance 200 uFsay. To a frequency of 50 Hz, the reactance XL= 2pfL -2 x 3-14 x 50 x 100 = 31400 Q; the reactance Xc - 1/2pf = l/(2 x 3.4 x 50 x 2000 x 10-6) = 1.6 Ohm.

Since L and are in series with V, very little of the varying voltage appears across C; practically the wholeof it appears acrossL. So the output voltage Vxy across a resistance R shown is a fairly steady or d.c. voltage

it has only a small 'ripple' of a.c. voltage, Fig. 39.7 (iii).Figure 39.8 shows a bridge circuitwhich produces full-wave rectification without the use of a centre-tapped

secondary as in Fig. 39.7 (i). As shown, four diodes are used. On one half of a cycle, when P is +ve relative toQ, only the diodes DI conduct. On the other half of the same cycle, only the diodes D 2 conduct. The varyingd.c. across , is thus similar to that shown in Fig. 39.7 (ii). In this circuit, however, only a 'smoothing'capacitor is used. Unlike the circuit in Fig. 39.7 (i), becomes charged to practically the peakvalue of thevarying d.c. voltage. The four-diode bridge rectifier thus provides a greater d.c. output voltage than the circuitin Fig. 39.7 (i).

Zener DiodeWhen the reverse bias or p.d. is increased across a p-n junction, a large increase in current is suddenly obtainedat a voltage Z, Fig. 39.9 (i). This is called the Zener effect, after the discoverer. It is partly due to the highelectric field which exists across the narrow p-n junction at the breakdown or Zener voltage Z, which dragsmore electrons from their atoms and thus increases considerably the number of electron-hole pairs.Ionisation by colli sion also contributes to the increase in carriers.


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Zener diodes are used as voltage regulators or stabilisers in circuits. In Fig. 39.9 (ii), a suitable diode D isplaced across a circuit L. Although the battery supply may fluctuate, and produce changes of current in Land D, ifR is suitably chosen, the voltage across D remains practically constant over a reverse current range oftens of milliamperes at the Zener voltage shown in Fig. 39.9 (i). The voltage across L thus remains stable.

The Transistor

The junction diode is a component which can only rectify. The transistor is a more useful component; it is acurrent amplifer. A transistor is made from three layers of p- and n-semiconductors. They are calledrespectively the emitter (E), base (B) and collector (C). Figure 39.10 (i) illustrates a p-n-p transistor, withelectrodes connected to the respective three layers. In a n-p-n transistor, the emitter is n-type, the base is p-type and the collector is n-type, Fig. 39.10 (ii). The base is deliberately made very thin in manufacture. Thetransistor is thus a three-terminal device.

Figure 39.10 shows the circuit symbols for p-n-p and n-p-n transistors. The arrows show the directions of

conventional current ( + ve charge or hole movement) between the emitter E and base B, so that electrons

would flow in the opposite direction. In an actual transistor, the collector terminal is displaced more than the

others for recognition or has a dot near it.

Current Flow in TransistorsThe transistor may be regarded as two p-n junctions back-to-back. Figure 39.11 (i) shows batteries correctly

connected to a p-n-p transistor. The emitter-base is forward-biased; the collector-base is reverse-biased; andthe base is common. This is called the common-base (C-B) mode of using a transistor. Note carefully the polaritiesof the two batteries. The positive pole of the supply voltage X is joined to the emitter E but the negativepole ofthe supply voltage Y is joined to the collector C. If batteries are connected the wrong way round to atransistor the latter may be seriously damaged. In the case of a n-p-n transistor, therefore, the negative pole of one

battery is joined to the emitter and the positive pole of the other is joined to the collector, Fig. 39.11 (ii).


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Consider Fig. 39.11 (i). Here the emitter-base is forward biased by X, so that positive charges or holes flowacross the junction from E to the base B. The base is so thin, however, that the great majority of the holes areurged across the base to the collector by the battery Y. Thus a current Ic flows in the collector circuit. Theremainder of the holes combine with the electrons in the n-base, and this is balanced by electron flow in the

base circuit, so that a small current IB is obtained here. From Kirchhoff's first law, it follows that, ifIE is theemitter current,

IE=IC + IB

Typical values for a.f. amplifier transistors are: IE = 1.0 mA,Ic = 0

.98 mA, I = 0 .02 mA.Although the action of n-p-n transistors are similar in principle to p-n-p transistors, the carriers of the

current in the former case are mainly electrons and in the latter case holes. Electrons are more speedycarriers than holes (p. 824). Thus n-p-n transistors are used in high-frequency and computer circuits,where the carriers are required to respond very quickly to signals.

Common-Emitter (C-E) Characteristics

A transistor can be arranged in one of three ways or modes in circuits. Figure 39.11 shows the common-base (C-B) mode, so-called because the base is common to the input (emitter-base) and output (collector-base) circuits.Figure 39.12 shows the common-emitter (C-E) mode. A third arrangement is a common-collector(C-C) mode.As we explain later, the common-emitter provides satisfactory current amplification and is widely used in a.f.amplifiers.

Figure 39.12 shows a circuit for obtaining the characteristics of a n-p-n transistor in the common-emittermode. X and Y may be batteries of 1.5 V and 4.5 V respectively, connected to potentiometers P and Q of 1kOhm and 5 kOhm. This enables the base-emitter p.d., VBEorVB) and the collector-emitter p.d., or Vc to bevaried. The p.d. is measured by high resistance voltmeters, preferably d.c. solid state voltmeter types capable ofmeasuring p.d. in steps such as 50 mV. The meter for base current, IB, should be a microammeter and for thecollector current,Ic , a milliammeter. Typical results are shown in Fig. 39.13 (i), (ii) and (iii).


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Output characteristic (Ic Ic, withIBconstant). The 'knee' of the curves shown in Fig. 39.13 (i) correspond toa low p.d. of the order of 0 .2 V. For higher p.d. the output current ICvaries linearly with Vc for a given basecurrent. The linear part of the characteristic is used in a.f. amplifier circuits, so that the output voltagevariation is then undistorted.

The output resistance r0 is defined as DVC/DIC, where the changes take place on the straight part of thecharacteristic. r0 is an a.c. resistance; it is the effective resistance in the output circuit for an a.c. signal input. Itshould be distinguished from the d.c. resistance, VC/IC, which is not required in amplifier circuit analysis.

The small gradient of the straight part of the characteristic shows that r0 is high. For example, suppose DV

= 2 V and DIC = 0.2 mA = 2 x 10 5 A. Then r0 - 2/(2 x 10-5) = 100000 Ohm. If a varying resistance load is usedin the output or collector circuit, the high value of r0 relative to the load shows that the output current is fairlyconstant. So the output voltage is proportional to the load resistance.

Transfer characteristic (Ic - IB, Vcconstant). The output currentIc varies fairly linearly with the inputcurrentIB, Fig. 39.13 (ii). The current transfer ratio b, or current gain, is defined as the ratio DI/DIB undera.c. signal conditions It should be distinguished from the d.c. current gain,IC/IB. From Fig. 39.13 (ii),

Input characteristic (IB VB, Vcconstant). The input resistance ri is defined as the ratio DV/DI. As the inputcharacteristic in Fig. 39.13 (iii) is non-linear, then r1varies. At any point of the curve, riis equal to the gradientof the tangent to the curve and is of the order of kilohms.

Current Amplification in C-E Mode

In general, the magnitude ofb, DIC/DIB, for the common-emitter circuit is high, from about 20 to 500 for manytransistors. Thus the base current is a sensitive control over the collector current.

We can obtain a rough value for b by assuming that when electrons are emitted from the n-emittertowards the p-base, a constant fraction a reaches the n-collector where a is typically 0-98 (see p. 829). Thus

Ic = aI. Now from p. 829,

Voltage Amplification and Power Gain

As we have just seen, the transistor in the common-emitter mode is a current amplifier. To change the output(a.c.) current to a voltage VO, a resistance load R can be used in the collector or output circuit. Figure 39.14shows also dia-grammatically the base bias necessary for no distortion ofV0 (p. 830).

We can illustrate the voltage amplification by supposing thatR = 5 kOhm, the input resistance ri = 2 kOhm,the input (a.c.) voltage is 10 mV or 0.01 V peak value, and the transfer ratio b = 50.

The peak a.c. currentIb flowing in the base circuit is:


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Transistor SwitchIn addition to its use as a current amplifier, the transistor can be used as a switch in computer circuits.Millions of switching operations are needed daily in working computers, so swift switches are required. Onthis account n-p-n transistors are preferred. Here the charge carriers are mainly electrons, Which have a muchgreater speed for a given voltage than holes or p-charges.

The basic circuit is shown in Fig. 39.20 (i). It consists of a n-p-n transistor connected in the common-emittermode, with a resistanceR in the output or collector circuit. Since this is a n-p-n transistor, the positive pole ofthe supply VCC is connected to the collector and the negative pole to the emitter E.

A typical output voltage (V0)-input voltage (V1) characteristic of the circuit is shown in Fig. 39.20 (ii). Atvery low input voltages the output voltage is practically + 6V, the supply voltage Vcc for the circuit. Atinput vol tages of more than a fraction of a volt, however, the output voltage is nearly zero. This is explainedshortly on page 837.

Sine Wave Input. Amplifier Use

Suppose the input Vi, is a sine wave voltage of peak value 6 V. Fig. 39.20 (Hi). For a large part of the + ve halfcycle, Viwill be greater than +0.4 V. So the output voltage V0 will be practically zero along PQ. When Vi is lessthan +0.4 V and negative on the - ve half of the cycle, V0 will be practically a constant high voltage RS asshown. So the output V0 is roughly asquare wave voltage.

To use the transistor as an amplifier, the output voltage V0 must have the same waveform as the input a.c.voltage Vi.This time the straight inclined line AB of the characteristic in Fig. 39.20 (ii) must be used. So, as

shown, (a) the base bias should correspond to the midpoint M of AB, a bias equal to OX or about 0-2 V, and(b) the input voltage to be amplified must have a peak value not greater than XB (0-1 V), otherwise theoutput waveform is distorted during part of the input cycle.


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States of TransistorWe can explain the characteristic curve by noting that, ifIC is the collector current flowing for a particularinput voltage, the output voltage V0, or Vce, is less than the supply voltage Vcc by the potential drop across R,which isICR. Thus

In general, Ic depends on the base current IC and this is governed by the base-emitter or input voltage Vi.Suppose Vi is very low or practically zero. Then Ic is practically zero, and the transistor is said to be

'cutoff'. From above, we can see that the output voltage VO is then practically equal to Vcc or high. See Fig.39.20 (ii). Conversely, suppose Vi is high so that the transistor is 'saturated', that is, any further increase in

base current produces no rise in Ic. The p.d. across R is then large and so the output voltage is practicallyzero from above. See Fig. 39.20 (ii).

Thus depending on the input voltage, the transistor can switch between two statescutoff or saturation.The output, voltage then switches between two levels, +VCC and practically 0. In the special type ofcomputer circuits known as logic circuits or logic gates, the binary digits '1' and '0' can be represented by +Vcc and 0 respectively, or by 0 and + Vcc, by this switching of states. It should be noted that the transistor acts'non-linearly', whereas it acts 'linearly' in amplifiers (p. 830).

Logic Gates

We can now discuss briefly some useful logic gates.Figure 30.21 (i) shows the circuit for an INVERTER gate. It consists of a transistor in the common-emitter mode

connection, with an appropriate load resistance R and base resistance rA. Suppose the input is a '1', forexample, + Vcc volt. A high base current then flows in rA, and as explained before, the transistor becomessaturated and the output is '0'. Conversely, if the input is '0' (zero volt), the transistor is cutoff and the outputis + Vcc or T. Thus the output is always the inverse or opposite of the input. This is shown in a so-called'truth table' in Fig. 39.21 (i), which also contains the symbol for the INVERTERgate.


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Figure 39.21 (ii) shows the circuit for a NOR gate. It is similar to the circuit in Fig. 39.21 (i) except thattwo inputs and two base resistors, rA and rB, are provided. If both inputs are '0' or zero volt, the transistor iscutoff; hence the output is '1' or +VCC volt. If either input or both inputs, A and B, are '1', then, withappropriate vales for rA and rB, the transistor saturates and the output is'0'.

These results are shown in the truth table in Fig. 39.21 (ii), together with the symbol for this gate. It iscalled a NOR gate because the output is if neither A nor is '1'; in all other cases the output is '0'.

Figure 39.22 (i) shows in symbol form an OR gate; it is made of a NOR gate followed by an INVERTER gate, sothat the output S1 of the NORgate is inverted to produce a final output S2. By writing the inputs A and in truthtable form as on p. 837, we find that S2 is a '1' if either A or is a T.

Figure 39.22 (ii) shows in symbol form an AND gate; it consists of two INVERTERgates followed by a NORgate.The truth table for S1 S2 and S3 shows that the output S3 is '1' only if A and are T.

Figure 39.22 (iii) shows the construction of a-NAND gate. The truth table shows that the output S3 is '1' ifA andB, individually or together, are not '1'.

PhototransistorAphotodiode is a junction diode sensitive to light. When the diode is reverse-biased, minority carriers flow inthe circuit and constitute a so-called 'dark' current. If the junction of the diode is now illuminated, the lightenergy produces more electron-hole pairs, which are then swept across the junction. The increased current whichflows is the light' current.

Aphototransistor is a transistor sensitive to light in which the base is usually left disconnected. When lightfalls on the emitter side, more electron-hole pairs are produced in the base. This is amplified by transistor

action, and a larger collector current is obtained. In principle the phototransistor is a photodiode plusamplifier.

Figure 39.23 shows a circuit in which a suitable p-n-p phototransistor is connected in series with arelay coil D and a d.c. supply voltage. When the phototransistor is illuminated, the increase in collectorcurrent closes the contacts of a magnetic relay. Current then flows in a circuit connected to the relay, and abell, for example, may then ring, Fig. 39.23. When the light is switched off, the falling current in the relay coilproduced an induced voltage in the same direction as the battery supply. This would raise the collector voltageand prevent the switch-off at the contacts. The diode OA81 across the coil acts as a safeguard. As soon as the risinginduced voltage becomes equal to the battery voltage the diode conducts, and prevents any further rise incollector voltage.

Field Effect Transistor (f.e.t.)

The junction transistor operates by the movement of two types of charge carriers, electrons (negative charges) andholes (positive charges). Further, the transistor has two p-n junctions. In this section we give a brief account of the

field effect transistor(f.e.t.). As we shall see, this type of transistor uses only one type of charge carrier whenworking and has only one p-n junction.

Figure 39.24 (i) shows the basic construction of the early f.e.t. It consists of a bar of n-type silicon (p-siliconcould also be used), with a p.d. VDS applied between one end called the source S and the other end called thedrain D. By diffusion of impurities, a heavily-doped p-region forms a p-n junction with the bar, and theelectrode at this p-region is called the gate G. The symbol for the n-type f.e.t. is shown in Fig. 39.24 (ii). Thearrow is reversed in the p-type f.e.t.


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Principle of f.e.t.

Consider the f.e.t. with batteries connected as shown in Fig. 39.25. The potential of the drain D is +ve relativeto the source S, so the electrons in the n-bar flow from S to D. This current is called the drain current ID.

The battery between the gate G and S makes the potential of G ve relative to S. Now the n-bar near the p-junction has a +ve potential relative to S. Thus the p-n junction near the gate is reverse-biased. Consequently, asshown diagram-matically in Fig. 39.25, a depletion region is obtained at the p-n junction (p. 826). Thewidth of the depletion region depends on the magnitude of the p.d. VGS when the p.d. VDS is constant. IfVGS,is made more -ve the wider is the depletion region at the p-n junction. This is shown diagrammatically inFig. 39.25.

The depletion region, which has no free electrons,narrows the conducting n-channel in the bar. WhenVGS is made more -ve, the current is reduced As itprovides a sensitive control over the drain current,small changes in gate voltage can produce largeamplification of the current. Thus signals, or a.c.voltages. can be amplified by the f.e.t. transistor.

F.E.T. Characteristics. F.E.T. AmplifierSome typical output characteristics (ID - VDS, VGS constant) and input characteristics (id - VGS, VDS constant)

are shown in Fig. 39.26 (i), (ii).

When VGS is kept constant, the drain current ID first increases linearly as the voltage VDS is increased from zero,since the bar then acts as an ohmic conductor. This corresponds to the line A in Fig. 39.26 (i). When VDS increasesfurther, the p-n junction at the gate becomes more reverse-biased, since the positive potential of the n-bar at the gateincreases relative to S and hence relative to G. The depletion region then widens, as stated above, and so the current IDbegins to increase at a slower rate. The slow rise of ID with VDS begins at X in Fig. 39.26 (i) and continues along thestraight line B.

When the gate voltage VGS is made more negative, the reverse-bias is increased. The current ID is now decreased.Figure 39.26 (ii) shows the effect on the input characteristics.

An a.f. amplifier circuit which uses the f.e.t. transistor is shown in Fig. 39.26 (iii). The load resistance is 10 kOhm,the temperature stabilisation is provided by the 2 kOhm resistance and the necessary bias for an undistorted output isprovided by the 1 MOhm resistor. The 0.1 uF capacitors prevents direct current or voltage reaching the input andoutput circuits.


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Junction Diode and Transistors

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