Transistor, its characteristics and constants

 Transistor

It is a semi-conductor device which plays the same role as that played by a vaccum tube triode. 

     Construction. It consists of an 'N-type' or 'P-type' crystal sandwitched in between two other types of crystals. The central piece is known as base while the others on the two sides are known as emitter and collector.

Fig. 1.

The emitter, collector and the base play the same role as that played by filament, plate and the grid of a vaccum tube triode. The base of the transistor has to be very thin and low doped as compared to the emitter and collector. This is required to minimise the loss of charged carriers due to recombination. Two types of transistors, 'N-P-N' and 'P-N-P' transistors, are shown in [Fig. 2(i) and (ii)] respectively along with their schematic diagrams. 

           (Click on image to see it clearly) 

Fig. 2. Working of a transistor. 

     Biasing. The emitter base circuit of the transistor is always forward biased while the collector base circuit is always reverse biased. 

     Fig. 2(i) shows the circuit diagram for an 'N-P-N' transistor. The emitter base circuit is forward biased by connecting negative terminal of a source of e.m.f. 'Veb' to emitter and positive terminal to base (P-type). The collector base circuit is reverse biased by connecting positive terminal of a source of e.m.f. 'Vcb' with collector C and negative terminal to the base. Three microammeters are connected in the circuit to note emitter current 'Ie', collector current 'Ic', and base current 'Ib'. Circuit diagram of 'P-N-P' transistor on similar guidelines is shown in Fig. 2(ii).

Transistor action

     Transistor action is the term applied to the process of explaining the passage of electric current through a transistor. 

     Let us consider the circuit diagram of 'N-P-N' transistor [Fig. 2(i)]. With 'Veb' applied as shown in [Fig. 2(i)], electrons in the emitter section are repelled and injected into base. Since P-region (base) is not highly doped and is very thin, most of the electrons diffuse through it and reach the collector section. About 5% of the electrons are lost as charge carriers because of combination with the holes in 'P-region'. Remaining electrons which have already crossed over to the collector section are rapidly swept by the collector voltage. For each electron flowing out of the collector and entering the positive terminal of 'Vcb' which again moves towards the base and the process is repeated. Thus, current through the transistor is due to motion of electrons from emitter to collector while conduction current in the external circuit is also due to motion of electrons but from collector to emitter. The collector current 'Ic' is approximately 0.95 'Ie' where 'Ie' is the emitter current. 

     In case of a 'P-N-P' transistor biased as shown in [Fig. 2(ii)], positive terminal of 'Veb' repels the hole in emitter region. As these holes cross over to the base which is 'N-type', some holes (about 50%) get lost by capturing free electrons. Remaining holes cross over to the collector region and are attracted by the negative terminal of 'Vcb'. As each hole reaches the collector terminal, an electron is emitted from the negative terminal of 'Vcb' and neutralises the hole. For each hole that is lost, a covalent bond near the emitter electrode breaks down and the liberated electron leaves the emitter electrode amd enters the positive terminal of the emitter battery. The new hole that is formed moves towards the base and undergoes the same process as explained above. So, current flowing through a 'P-N-P' transistor takes place the conduction of holes from emitter to collector, while conduction current in the external circuit is carried by the electrons. 

     The emitter current 'Ie', collector current 'Ic', and base current 'Ib' (due to neutralisation of hole in base region) are shown by micro-ammeters, M₁, M₂, M₃ are connected in the circuits. It will be onserved that 

                  Ie = Ic + Ib

Modes of transistor operation

There are three electrodes of a transistor. One of these electrodes is held common or grounded. The term 'common' is used with that electrode which is common to both input and output circuits. The common electrode is generally earthed. Therefore, the word grounded is also used. According these are three modes for operation of transistor. The input and the output impedances of the transistor vary from one mode to the other. 

     (i) Common-base operations. In this type of operation base terminal is common to both input and output circuits. Transistor circuit and its equivalent vacuum tube circuits are shown in [Fig. 3(i) and (ii)] respectively. In this case, the input impedance is high. 

Fig. 3. Common base operation. 

    (ii) Common-emitter operation. In this type of operation the emitter forms the common electrode. A common non emitter arrangement for a transistor and vacuum tube are shown in [Fig. 4(i) and (ii)]. In this case, the input impedance is low while the output impedance is medium to high. 

Fig. 4. Common emitter operation. 

   (iii) Common-collector operation. The collector forms the common terminal in input and output circuits are shown in [Fig. 5 (i) and (ii)]. In this case, the input impedance is high and the output impedance is low. 

Fig. 5. Common-collector operation. 

     There can be a number of ways in which transistor characteristics can be plotted. Two very commonly used characteristics of a transistor are given below :

Transistor characteristics

Corresponding to the three types of connections for operation of transistor, we have a three types of transistor characteristics. 

     1. Common base connection (CB mode). The circuit diagram of CB mode of N-P-N transistor is shown in Fig. 6(i) while that of P-N-P transistor is shown in Fig. 6(ii). Three milliammeters A, B and C indicate emitter current 'Ie', base current 'Ib' and collector current 'Ic' respectively. Veb is the emitter base voltage while Vcb is the collector base voltage. It may be noted that emitter circuit is forward biased while collector circuit is reverse biased. Circuit between emitter and base is the input circuit while that between collector and base is the output circuit. 

Fig. 6. Circuit diagram. 

     Following characteristics can be drawn from these connections. 

     (a) Input characteristics. Input characteristics of a transistor, is common base operation is a graph between emitter current (Ie) and emitter-base voltage (Veb) at constant collector-base voltage (Vcb). 

     Input characteristics for N-P-N and P-N-P transistors in common base mode are shown in Fig. 7(i) and (ii), respectively. 

     Input characteristics can be yield the value of input resistance of the transistor. 

     Input resistance (ri) of a transistor in common base operation is defined as the ratio between change in emitter-base voltage (∆Veb) to the change in emitter current (∆Ie) at constant collector-base voltage (Vcb). 

            ri = [∆Veb/∆Ie]Vcb = Constant

Unit of ri is 'ohm'. 

Fig. 7. Input characteristics. 

    (b) Output characteristics. Output characteristics of a transistor in common base operation is a graph between collector-base (Vcb) voltage and the collector current (Ic) at constant emitter current (Ie). 

     Output characteristics for N-P-N transistor and P-N-P transistor in common base operation are shown in Fig. 8(i) and (ii), respectively. 

Fig. 8. Output characteristics (CB mode). 

     Output characteristics can be yield the value of output resistance (r0) of the transistor. 

     Output resistance of a transistor in common base operation is defined as the ratio between change in collector-base (∆Vcb)  voltage to the change in collector current (∆Ic) at constant emitter current (Ie). 

                r0 = [∆Vcb/∆Ic]Ie = Constant

Unit of r0 is 'ohm'. 

     (c) Transfer characteristics. It is a graph between collector current (Ic) and emitter current (Ie) keeping collector-base (Vcb) constant. 

Fig. 9. Transfer characteristics. 

     Transfer characteristics is shown in Fig. 9. This characteristics is very useful in determining the effect of change in current of input circuit (due to signal) in the output. 

     2. Common emitter connection. The circuit diagrams for CE connection of N-P-N transistor and P-N-P transistor are shown in Fig. 10 (i) and (ii), respectively. In the case, the emitter is common to both input and output circuits. Circuit between base and emitter is the input circuit while that between collector and emitter is the output circuit. 

Fig. 10. Circuit diagram. 

     Following characteristics can be drawn from these connections. 

(a) Input characteristics

     Input characteristics of a transistor in common emitter operation in the graph between emitter base voltage (Veb) and base current (Ib) at a constant, collector emitter (Vce) voltage. 

     Input characteristics of N-P-N transistor and P-N-P transistor in common emitter operation are shown in Fig. 11 (i) and (ii), respectively. 

Fig. 11. Input characteristics (CE mode). 

     We can calculate the value of input resistance (ri) from the input characteristics. 

     Input resistance of a transistor in common emitter operation is defined as the ratio between change in emitter-base (∆Veb) voltage to change in base current (∆Ib) at constant collector-emitter (Vce) voltage. 

                ri = [∆Veb/∆Ib]Vce = Constant

Unit of ri is 'ohm'. 

(b) Output characteristics

     Output characteristics of a transistor in common emitter operation is a graph between collector-emitter (Vce) voltage and collector current (Ic) keeping base current (Ib) constant. 

     Output characteristics of N-P-N transistor and P-N-P transistor in common emitter operation are shown in Fig. 12 (i) and (ii), respectively. 

     We can calculate the value of output resistance (r0) from output characteristics. 

Fig. 12. Output characteristics (CE mode). 

     Output resistance of transistor in common emitter operation is defined as the ratio between change in collector emitter (∆Vce) voltage to the change in collector current (∆Ic) at constant base current (Ib). 

                r0 = [∆Vce/∆Ic]Ib = Constant

Unit of r0 is 'ohm'. 

(C) Transfer characteristics

     It is a graph between collector current (Ic) and base current (Ib) at constant collector emitter voltage, Vce . 

          It is a straight line as shown in Fig. 13.

Fig. 13. Transfer characteristics. 

3. Common collector connection : (CC mode) 

     The circuit diagrams of CC mode of N-P-N and P-N-P transistors in common collector mode of connections are shown Fig. 14 (i) and (ii), respectively. 

     Collector base circuit forms the input circuit while the emitter-collector circuit forms the output circuit. Generally, its input characteristics are not drawn because of low voltage and power gain in this mode of connection. 

Fig. 14. Common collector mode. 

Transfer characteristics

     Transfer characteristics of a transistor in common collector mode of connections is a graph between base current (Ib) and emitter current Ie at constant emitter collector (Vce) voltage. 

     The transfer characteristics is shown in Fig. 15. 

Fig. 15. Transfer characteristics. 

Transistor constants

Depending upon the mode of operations the performance of a transistor is specified in terms of some constants known as transistor constants. 

Important notes 

1. The base region of a transistor is always narrow. 
2. The base region is always low doped. 
3. Input circuit of a transistor is always forward biased while the output circuit is always reverse biased. 

Key words

1. Transistor. A semi-conductor device having one type of semi-conductor sandwiched between two other types of semi-conductor. 

2. Transistor action. Process of conduction of a transistor. 

3. Transistor region. A small region having a potential gradient due to contact potential. 

Key Formulae

Transistor constants

   

     (a) Current amplification factor or forward current transfer ratio (∝). 

   

     (i) ∝ = [∆Ic/∆Ie]Vcb = constt. = [Ic/Ie]Vcb = constt. 

                [For common base operation]

(ii) β = [∆Ic/∆Ib]Vcb = constt. = [Ic/Ib]Vcb = constt. 

               [For common emitter operation]

     (b) Current transfer ratio (γ) 

γ = [∆Ie/∆Ib]Vce = constt. = [Ie/Ib]Vce =constt.

                [For common collector operation]

(c) Relation between transistor constants

(i) β terms of ∝ : β = ∝/1 - ∝

(ii) ∝ in terms of β : ∝ = β/1 + β

(iii) γ in terms of ∝ : γ = 1/1 - ∝

(iv) γ in terms of β : γ = 1 + β