Question

Draw a circuit diagram of an n-p-n transistor with its emitter base junction forward biased and base collector junction reverse biased. Describe briefly its working. 

Explain how a transistor in active state exhibits a low resistance at its emitter base junction and high resistance at its base collector junction. 

Draw a circuit diagram and explain the operation of a transistor as a switch.

Answer 1

Base Current and Collector Current: Under forward bias of emitter-base junction, the electrons in emitter and holes in base are compelled to move towards the junction, thus the depletion layer of emitter-base junction is eliminated. As the base region is very thin, most electrons (about 98%) starting from emitter region cross the base region and reach the collector while only a few of them (about 2%) combine with an equal number of holes of base-region and get neutralised. As soon as a hole (in P-region) combines with an electron, a covalent bond of crystal atom of base region breaks releasing an electron-hole pair. The electron released is attracted by positive terminal of emitter battery V_EE, giving rise to a feeble base current (I ). B  Its direction in external circuit is from emitter to base. The hole released in the base region compensates the loss of hole neutralised by electrons. 

The electrons crossing the base and entering the collector, due to reverse biasing of collector-base junction, are attracted towards the positive terminal of collector battery V_CC. In the process an equal number of electrons leave the negative terminal of battery V_CC  and enter the positive terminal of battery V_EE. This causes a current in collector circuit, called the collector current. In addition to this the collector current is also due to flow of minority charge carriers under reverse bias of base-collector junction. This current is called the leakage current.

 Thus, collector current is formed of two components: 

(i) Current (I_nc) due to flow of electrons (majority charge carriers) moving from emitter to collector. 

(ii) leakage current (I_leakage) due to minority charge carriers, i.e. ,  I_c  = I_nc + I_leakage. 

Emitter Current: When electrons enter the emitter battery V_EE  from the base causing base current or electrons enter the collector battery V_CC  from the collector causing collector current, an equal number of electrons enter from emitter battery V_EE  to emitter, causing the emitter current. The process continues. 

Relation between Emitter, Base and Collector Currents: 

Applying Kirchhoff’s I law at terminal O , we get 

I_E = I_B + I_C 

That is, the emitter current I_E  is the sum of base current I_B  and the collector current I_C. This is the fundamental relation between currents in the bipolar transistor circuit.

Transistor as a Switch

A switch is a device which can turn ON and OFF current is an electrical circuit. A transistor can be used to turn current ON or OFF rapidly in electrical circuits. Operation: The circuit diagram of n-p-n transistor in C_E configuration working as a switch is shown in fig. V_BB  and V_CC  are two dc supplies which bias base-emitter and emitter collecter junctions respectively. Let VBB  be the input supply voltage. This is also input dc voltage (V_C ). The dc output voltage is taken across collector-emitter terminals, R_L  is the load resistance in output circuit.

Applying Kirchhoff’s second law to input and output meshes (1) and (2), we get

Let us see the change in V₀  due to a change in V_i. In case of Si transistor; the barrier voltage across base-emitter junction is 0.6V. Therefore, when V_i is less than 0.6V, there is no collector current (I_C = 0), so transistor will be in cut off state. Hence, from (iv) with I_C = 0; V₀ = V_CC. 

When V_i  becomes greater than 0.6V, I_C  begins to flow and increase with increase of V_i. Thus, from (iv), V₀  decreases upto V_i = 1V; the increase in I_C  is linear and so decrease in output voltage V₀  is linear. Beyond V_i = 1V, the change in collector current and hence in output voltage V₀  is non-linear and the transistor goes into saturation. With further increase in V_i, the output voltage further decrease towards zero (though it never becomes zero). 

If we plot V₀  versus V_i, we get the graph as shown in fig. [This characteristics curve is also called transfer characteristic curve of base biased transistor.] 

The curve shows that there are non-linear regions. (i) between cut off state and active state and (ii) between active state and saturation state; thus showing that the transitions (i) from cut off to active state and from active to saturation state are not sharply defined.

Now we are in the position to explain the action of transistor as a switch. When transistor is non-conducting (I_C = 0), it is said to be ‘switched off’ but when it is conducting (I_C  is not zero); it is said to be ‘switched ON’. 

As long as input voltage V_i  is low and unable to overcome the barrier voltage of the emitter base junction, V₀  is high (I_C = 0 and V₀ V_CC), so the transistor is ‘switched OFF’ and if it is high enough to derive the transistor into saturation (I_C  is high and so V₀ (V_CC - I_C R_L) is low, very near to zero, so the transistor is ‘switched ON’. Thus we can say low input switches the transistor is OFF state and high input switches it ON. 

The switching circuits are designed in such a way that the transistor does not remain in active state.