Transistor Action

The emitter-base junction of a transistor is forward biased whereas collector-base
junction is reverse biased. If for a moment, we ignore the presence of emitter-base
junction, then practically* no current would flow in the collector circuit because of the
reverse bias.

However, if the emitter-base junction is also present, then forward bias on
it causes the emitter current to flow. It is see that this emitter current almost entirely
flows in the collector circuit. Therefore, the current in the collector circuit depends upon
the emitter current.

If the emitter current is zero, the collector current is nearly zero.
However, if the emitter current is 1mA, then collector current is also about 1mA.

This is
precisely what happens in a transistor. We shall now discuss this action for
–npn and pnp transistors.

(i) Working of npn transistor:

Fig.1 shows the npn transistor with forward
bias to emitter-base junction and reverse bias to collector-base junction. The forward
bias causes the electrons in the n-type emitter to flow towards the base. This constitutes
the emitter current I _E .

As these electrons flow through the p-type base, they tend to
combine with holes. As the base is lightly doped and very thin, therefore, only a few
electrons (less than 5%) combine with holes to constitute base** current I B . The
remainder (***more than 95%) cross over into the collector region to constitute collector
current I C . In this way, almost the entire emitter current flows in the collector circuit. It is
clear that emitter current is the sum of collector and base currents i.e.

I _E = I _B + I _C

…………………………………………………………………………………………………………………………..
*In actual practice, a very little current (a few µA) would flow in the collector circuit. This
is called collector cut off current and is due to minority carries.
**The electrons which combine with holes become valence electrons. Then as valence
electrons, they flow down through holes and into the external base lead. This
constitutes base current I B .
****The reasons that most of the electrons from emitter continue their journey through
the base to collector to form collector are: (i) The base is lightly doped and very thin.
Therefore, there are a few holes which find enough time to combine with electrons. (ii)
The reverse bias on collector is quite high and exerts attractive forces on these
electrons.

(ii) Working of pnp transistor:

Fig.2 shows the basic connection of a pnp
transistor. The forward bias causes the holes in the p-type emitter to flow
towards the base. This constitutes the emitter current I E . As these holes
cross into n-type base, they tend to combine with the electrons. As the
base is lightly doped and very thin, therefore, only a few holes (less than

5%) combine with the electrons. The remainder (more than 95%) cross
into the collector region to constitute collector current I C . In this way,
almost the entire emitter current flows in the collector circuit. It may be
noted that current conduction within pnp transistor is by holes. However, in
the external connecting wires, the current is still by electrons.
Importance of transistor action. The input circuit (i.e. emitter-base
junction) has low resistance because of forward bias whereas output
circuit (i.e. collector-base junction) has high resistance due to reverse
bias. As we have seen, the input emitter current almost entirely flows in
the collector circuit. Therefore, a transistor transfers the input signal
current from a low-resistance circuit to a high-resistance circuit. This is the
key factor responsible for the amplifying of the transistor. We shall discuss
the amplifying property of transistor later in this chapter.

Note.

There are two basic transistor types :

the bipolar junction transistor (BJT) and field.
Effect transistor (FET). As we shall see, these two transistor types differ in both their
operating characteristics and their internal construction. Note that when we use the
term transistor, it means bipolar junction transistor (BJT).

The term comes from the
fact that in a bipolar transistor, there are two types of charge carriers (viz. electrons and
holes) that play part in conductions. Note that bi means two and polar refers to
polarities. the field-effect transistor is simply referred to as FET.

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