(i)That 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 (.)

(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.

(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.

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