Describe a transistor and explain its working.

Transistor : It is a device which consists of two p-n junctions. Transistor means transfer of resistance from input to output. It is also called a junction Transistor.
i) Emitter : The section at one end of the transistor is called Emitter (E). It is heavily doped and supplies the majority charge carriers.
ii) Base : The section at middle of transistor is called Base (B). It is very thin and lightly doped so that it may pass most of the injected charge carriers to collector.
iii) Collector : The section at other end of the transistor is called collector (C). It is moderately doped and collects the majority charge carriers through Base.
1) Working of p-n-p Transistor :
i) In p-n-p transistor, the emitter and collector are of p-type and base is of n-type. The emitter base junction is forward biased and collector base junction is reverse biased as shown.

ii) The holes in the emitter are repelled by positive terminal of V_EB, cross the emitter - base junction and enter the base region, This constitutes the Emitter current (I_E).
iii) As the base is lightly doped only a few holes recombine with electrons in base and constitute a small base correct (I_B).
iv) Majority of holes will penetrate through the base and enters the collector region. The collector sweeps the holes that enter the collector region giving large collector current (I_C).
v) For each hole reaching the collector, an electron is released from the negative terminal of V_CB to neutralize the hole.
vi)With this energy a covalent bond near the emitter breaks and electron-hole pair is formed. Hole enters into emitter and electron enters to the positive terminal of V_EB. This process continues. Current conduction inside the transistor is due to holes and in the external circuit is due to electrons.
${\mathrm{I}}_{\mathrm{E}}={\mathrm{I}}_{\mathrm{B}}+{\mathrm{I}}_{\mathrm{C}}$
2. Working of n-p-n transistor :
i) In n-p-n transistor, the emitter and collector are of n-type and base is of p-type. The emitter Base junction is forward biased and collector base junction is reverse biased as shown.

ii) The electrons in the emitter are repelled by negative terminal of V_EB, cross the emitter - base junction and enters the base region. This constitutes the Emitter current (I_E).
iii)As the base in lightly doped only a few electrons recombine with holes in base and constitute a small base current (I_B).
iv) Majority of electrons will penetrate through the base and enters the collector region. The collector sweeps all the electrons that enter the collector region giving large collector current (I_C).
v) The electrons collected by the collector move towards the positive terminal V_CB and further into the positive terminal of V_EB and finally into emitter region. This process continues.
vi) Current conduction inside the transistor and in the external circuit is due to electrons only.
$\therefore {\mathrm{I}}_{\mathrm{E}}={\mathrm{I}}_{\mathrm{B}}+{\mathrm{I}}_{\mathrm{C}}$