What is transistor ? How is it formed ? Show with the help of diagram action of npn transistor

Explanation

Transistor: It is an electronic device which is obtained by slicing either a thin layer of n-type semiconductor between two thick layers of p-type semiconductor or a thin layer of p-type semiconductor between two thick layers of n-type semiconductor.

In transistor, central layer is lightly doped and known as base while one of the outer layers heavily doped is known as an emitter and other is known as collector. The base is the junction where the proper interaction between the emitter and collector takes place. p-n-p Transistor: Biasing arrangement for p-n-p transistor is shown in Figure in which positive terminal of the battery `V_(EB)` is connected to the emitter and negative terminal of the battery is connected to the base. Similarly negative terminal of the battery `V_(CB)` is connected to collector and its positive terminal is connected to base. Because emitter -base junction is forward biased, holes from the emitter region move towards the base whereas electrons from the base region move towards emitter. Since the thickness of the base is very small and it is lightly doped, a small percentage of these holes `(lt 5%)` recombine with electrons in the base region and remaining holes are pulled by the collector region which is reversed biased and is at high negative potential. These holes constitute the collector current. The collector current is slightly less than emitter current.

As soon as a hole recombines with an electron in the base, an electron from negative terminal As soon as a hole recombines with an electron in the base, an electron from negative terminal of the battery `V_(EB)` enters the base and gives rise to base current. For each electron -hole recombination, a covalent bond in the emitter region breaks and an electron-hole pair is generated. Electrons so generated as swept by the positive terminal of the battery `V_(EB)` resulting in the emitter current `I_(E)`. Thus `I_(E)= I_(B)= I_(C)`. Inside a p-n-p transistor, current is due to holes whereas current in the external circuit is due to electrons.