NPN Transistor, construction and working

What is a NPN Transistor?

Symbol of NPN Transistor
Symbol of NPN Transistor

NPN transistor is most commonly used transistor because the majority carrier in NPN transistor is electrons and the mobility of the electrons is higher than the mobility of holes.

Construction of NPN Transistor:

Construction of NPN Transistor
Construction of NPN Transistor

It consists of three terminals emitter, base and collector. It consists of P type thin layer placed between two N type layers. 

The electrons which is the majority charge carriers flow from emitter to collector. The P type which is the base is lightly doped and Emitter is moderately doped and collector is heavily doped.

Two diode Model of NPN transistor:

Two diode model of NPN Transistor
Two diode model of NPN Transistor

A diode is a two terminal device which allows the current to flow in only one direction and resists the flow in the other direction. NPN Transistor is three terminal device which looks like two PN Junction diodes connected back to back. 

These two diodes behave like collector base junction and base emitter junction. When two diodes are merged and one terminal is taken as common it results in three terminal device.

NPN Transistor circuit:

NPN Transistor circuit

As shown in figure the base emitter circuit is forward biased and collector base circuit is reverse biased. Collector is always to the positive terminal and emitter is connected to the negative terminal. The load resistance and a resistance at the base terminal are used to limit the current flow.

Working of NPN Transistor:

Working of NPN Transistor
Working of NPN Transistor

Two batteries are used. The negative terminal of one battery is connected to the emitter and the positive terminal of the same battery is connected to the base. Thus the emitter base terminal is forward biased. Emitter which is the N type is moderately doped and base which is P type is lightly doped. 

So when emitter base is forward biased the majority carrier in N type which is holes, move from emitter terminal to base. This movement of majority charge carriers results in emitter current IE. The potential barrier is less at the junction.  

Some of the electrons combine with the holes in base and the movement of the remaining charge carriers constitutes the base current IB. Thus the emitter current is the sum of the base current and the collector current.

The positive terminal of the second battery is connected to the collector and negative terminal to the base. Thus base collector circuit is reverse biased. The electrons which reaches the collector combines with the holes from the positive terminal to which the collector is connected. 

At the same time electrons from emitter reaches again to the collector. This process continues. This movement of electrons forms the emitter, base and collector current.

So a small change in the emitter base bias voltage results in large change in emitter collector current. Thus they are used as amplifiers.

Post a Comment