Electronic Devices - BJT Devices - Discussion

If a bjt is not doped, it will be an instrinsic material.

Why BJT has to be doped?

What will happen if it is not doped, What's the purpose?

As we known that transistor is from third generation semiconductor family which consists of three regions. Let take a npn transistor, here emitter is heavily doped compared to base.

And collector is moderately doped region. When we give a bias voltage to drive transistor, electrons are tried to move from emitter (n) to base region (p) whereas emitter is heavily doped the width of the conduction band is high compared to base.

Hence electrons fall from high level to low level causes diffusion current. Hence base is lightly doped compared to collector, so electrons can't jump and recombination will takes place, so to avoid this we use an external bias at collector to drive electrons from base to collector and hence drift current will produce.

So according to walter and shockley in base the recombination must be very low to drive more current to the collector, hence to decrease recombination, width of the base must be low.

For npn width of base < length of emitter, base thickness is approximately equal to junction thickness (0.1ua). Hence base current = 2% emitter current.

How is it possible?

The centre layer is the base of the transistor. The base has very small size. Therefore, the size becomes very less, Therefore the ratio is 150:1.

how? explain