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Saturday, 11 March 2017

Semiconductor Electronics - III


Reverse Biasing of a pn-junction:
If the positive terminal of a battery is connected to the n-side and negative terminal to the p-side, then pn-junction is said to be reverse biased. The majority charge carriers move away from the junction. The potential barrier offers high resistance during the reverse bias. However, due to the minority charge carriers a small current, called reverse or leakage current flows in the opposite direction. Thus junction diode has almost a unidirectional flow of current.

Action of a transistor:
When the emitter-base junction of an npn-transistor is forward biased, the electrons are pushed towards the base. As the base region is very thin and lightly doped, most of the electrons cross over to the reverse biased collector. Since few electrons and holes always recombine in the base region, so the collector current  IC is always slightly less then emitter current IE.
IE = IC + IB
Where IB is the base current.

Three Configurations of a Transistor:

A transistor can be used in one of the following three configurations:
a) Common-base (CB) circuit.
b) Common-emitter (CE) circuit.
c) Common-collector (CC) circuit.

Current Gains of a Transistor:
Usually low current gains are defined:
a) Common base current amplification factor or ac current gain a :
It is the ratio of the small change in the collector current to the small change in the emitter current when the collector-base voltage is kept constant.
b) Common emitter current amplification factor or ac current gain b  :
It is the ratio of the small change in the collector current to the small change in the base current when the collector emitter voltage is kept constant.

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