For a transistor to works as an amplifier the two p-n junction must be correctly
biased with an external D.C
voltages. The base-emitter junction is
forward biased and the base-collector junction is reversed biased. Let us
consider a npn transistor, the operaion
for the npn and pnp is same except that the holes and electrons , the current
direction and the polarity of the applied biased voltage is reversed.
Looking at what goes on inside the n-p-n
transistor when it is forward biased or reversed biased. The forward biasing
from the base-emitter region lowers the narrows the depletion region and the
reversed biasing from base to collector widens the depletion layer.
In the
n-type region the majority of the carriers are conduction electrons which
diffuse across the base emitter junction into the p-type base region just as in
forward biased diode.
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