The Bipolar Junction Transistor is
more difficult to understand. It also consists of several
layers of semiconducting materials creating two p-n
junctions. Unlike the FET, the BJT does not conduct if no
voltage is applied across the base-emitter junction. The
device is designed to be controlled by biasing the
base-emitter junction. Applying a positive voltage to this
junction biases the base-emitter so that it turns on. The
conduction of the base-emitter diode creates a favorable
condition in the base so that current can flow from the
collector to the emitter. The amount of current flowing is
very sensitive to the biasing of this junction. If the base
region were not there the device would consist of a single
doped semiconducting material through which current would
flow easily. The presence of the base region sandwiched
between a small emitter and a larger collector allows the
current to be controlled through the base-emitter junction
with a very small signal. Opposite to the depletion-mode
FET (but like the enhancement mode devices) the current
flowing through the BJT flows more and more as the voltage
across the base-emitter voltage is INCREASED.
