Environmental Engineering Reference
In-Depth Information
1.4.1
Basic Operation
To understand the basic operation of a bipolar transistor let us consider
the simplified scheme in Fig. 1.17 where the emitter terminal is connected to
ground. The base-emitter junction acts as a diode and a current flows if the
junction is forward biased. In such a situation, that is a current of
majority carriers (holes in this case) flows from the base terminal across the
base-emitter junction. Meantime, a current of electrons flows from the
emitter across the base-emitter junction and enters the base thus diffusing
towards the base-collector junction. Due to the different doping levels
electrons that diffuse into the base are much more than just holes that diffuse
into the emitter [1]-[3].
If is larger than 0.2-0.3 V, the excess of electrons in the base is subject
to a negative electric field imposed by the collector voltage. When those
electrons appear at the base-collector junction, they are pushed into the
collector region. Since the base width, is small, electrons coming from
the emitter do not have the possibility to recombine with holes in the base
and almost all are pushed into the collector.
In such a situation, the small base current is mainly determined by holes
while electrons coming from the emitter mainly determine the large collector
current. Consequently, the emitter current is the sum of those two
contributions. Under this condition the transistor is said to operate in the
active region.
The collector current, is caused by the base-emitter voltage and, as for
a diode, it has an exponential relationship that is
