Video Transcript
The base of a transistor is
blank. (A) Heavily doped. (B) Moderately doped. (C) Lightly doped. (D) There is not enough information
to determine the answer.
This question is asking us about
the doping of the base of a transistor. Recall that transistors consist of
three different parts: a collector, a base, and an emitter. When we draw these components for
an npn transistor, it would look something like this. Regardless of transistor type,
either npn or pnp, the doping present in the semiconductors that make it will be the
same.
In our npn transistor, the
collector and emitter will be much more strongly doped than the base. This is to say, the collector and
emitter will have more impurities in their construction, which will result in a
higher electrical conductivity. The base is much more weakly doped
compared to the collector and emitter, meaning that electrical charge will have a
more difficult time passing through it.
Also, for real transistors, not the
diagrams we tend to use, the base is very thin. This thinness, combined with a low
doping, helps minimize the chances for recombination of charge carriers in the
transistor, which would otherwise increase the base current. Ideally, we want the base current
to be much lower than the collector or emitter currents. Therefore, the base of a transistor
is lightly doped. The correct answer here is (C).
