<electronics> A transistor made from a sandwich of n- and
p-type semiconductor material: either npn or pnp. The
middle section is known as the "base" and the other two as the
"collector" and "emitter". When used as an amplifying
element, the base to emitter junction is in a "forward-biased"
(conducting) condition, and the base to collector junction is
"reverse-biased" or non-conducting. Small changes in the base
to emitter current (the input signal) cause either holes
(for pnp devices) or free electrons (for npn) to enter the
base from the emitter. The attracting voltage of the
collector causes the majority of these charges to cross into
and be collected by the collector, resulting in amplification.
