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.