bipolar transistor

    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.

   Contrast field effect transistor.

   (1995-10-04)