Source : Free On-Line Dictionary of Computing

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)