With the aid of a diagram, describe the operation of NPN transistor

 

The figure above shows a NPN transistor with a forward bias to the emitter base junction and reverse bias to the collector base junction. The forward bias causes the electrons in the N type emitter to flow towards the base. This constitutes the emitter current Ie. As these electrons flow through the P type base, they tend to combine with holes to become valence electrons. These valence electrons flow down through the holes and into the external base lead thus constituting base current Ib. Since the base is lightly doped and very thin, only a few electrons less than 5% combine with holes to form base current Ib. The remainder which is more than 95% cross over into the collector region to constitute collector current Ic. in this way it is concluded that almost the entire emitter current flows to the colloctor circuit.