Basic Transistor Operation

– For correct operation, the two pn junctions must be correctly biased with external dc voltages.

– Operation of the pnp is similar as that of npn, but the roles of electrons and holes, bias polarities, and current directions are all reversed.

– The figure below shows the correct biasing of a BJT.

– Note the base-emitter (BE) junction is forward biased and the base-collector (BC) junction is reverse biased.

– The forward bias from base to emitter narrows the BE depletion region.

– The reverse bias from base to collector widens the BC depletion region.

– The heavily doped n­-type emitter region is packed with conduction-band (free) electrons.

– The free electrons from the emitter diffuse easily through the forward biased BE junction into the p-type base region

– In the base, the electrons become minority carriers (like in a forward biased diode).

– The base region is lightly doped and very thin, so it has a limited number of holes.

– Because of that light doping, only a small percentage of all the electrons flowing through the BE junction can combine with the available holes in the base.

– These relatively few recombined electrons flow out of the base lead as valence electrons, forming the small base electron current.

– Most of the electrons flowing from the emitter into the lightly doped base region do not recombine, but diffuse into the BC depletion region.

– Once here, they are pulled through the reverse-biased BC junction by the electric field set up by the force of attraction between the positive and negative ions.

– Electrons now move through the collector region, out through the collector lead, and into the positive terminal of the collector voltage source.

– This forms the collector electron current. The collector current is much larger than the base current.

– This is the reason transistors exhibit current gain.

– From graph above:

I_E = I_C + I_B

– Capital letters indicate dc values.