These free electrons easily diffuse through the forward-based BE junction into the lightly doped and very thin p-type base region, as indicated by the wide arrow. The heavily doped n-type emitter region has a very high density of conduction-band (free) electrons. To understand how a transistor operates, let’s examine what happens inside the NPN structure. This condition is called forward-reverse bias. Notice that in both cases the base-emitter (BE) junction is forward-biased and the base-collector (BC) junction is reverse-biased. See Also: OPM BiasingĪ bias arrangement for both NPN and PNP BJT for operation as an amplifier.
The operation of the PNP is the same as for the NPN except that the roles of the electrons and holes, the bias voltage polarities, and the current directions are all reversed. In this section, we mainly use the NPN transistor for illustration. In order for a BJT to operate properly as an amplifier, the two PN junctions must be correctly biased with external de voltages. How does a bipolar junction transistor work?
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