These diagrams show that the emitter current ( I E) is the sum of the collector current ( I C) and the base current ( I B), expressed as follow:Īs mentioned before, I B is very small compared to I E or I C. The capital-letter subscripts indicate dc values. Notice that the arrow on the emitter inside the transistor symbols points in the direction of conventional current. The directions of the currents in an NPN transistor and its schematic symbol are shown in above Fig these for a PNP transistor are shown in above Fig. The emitter current is slightly greater than the collector current because of the small base current that splits off from the total current injected into the base region from the emitter. The free electrons move through the collector region, into the external circuit, and then return into the emitter region along with the base current, as indicated. As the free electrons move toward the reverse-biased BC junction, they are swept across into the collector region by the attraction of the positive collector supply voltage. Most of the free electrons that have entered the base do not recombine with holes because the base is very thin. When the electrons that have recombined with holes as valence electrons leave the crystalline structure of the base, they become free electrons in the metallic base lead and produce the external base current. The base has a low density of holes, which are the majority carriers, as represented by the white circles.Ī small percentage of the total number of free electrons injected into the base region recombine with the holes and move as valence electrons through the base region and into the emitter region as hole current, indicated by the red arrows. 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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