Monday, 8 April 2013

Basic NPN and PNP Transistor Operation

For correct operation if a transistor, the two pn junctions must be correctly biased with  the externally applied dc voltages.

Operation of the pnp transistor is same as that of a npn transistor, but the change is that the  roles of electrons and holes, bias polarities, and current directions are all reversed.

The figure shows the correct biasing of a BJT circuit.

The base-emitter (BE) junction should be forward biased and the base-collector (BC) junction is reverse biased for the proper working of the BJT..

Due to the forward bias condition, base to emitter narrows the BE depletion region. The reverse bias voltage applied from base to collector will leads to the widening of  the BC depletion region. The heavily doped n¬-type emitter region is packed with conduction-band (free) electrons and it will emit electrons when the emitter-base junction is forward biased. The free electrons from the emitter diffuse through the forward biased BE junction into the base region (p-type). The electrons become minority carriers (like in a forward biased diode) in the base region. Since the base region is lightly doped and very thin, it has a limited number of holes only. Only a small percentage of all the electrons flowing through the BE junction can combine with the available holes in the base region because of that light doping. A small base electron current is formed due to these relatively few recombined electrons flow out of the base lead as valence electrons. Most of the electrons flowing from the emitter to the base region will not recombine. These electrons will diffuse into the BC depletion region.

These electrons are pulled through the reverse-biased BC junction by the electric field set up. It is mainly due to the force of attraction between the negative and positive ions.

Electrons can move through the collector region, out through the collector lead, and into the positive terminal of the collector voltage source due to the externally applied voltage.

The collector electron current is thus formed. It is clear that the collector current (Iccr) is much larger (higher) than the base current.

Due to this reason, transistors exhibit current gain.

It is clear that the emitter current is higher than the base and collector currents. So the emitter current will constitute the base and the collector currents.

Hence,

IEcr = ICcr + IBcr

The Capital letters shows the dc values of current.

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