Turn-on, turn-off, and storage delay:

The Bipolar transistor shows a few delay characteristics while turning on and off. Most of the transistors, and particularly power transistors, show long base storage time that limits maximum frequency of operation in switching applications. One technique for reducing this storage time is using a Baker clamp.

Transistor 'alpha' and 'beta':

The proportion of electrons capable to cross the base and reach the collector is a measure of the BJT (bipolar junction transistor) efficiency. The heavy doping of the emitter region and light doping of the base region origin several electrons to be injected from the emitter into the base than as compared to the holes to be injected from the base into the emitter. The common-emitter current gain is presented by β_F or h_fe; it is approximately the ratio of the DC collector current to the DC (direct current) base current in forward-active region. It is generally greater than 100 for small-signal transistors but can be smaller in transistors intended for high-power applications. An additional significant parameter is the common-base current gain, α_F. The common-base current gain is almost the gain of current from emitter to collector in the forward-active region. This ratio generally has a value that is close to unity; approximately between the 0.98 and 0.998. Alpha and beta are more precisely related through the following identities (NPN transistor):

α_F = I_c/I_E

β_F = I_c/I_B

β_F = α_F / (1- α_F) ↔ α_F = β_F / (β_F +1)

Posted Date: 1/10/2013 6:38:25 AM | Location : United States

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