@Prajakta Kelapure • 16 Sep, 2014
Transistor current components (Very very Important)

1. Current conduction in N-P-N transistor is because of electrons and that in P-N-P is due to holes. The conventional flow of current is in the opposite direction.

2. The current flowing through the emitter is called as the emitter current (IE).this emitter current is composed of two components one because of holes IhE and theother because of electrons IeE

IE = IhE + IeE

3. Since the emitter is heavily doped and the base is lightly doped thus the emitter current because of electrons is so very small as compared to the hole current.

4. All the holes crossing the emitter junction do not reach the collector as some of the recombine in the base region.

5. Ihc is the hole current in the collector region. Now some electrons recombine in the base region so this current Ihc is smaller than the IhE . the difference is called as the base current Ib.

6. If the emitter were open circuited IE would have been zero, and Ihc would also be zero. Thus the base collector would act as the reverse biased diode and the collector current would be equal to reverse saturation current ICB0 or IC0.

7. Thus when emitter is closed then

IC= Ihc+IC0

8. In the active mode of operation the collector current is given as :

IC=α IE+IC0 , where α is the large signal current gain.

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