Mumbai University > FE > Sem 1 > Basic Electrical and Electronics Engineering

Marks: 4 M

Year: May 2014

The emitter to base junction of the transistor is forward biased by applying a voltage VBB. ∴ Width of depletion region at this junction reduces. The supply voltage Vcc makes the collector to base junction reverse biased. This will increase the depletion region at the function. Due to forward biased emitter to base junction, electrons in the n-type emitter flow toward p-type base. This constitutes the emitter current IE. These electrons cross the emitter to base junction and enter into the thin and lightly doped base region, where they tend to combine with the holes. Holes also flow from p type base to n-type emitter. But as the base is more lightly doped than the emitter, almost all the current flowing across the emitter to base junction consists of electrons entering the base from the emitter. Therefore electrons are majority carries in an n-p-n transistor.

As the base is thin and lightly doped, it consists of the few holes. Therefore out of the injected electrons from the emitter, very few recombine with the holes in the base region. This constitutes the base current IB. The base current is therefore small as compared to the emitter current. The remaining large no. of electrons will cross the depletion region of the collector to base junction and pass through the collector region of the collector region to the positive end of external supply voltage Vcc. ∴ the collector current Ic is larger than the base current IB. Due to reverse biased collector junction the depletion region penetrates deeply into base region.