In transistor, emitter-base junction is forward biased with low voltage battery and the collector base junction is reverse biased with high voltage battery. The current in forward biased circuit is measured in milli ampere while the current in the reverse biased circuit is measured using micro ampere.

If the forward bias voltage is made large, the majority carriers in the emitter will drift towards the collector through the base region with large velocity. Since the carriers number is also large, heat produced results in the breakage of covalent bond resulting in transistor getting spoilt completely. However, in case of reverse bias, even though voltage is high, it simply collects the carriers coming from emitter through the base. In this case, there is no heating effect and hence no breakage of bond in the transistor.