Explain physically how a p-n junction functions as a rectifier.
Ans: When a P-N junction diode is forward-biased and applied voltage is increased from zero, hardly any current flows through the device in beginning. It is so as external voltage is being opposed by the internal barrier voltage VB whose value is 0.7 V
For Silicon and 0.3 V for Ge. As soon as VB is neutralized, current through diode increases rapidly with increasing applied battery voltage. So, in forward bias condition, P-N junction diode behaves just like a closed switch. When a P-N junction diode is reverse biased, majority carriers are blocked and only a small current (because of minority carriers) flows through the diode. It's of the order of Nano amperes (nA) for Si and microamperes (µA) for Ge. For all practical purposes, this current is almost negligible. So, in Reverse bias condition, P-N junction diode behaves like an open switch. So, above discussion shows that the P-N junction functions as a rectifier.