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 V_B whose value is 0.7 V

For Silicon and 0.3 V for Ge. As soon as V_B 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.