What is p-n junction ?

Answer 1

a p-n junction is a term in solid state electronics.
a diode is the classic p-n junction where two different elements are joined together to form a solid piece.
electricity will flow through the junction in one direction only and is blocked the other way.
this is used in rectification, allowing only the positive half cycle to pass through and blocking the negative half cycle. four diodes placed in a diamond shape will provide full rectification and by adding a capacitor across the output a smooth dc supply can be achieved. the transistor goes one better by adding another junction, p-n-p or n-p-n.

Answer 2

Read: http://en.wikipedia.org/wiki/P-n_junction