It is an interference between two semiconductor materials inside a semiconductor.
There are holes and electrons present in a semiconductor. Holes are the absence of electrons.
Reverse biasing:
In a p-n junction, when p-type is connected to negative terminal of battery and n-type is connected to positive terminal the current is reverse biased.
A reverse bias strengthens the potential barrier and therefore obstructs the flow of charge carriers. So, the current flow is very low.