In a PN junction diode holes are the majority carriers on P side whereas electrons are the majority carriers on n-side. The process of diffusion takes place due to which the majority carriers diffuse from one region to the other so the P-region becomes less positive and the n-region becomes less negative.
An imaginary battery is developed across the junction which prevents further movement of majority carriers and the voltage so developed is known as the potential barrier. The layer is known as the depletion layer.
A PN junction Diode is said to be reverse biased when P-side is connected to the –Ve terminal and n-side is connected to the +Ve terminal of the battery.
In this biasing, the majority carriers are pulled away from the junction and hence no current flows through the junction due to majority carriers. The current due to minority carriers is present but it is of very small value of the order of mA.
This reverse current has a very small value at the start but it shoots up at a particular voltage known as the break down voltage. The cause of this rise is that the break down voltage, large no. of covalent bonds break up due to which a large number of charge carriers are generated (Avalanche Breakdown). At this breakdown there is an increase in the value of current as shown.
This is how the depletion layer or potential barrier is formed in the pn junction diode.