plz define barrier potential.

In a PN junction the depletion layer is formed due to diffusion of holes and electrons. The diffusion causes the P type material to be negatively charged and the N type material to be positively charged. 

The electric field existing within the depletion layer due to the charges on the P and N blocks gives rise to a potential difference across the depletion layer. This potential difference is called depletion layer. For Si diode, barrier potential is between  0.5V to 0.7V and for germanium diodes it is 0.1V to 0.3V.