the semi conductor diode is formed by simply bringing n and p tripe material together (constructed from same base ge and si), at the instant the two material are joined the electrons and holes in the region and functions will combine resulting in lack of carriers in the reason of junctions. The reason of uncover positive and negative ions is called the depletion reasons due to the depletion of carriers in this region.
In absence of applied bias voltage the net flow of charge in any directions for the semi conductor diode is zero, hence no current is flow.
If an external voltage v is applied is the p-n junctions such that positive terminal to n type and negative terminal to p type material , then the depletions layer will increased and only reverse leakage current minority carries will flow.
A forward condition is establish by Appling the positive potential p type material and negative potential to n type material and holes in the p toe material with the ions near the boundary and reduce the width the depletion regions. As the applied bias increase in magnitude, the depletions regions will continue to decrease in Edith until a flood of electrons can pass through the junctions, resulting in an exponential rise in the current in the forward biased regions of characteristics.