What is meant by pn diode and how depletion layer is formed between the pn junction ?
In a pn junction, without an external applied voltage, an equilibrium condition is reached in which a potential difference is formed across the junction. This potential difference is called built-in potential Vbi.
After joining p-type and n-type semiconductors, electrons near the pn interface tend to diffuse into the p region. As electrons diffuse, they leave positively charged ions (donors) in the n region. Likewise, holes near the pn interface begin to diffuse into the n-type region, leaving fixed ions (acceptors) with negative charge. The regions nearby the pn interfaces lose their neutrality and become charged, forming the space charge region or depletion layer (see figure A).
The electric field created by the space charge region opposes the diffusion process for both electrons and holes. There are two concurrent phenomena: the diffusion process that tends to generate more space charge, and the electric field generated by the space charge that tends to counteract the diffusion. The carrier concentration profile at equilibrium is shown in figure A with blue and red lines. Also shown are the two counterbalancing phenomena that establish equilibrium.