<P> After joining p - type and n - type semiconductors, electrons from the n region near the p--n interface tend to diffuse into the p region leaving behind positively charged ions in the n region and being recombined with holes, forming negatively charged ions in the p region . Likewise, holes from the p - type region near the p--n interface begin to diffuse into the n - type region, leaving behind negatively charged ions in the p region and recombining with electrons, forming positive ions in the n region . The regions near the p--n interface lose their neutrality and most of their mobile carriers, forming the space charge region or depletion layer (see figure A). </P> <P> 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 . </P> <P> The space charge region is a zone with a net charge provided by the fixed ions (donors or acceptors) that have been left uncovered by majority carrier diffusion . When equilibrium is reached, the charge density is approximated by the displayed step function . In fact, since the y - axis of figure A is log - scale, the region is almost completely depleted of majority carriers (leaving a charge density equal to the net doping level), and the edge between the space charge region and the neutral region is quite sharp (see figure B, Q (x) graph). The space charge region has the same magnitude of charge on both sides of the p--n interfaces, thus it extends farther on the less doped side in this example (the n side in figures A and B). </P> <P> In forward bias, the p - type is connected with the positive terminal and the n - type is connected with the negative terminal . </P>

Explain the operation of pn junction diode in forward bias and reverse bias