<P> Electrons and holes diffuse into regions with lower concentrations of electrons and holes, much as ink diffuses into water until it is uniformly distributed . By definition, N - type semiconductor has an excess of free electrons compared to the P - type region, and P - type has an excess of holes compared to the N - type region . Therefore, when N - doped and P - doped pieces of semiconductor are placed together to form a junction, electrons migrate into the P - side and holes migrate into the N - side . Departure of an electron from the N - side to the P - side leaves a positive donor ion behind on the N - side, and likewise the hole leaves a negative acceptor ion on the P - side . </P> <P> Following transfer, the diffused electrons come into contact with holes on the P - side and are eliminated by recombination . Likewise for the diffused holes on the N - side . The net result is the diffused electrons and holes are gone, leaving behind the charged ions adjacent to the interface in a region with no mobile carriers (That's why it is called the depletion region; carriers are being depleted). The uncompensated ions are positive on the N side and negative on the P side . This creates an electric field that provides a force opposing the continued exchange of charge carriers . When the electric field is sufficient to arrest further transfer of holes and electrons, the depletion region has reached its equilibrium dimensions . Integrating the electric field across the depletion region determines what is called the built - in voltage (also called the junction voltage or barrier voltage or contact potential). </P> <Dl> <Dd> Mathematically speaking, charge transfer in semiconductor devices is due both to conduction driven by the electric field (drift) and by diffusion . For a P - type region, where holes conduct with electrical conductivity σ and diffuse with diffusion constant D, the net current density is given by </Dd> </Dl> <Dd> Mathematically speaking, charge transfer in semiconductor devices is due both to conduction driven by the electric field (drift) and by diffusion . For a P - type region, where holes conduct with electrical conductivity σ and diffuse with diffusion constant D, the net current density is given by </Dd>

What are the main components of depletion layer