PN Junction

Formation of PN Junction

   

Introduction to formation of PN junctions

The joining of n-type and p-type semiconductor materials form PN junction. Based on the fact that the p-type region has a high concentration of holes and the n-type region has a high concentration of electrons, electrons diffuse side to the p-type side from the n-type. Likewise, the flow of holes by diffusion occur from the p-type to the n-type side. In the case that the electrons and holes not being charged, the process of diffusion would continue till the electrons and holes concentration is the same on the two sides, like what happens when two gasses come in contact together. Yet, in the pn junction, after the movement of electrons and holes to the junction’s other side, exposed charges are left behind on sites of dopant atom, that are not able to move and fixed in the crystal lattice.   Positive ion cores are exposed in the n-type side while negative ion cores are exposed on the p-type side. E (electric field) forms between the n-type material’s positive ion cores  and the p-type material ‘s negative ion cores. The depletion region is the name of this region due to the electric field quickly sweeping out free carriers, leading to the depletion of free carriers in the region. Due to E, V_bi (built-in potential) is formed at the junction.

Carrier Movement in Equilibrium

An equilibrium state between carrier generation, diffusion, recombination and drift in the existence of electric field in the depletion region is represented by the pn junction which has no external inputs. Some carriers can still cross the junction by diffusion, in spite of the existence of the electric field that forms the impairment to diffusion of carriers across the electric field. But statistically, some carriers will possess a high velocity and travel in a net direction appropriate such for crossing the junction. After a majority carrier move across the junction, it turns into a minority carrier. The carrier will continue diffusing away from the junction and can move a distance equal to the diffusion length on average before it recombines. The diffusion current is the current caused by the carriers diffusion across the junction. Note that in a real pn junction the velocity and number of the carriers is much bigger and the number are much larger for carriers crossing the junction. Minority carriers that reach the diffusion region’s edge are swept across the region by the depletion region’s electric field. The drift current is the current causing this. The drift current in equilibrium is restricted by the number of minority carriers that are generated thermally within a junction’s diffusion length. The device’s net current is equal to zero in equilibrium. The electron diffusion current and the electron drift current balance out exactly (otherwise a net buildup of electrons would happen on one side or the other of the device). Likely, the hole drift current and the hole diffusion current balance out each other as well.