Why, in the absence of an external electric field, the electrons do not pass through the p – n junction from the n-semiconductor to the n-semiconductor?
In a semiconductor of the p-type, which is obtained by means of an acceptor impurity, the concentration of holes is much higher than the concentration of electrons. In an n-type semiconductor, which is obtained by means of a donor impurity, the concentration of electrons is much higher than the concentration of holes. If contact is established between two such semiconductors, then a diffusion current will arise – the main charge carriers (electrons and holes) randomly flow from the region where there are more of them to the region where there are fewer of them and recombine with each other. As a consequence, there will be practically no free (mobile) majority charge carriers near the boundary between the regions, but impurity ions with uncompensated charges will remain. The region in the p-type semiconductor, which is adjacent to the boundary, receives a negative charge carried by the electrons, and the boundary region in the n-type semiconductor receives a positive charge carried by the holes (more precisely, it loses the negative charge carried by the electrons).
Thus, at the semiconductor boundary, two layers with space charges of opposite sign are formed, which generate an electric field in the transition. This field induces a drift current in the opposite direction to the diffusion current. In the end, a dynamic equilibrium is established between the diffusion and drift currents, and the change in space charges stops. Depleted regions with stationary space charges are called the p – n-transition.
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