Forbidden zone of energy

Some surface States create near the middle of the forbidden zone of energy levels that are levels of recombination traps. Recombination traps can only be a fast surface States, since the transition time of carriers on a slow surface States is very large. The phenomenon of surface recombination is commonly characterized by the surface recombination speed of charge carriers, which is defined as the ratio of the flux density of charge carriers at the semiconductor surface to the excessive concentration of these carriers at the surface.

Thus, the rate of surface recombination shows how the number of charge carriers recombines the 1 s on the surface of the semiconductor area of 1 cm2. The dimension of the rate of surface recombination is the same as the dimension of speed, i.e., cm/s speed.

The density of surface States in General and of surface States, which are the recombination traps, in particular, depends on the surface treatment of the semiconductor and the properties of the external environment with which it is in contact. Since the density of surface States is usually large, the recombination of carriers at the surface is much faster (more intense) than in the volume of the semiconductor. At small dimensions of the semiconductor crystal, the phenomenon of surface recombination will significantly reduce the effective life time of charge carriers.

In addition, the density of surface States may change with time due to evaporation or condensation of moisture on the surface of the crystal, due to the possible migration of adsorbed impurities on the surface, etc. These processes, leading to changes in the effective life time of charge carriers, can be a cause of instability of parameters and characteristics of semiconductor devices.

Thus, in the manufacture of semiconductor devices it is necessary, first, to choose a method of surface treatment of semiconductor crystals in which the velocity of surface recombination is minimal, and, secondly, to find a method for long-term preservation of the achieved values of rate of surface recombination. The last task is usually solved by coating the crystal surface special coating and sealing device into the housing.