Information on the physical states of a semiconductor surface can be obtained by investigating the behavior of carriers distributed near the surface, e.g. by measuring the impedance of a diode which consists of two metal electrodes, one set near the surface and the other on the bulk of the semiconductor. It can be also obtained from the impedance measurements on a Schot-tky barrier diode fabricated by contacting an electrode directly with the surface. Similar information can be obtained from the MIS junction with a narrow air gap as an insulating layer, which is formed by allowing the electrode to approach the surface. In this case, the impedance of such assembly is similar to that of a diode with a thin insulating film on the semiconductor surface, or a MOS diode. Theories proposed by Lehovec et al. have been used to explain the characteristics of the semiconductor surface, but they are not always correct. A corrected version of their theories has been used to construct an equivalent circuit for the semiconductor surface. The experimental results could be satisfactorily explained on the basis of this equivalent circuit.
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