IMPEDANCE STUDIES AT SEMICONDUCTOR ELECTRODES - CLASSICAL AND MORE EXOTIC TECHNIQUES

Unique characteristics of semiconductor electrodes are their large photosensitivity and the fact that an applied potential can penetrate quite sizeably into the electrode material (depletion layer ∼ 1 μm). These peculiar features have favored the development of many specific impedance or transient techniques. The classical electric impedance is mostly dominated by the capacitance of the depletion layer in the semiconductor, allowing for the determination of the flatband potential. More complex behaviors are often associated with the occurrence of surface states in the forbidden gap. Opto-electric impedance techniques include photocurrent and photopotential response to a modulated illumination. They provide additional information on the interface kinetics. Electro-optic impedance techniques include absorption and luminescence response to a potential modulation. Infrared absorption response provides detailed information on the nature of the charge being diplaced upon potential modulation (free carriers or surface states). The concentration of free carriers near the interface may also be probed by the surfrace conductivity, measured at optical, microwave or d.c. frequencies. These "parallel" probes can be used in response to a light modulation or a potential modulation. Since they give a direct measure of the charge (and not the current) they may be especially interesting at the lower frequencies. © 1990.