SIMPLE ANALYTICAL MODEL OF BIAS DEPENDENCE OF THE PHOTOCURRENT OF METAL-SEMICONDUCTOR-METAL PHOTODETECTORS

The current-voltage (I-V) characteristics of metal-semiconductor-metal (MSM) photodetectors under various light intensities are examined. Th e current shows an initial increase followed by saturation and a subse quent sharp increase as bias increases. We propose a theoretical model for bias dependence in all regions of operation except for breakdown, based on drift collection of carriers in the depleted regions under t he contacts and diffusion and recombination in the undepleted region. This is based on the solution of the diffusion equation in the undeple ted area between the two contacts of the MSM structure. The solution i s subject to boundary conditions on excess minority carriers at the ca thode end and continuity of current at the anode end. The latter is wr itten in terms of a parameter, denoted as effective diffusion length, which describes the collection efficiency of carriers at the anode. Th e closed-form solution thus derived corroborates with physical expecta tions in several limiting cases. To compare theory with experiment, we propose methods to extract parameters that are used to normalize the I-V curves and calculate depletion widths under different light intens ities, from current- and capacitance-voltage measurements. A close mat ch between experimental and theoretical results is observed, and possi ble breakdown mechanisms are discussed. (C) 1996 Optical Society of Am erica