In this report the results of theoretical study of peculiarities of photocarrier transport in periodic semiconductor multilayer variable-gap structures are presented. It is investigated in detail the case of sawtooth-like photosensitive structure with linear coordinate dependence of energy gap within each layer. The stucture is supposed to be illuminated with polychromatic light which uniformly generates photocarriers. The problem is solved in quasi-neutral approximation on the basis of diffusion-drift equation complemented by appropriate boundary conditions being followed from the continuity of carrier concentrations and current densities at multilayer variable-gap structures interfaces.
Band-gap grading due to spatial variation of semiconductor composition gives rise to appearance of quasielectric built-in fields which significantly affect the photocarrier transport. The coordinate dependence of photocarrier concentration manifests the nonmonotonic shape with its maximum being reached in the regions of smallest band gap.The presence of quasielectric fields gives rise to reduction of the sweep-out effect and to increase of the photosensitivity at large values of bias voltage. The photoelectric amplification coefficient of such photoresistors at high voltages is shown to be much greater than that based on an homogeneous material.

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1. Modeling of an IR variable-gap photodiode with the trapezoid-like profile of energy band diagram