Stationary charge transport in metal-semiconductor-metal (MSM) structures

Abstract

Important physical processes affecting charge transport in the metal-semiconductor-metal (MSM) structure have been studied from the diffusion theory point of view, in combination with boundary conditions based on thermionic emission theory. At current densities exceeding about 0.1 A cm−2, the stationary unipolar charge transport of injected charge carriers, to which the problem reduces if thermal and avalanche generation of electron-hole pairs can be neglected, is described by the conduction current equation and Poisson's equation. Charge conduction through a semiconductor with nonuniform electric field and mobile-charge distribution is described using the low-field diffusion constant and field-dependent mobility. The current-dependent concentration of mobile charges at the M-S interfaces represent the boundary conditions. Physical processes in four MSM structures (PtSi-n Si–PtSi), differing in doping concentration (ND = 4.4 and 12 × 1014 cm−3) and semiconductor width (L = 4 and 10 μm), have been investigated numerically at two different temperatures (Tc = 300 and 423°K). At current densities exceeding about 5 A cm−2, the investigated structures can be regarded as trap free, if they have a semiconductor trap density of the order of 1012 cm−3 or lower. The diffusion and space charge of injected carriers significantly affect the electrical characteristics of the structures. Depending on the semiconductor temperature, donor concentration, and current density, the width of the diffusion-affected region of the semiconductor varies from 0.2 to 0.8 μm. At small current densities, the structure current increases exponentially with applied voltage; at current densities exceeding 1–5 A cm−2, this current increase is significantly reduced because of the space-charge effect of the mobile carriers. For still greater applied voltages, the I-V characteristic levels off almost completely. In comparison with the room-temperature characteristic, the high-temperature characteristic is shifted towards slightly lower voltages at small currents, while the large space-charge effect of the mobile carriers at high current levels can cause these I-V characteristics to intersect.