Steady-state response of silicon radiation detectors of the diffused p-n junction type to x rays. I: Photovoltaic mode of operation

Abstract

The response to x rays of silicon radiation detectors of the p-n junction type was investigated with special consideration of their dependence on the applied voltage. In agreement with theory, the photocurrent, Ig, was found to consist of a voltage-independent part mainly determined by the average diffusion length of minority carriers in the base layer, and a voltage-dependent part which is proportional to the width of the depletion region, w. Due to the voltage dependence of w,Ig increases with increasing voltage applied, but its relative change produced by different voltages is independent of exposure rate and quality of radiation. Exposure rate and energy dependence of Ig expressed in relative values are thus independent of applied voltage. Silicon radiation detectors, used as photodiodes can therefore be useful for monitoring of radiations at exposure rates larger than 1 R/min, taking advantage of the possibility to increase current sensitivity by increasing the voltage V and to increase the voltage signal by increasing the load resistance RL. There are however limitations in increasing Vand RL because of the increasing noise with increasing dark current and some dependence of measured current signals on RL. The temperature coefficient of Ig is positive and independent of RL, but shows some small voltage dependence. In the temperature range between 25 and 50 ?C, the average temperature coefficient is approximately 0.35 percent per degree centigrade. A value of the average diffusion length of minority carriers in the base layer has been derived from the measured voltage dependence of Ig.