Steady-State Response of Silicon Radiation Detectors of the Diffused P-N Junction Type to X rays. II: Photodiode Mode...

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, I , 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, I _g 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 I 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 R . There are however limitations in increasing V and R . because of the increasing noise with increasing dark current and some dependence of measured current signals on R . The temperature coefficient of I is positive and independent of R , 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 I .