Temperature Dependence of Photocurrents Produced by X and Gamma Rays in Silicon Radiation Detectors

The temperature dependence of dc photocurrents produced by x and gamma rays in silicon radiation detectors of the n-p, p-i-n, and surface-barrier type was investigated in a temperature range between 20 and 50 °C. Photodiode photocurrents, assumed as being equal to the generated currents I _g, showed a positive temperature dependence in all detectors investigated. Their temperature coefficient at 25 °C varied between +0.004 per °C and + 0.002 per °C. The temperature dependence of short-circuit currents I _sc measured by a compensation method, was positive and nearly linear for n-p type detectors but nonlinear and negative for p-i-n and surface-barrier type detectors. It is shown, that this different behavior of individual detectors is due to the influence of the strongly temperature-dependent junction current I which under the short-circuit mode is drained off the generated current I . The junction current is a function of the internal series resistance R and the junction resistance R of the irradiated detector (I = I R /R ). With increasing resistance ratio R /R , the current ratio I /I increases and the temperature coefficient α_sc of the short-circuit current decreases. Temperature coefficients (α_sc)₂₅ measured in the different detectors at 25 °C and a current density 6 × 10^-7 A/cm² decreased with increasing resistance ratio from + 0.004 per °C to - 0.005 per °C. Resistance ratios R /R of the detectors investigated ranged between 0.01 and 0.24 approximately. Thus, (α_sc)₂₅ measured in an individual detector can be changed by changing its effective series resistance. A decrease of (α_sc)₂₅ with increasing I _sc was observed in detectors with larger resistance ratios. This was apparently due to the voltage dependence of R at higher junction voltages produced by larger short-circuit currents.