Charge Transport in Semiconductor Detectors with Trapping and Detrapping

The transient response of surface barrier detectors fabricated from semi-insulating CdTe has been analyzed under the conditions of trapping and detrapping. An analysis of the response of these devices to alphas allows measurement of material parameters pertinent to gamma detection and also verifies a theoretical model of trapping and detrapping. Actual and theoretical pulses presented here have a fast rising component followed by a slow rising component, the latter being associated with the release of carriers from traps. Decreasing the electric field increases the rise time and relative amplitude of the slow rising component. However, under high field conditions when detrapping times are much larger than the transit times and comparable to or greater than the trapping times, the rise time of the slow component of the pulse is nearly independent of electric field and is proportional to exp ¿A/kT where ¿A is the depth of the trap. In addition, when the detrapping time is much larger than the transit time, decreasing the temperature increases the rise time of the slow component but does not change its relative amplitude. This analysis shows that although 100% collection efficiencies may be attained in conventional solid state detectors at elevated temperatures, this may not be the case at reduced temperature. Degradation in the performance of germanium spectrometers below 20°K may be explained by the trapping and detrapping of carriers.