Transients In Stressed Ge : Ga Photoconductors Under High Background For Pacs

The photoconductors used in the integral field spectrometers of the PACS instrument onboard the Herschel space observatory consist of stressed gallium doped germanium crystals featuring cut-off wavelengths of 127 mu m and 205 mu m. The measured transient responses of these Ge:Ga photoconductors to a step change in the incident photon flux level as well as a test setup that allows creation of transients by different methods are presented in this paper. The transient response of extrinsic photoconductors is caused by charge carriers drifting or diffusing to a contact region and recombining. This limits the initial gain of the device. Because of potentially long time constants, the transient behavior presents a serious challenge to high-sensitivity, low-temperature extrinsic semiconductors. In particular at low IR photon fluxes it usually is impossible for the detector to reach steady-state behavior during a reasonable observation time.However, since the time constants depend on the inverse photon flux, theory suggests the transient times for the high thermal background levels anticipated for PACS to be of the order of tens of milliseconds. Experimentally we find the response time to be limited by the transition time between the different infrared fluxes. The experimental studies on the transients are accompanied by numerical calculations. The results support the prediction that transients are not expected to play a major role for the low signal regime in PACS.