Ge:Ga far-infrared photoconductors for space applications

We have fabricated 0.5 x 0.5 x 0.5-mm(3) Ge:Ga photoconductors for the [OI] 63-mu m line channel of the Far-Infrared Line Mapper (FILM) on the Infrared Telescope in Space (IRTS), These photoconductors have a high responsivity of 10-20 A/W (eta G of 0.13-0.25) and a good noise equivalent power (NEP) of about 2 x 10(-17) W/root Hz for step change in photon influx at a bias field of 1.8 V/cm and operational temperature of 2 K under a low background photon influx of 1.1 to 2.2 x 10(6) photons/s. The photoconductors show slow transient response with a time constant of about 10 s at low temperature under low background conditions, but they have almost no hook response. The responsivity to chopped light decreases as the chopping frequency increases from 7.5 to 210 Hz. We infer that this is due to what we call the ''moderately fast response''. Two time constants, tau(a) similar to 10 s and tau(m) similar to 0.23 s, derived from the two-region model of a Ge:Ga photoconductor, correspond to the slow transient response and the ''moderately fast response'', respectively.