FAR-INFRARED PHOTOCONDUCTIVITY IN HIGH-PURITY EPITAXIAL GaAs

Extrinsic far-infrared photoconductivity has been observed at 4.2°K in high-purity n-type epitaxial layers of GaAs grown on Cr-doped semi-insulating GaAs substrates. Measurements of the responsivity and noise in the detection system at 300 Hz in a 1-Hz bandwidth yield an NEP of 1.2 × 10−11 W at 195 μ, 1.4 × 10−12 W at 337 μ and 6 × 10−11 W at 902 μ. The time constant of the detector has been determined to be shorter than 1 μsec using a Ge avalanche modulator to chop the incident radiation. A time constant of about 5 nsec has been measured using impact impurity ionization in the GaAs.