HIGH-SPEED AL0.2GA0.8AS GAAS MULTI-QUANTUM-WELL PHOTOTRANSISTORS WITHTUNABLE SPECTRAL RESPONSE/

We have measured the high-frequency characteristics and temporal respo nse of a GaAs/AlGaAs heterojunction phototransistor with a GaAs/Al0.2G a0.8As multi-quantum-well collector. The quantum wells offer tunabilit y of the photoresponse (10 nm for a bias change of 4 V) and a negative differential resistance in the photocurrent-voltage characteristics. Measured values of f(T) and f(max) are 20 and 6 GHz, respectively. The temporal response to short-pulse optical excitation is characterized by a linewidth of 46 ps. Such devices are attractive candidates for ma king optically induced oscillators.