A 27 picosecond photodetector with a modulation doped AlxGa1-xAs/GaAs heterostructure

A sensitive ultrahigh speed photodetector with an AlxGa1-xAs/GaAs modulation doped heterostructure field effect transistor has been developed. With a gate length\geq 20\microm, a gate-drain spacing\geq 8\microm, and a gate-width of 450\microm, the detector-showed an impulse response with a rise time of 12psec and a full width at half maximum of 27psec. When tested by a GaAs diode laser (\lambda \simeq 8200Å), the detector showed an ac optical gain of 6.3. The observed optical gain is attributed to an increase of the high-mobility electrons. The response time is limited by the electron transit time which is determined by the built-in electric field rather than the applied field. This allows picosecond photodetection at extremely low bias voltages (IV). The detectors should find wide applications, such as in gigabit rate optical communication, monolithic optoelectronic integrated circuits, and measurements of the dynamic response of semiconductor lasers.