Low temperature growth of GaAs (LT-GaAs) near 200 C results in a recombination lifetime of nearly 1 ps, compared with approximately 1 ns for regular temperature~ 600 C grown GaAs (RT-GaAs), making it suitable for ultra high speed detection applications. However, LT-GaAs detectors usually suffer from low responsivity due to low carrier mobility. Here we report electro-optic sampling time response measurements of a detector that employs an AlGaAs heterojunction, a thin layer of LT-GaAs, a channel of RT-GaAs, and a vertical electric field that together facilitate collection of optically generated electrons while suppressing collection of lower mobility holes. Consequently, these devices have detection efficiency near that of RT-GaAs yet provide pulse widths nearly an order of magnitude faster—~ 6 ps for a cathode-anode separation of 1.3 μm and~ 12 ps for distances more than 3 μm. View Full-Text