PICOSECOND PULSE RESPONSE CHARACTERISTICS OF GAAS METAL-SEMICONDUCTOR-METAL PHOTODETECTORS

We present a comprehensive theoretical and experimental analysis of the current response of GaAs metal-semiconductor-metal Schottky photodiodes exposed to 70 fs optical pulses. Theoretical simulations of the carrier transport in these structures by a self-consistent two-dimensional Monte Carlo calculation reveal the strong influence of the distance between the finger electrodes, the external voltage, the GaAs layer thickness and the excitation intensity on the response time and the corresponding frequency bandwidth of these photodetectors. For many experimental conditions, the model demonstrates a clear temporal separation of the electron and hole contributions to the output current due to the different mobilities of the two carrier types. For a diode with an electrode separation of 0.5-mu-m, an electric-field strength above 10 kV/cm and low intensity of the incident light the theory predicts a pulse rise time below 2 ps, an initial rapid decay as short as 5 ps associated with the electron sweep out and a subsequent slower tail attributed to the hole current. For weaker electric fields and/or higher light intensities a significant slowing down of the detector speed is predicted because of effective screening of the electric field by the photoexcited carrier. Heterostructure layer-based devices are shown to provide superior performance compared to diodes manufactured on bulk substrates. Experimental data obtained by photoconductive or electro-optic sampling on diodes with electrode separation between 0.5 and 1.2-mu-m agree fairly well with the theoretical predictions.