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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 μm, 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 carriers. 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 μm agree fairly well with the theoretical predictions.

GaAs metal-semiconductor-metal photodiodes fabricated on GaAs grown at low substrate temperatures (200 °C) have been investigated in the time domain by electro-optic sampling. It could be shown that these diodes have a faster response, a considerably reduced long time tail, and can be used at larger bias than comparable diodes produced on GaAs grown at 700 °C. Temperature-dependent measurements show that the tail can be described by hopping conductivity and disappears below 50 K.

Abstruct-The impulse response of interdigitated metalsemiconductor-metal photodetectors fabricated on an Fe-doped InGaAs absorbing layer and an Fe-doped InP barrier enhancement layer is investigated. For ultra-short pulse excitation of 150 fs at .h = G 2 0 nm the photoresponse is found to be less than 13 ps FWHM for detectors with 1 pm finger spacing. Above a certain level of illumination, the peak amplitude increases sublinearly and the relative contribution of the tail to the detector response is appreciably enhanced. The screening of the electric field by photo-generated space charges is responsible for this nonlinearity. For detectors with 5 pm finger spacing illuminated with 1.3 pm light pulses (FWHM = 33 ps), space charge perturbation of the impulse response manifests itself by a decrease of the FWHM and an increase of the fall time with increasing illumination level. The practical consequences for the performance of MSM detectors in various applications are discussed.

Metal-semiconductor-metal photodiodes were optimized as pulse generators for time-resolved electrical measurements at low temperatures and high magnetic fields, using a laser diode as the light source. For the photoconductor, a heterostructure consisting of high-quality GaAs and low-temperature grown GaAs was used. The dependence of the photocurrent pulses on the applied magnetic field was determined. A two-dimensional electron gas in a magnetic field served as the device under test for the time-resolved measurements. We observed a splitting of the incident pulse into two transmitted pulses resulting from modal dispersion in the two-dimensional electron gas.