Graduate Thesis Or Dissertation

Modeling and testing of semi-insulating gallium arsenide interdigitated photodetectors

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  • High speed photodetectors are a necessary element in broad band digital and analog optical communication systems. In this thesis easily integrable planar high speed photodetectors made on undoped semi-insulating (SI) GaAs substrates are modeled and tested. The fabrication process of the detectors is fully compatible with GaAs metal-semiconductor field effect transistor (MESFET) processing technology. Interdigitated fingers are used as the contacts to achieve both high sensitivity and large bandwidth. Detectors made with both ohmic and Schottky contacts are fabricated and tested. The equivalent circuit elements of the interdigitated structure are modeled using accurate lumped element circuit models associated with the various discontinuities of the structure. The results of the model agree well with the experimental results as well as with other published results. Numerical simulation of the SI-GaAs metal-semiconductor- metal (MSM) photodetector is performed. The carriers are tracked after an ideal optical pulse is applied and the intrinsic current as a function of time is computed. Then the influence of all the external circuit elements is included and the output current across the load resistor is computed. The simulated response is compared with other published models. The electrical and optical characteristics of the detectors are measured. For ohmic contact detectors, the dark current increases linearly with bias until some critical field is reached beyond which the dark current increases nonlinearly with bias. The time response of the detectors is measured with a 10 ps pulsed laser operating at - 600 nm and also with a pulsed GaAs /AlGaAs semiconductor laser operating at 850 nm. The ohmic and Schottky contact detectors have approximately the same rise time. The fall time of the Schottky contact detector is much smaller than the fall time of ohmic contact detector. The long fall time of the ohmic detector does not depend on the spacing between contacts. This long fall time is due to the large barrier that exists near the ohmic metal/SI-GaAs cathode contact. No such barrier exists for SI-GaAs MSM photodetector. The simulated impulse response of the SI-GaAs MSM photodetector is compared with the measured impulse response.
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