Gallium Arsenide Metal-Semiconductor

The physical properties of GaAs and InGaAs metal -semiconductor-metal (MSM) photodetectors are investigated experimentally and theoretically. The studies focus on three fundamental characteristics of these devices: (1) dark current, (2) speed performance, and (3) responsivity. A quanum-mechanical model is developed to describe the dark current of the photodetector. It is shown that a thermionic emission expression can be derived from this model. There is good agreement between the theoretical model and experimental data obtained for detectors fabricated in the laboratory. The optimum metal barrier height leading to the lowest dark current is approximately one-half the semiconductor bandgap energy. An experimental photodetector with islands which shows an enhanced photocurrent is fabricated and characterized. Application of small Au islands is shown to increase the photocurrent and responsivity by more than an order of magnitude in some cases. Based on findings of this investigation, a high frequency photoconductive device is proposed. Photodetectors with 0.1-1 μm finger spacing are simulated. The intrinsic device is first analyzed and output current versus time is obtained. Monolithic and discrete parasitic circuits are then introduced to more closely simulate realistic devices. The output voltage versus time for these devices is analyzed. The parasitic circuit degrades speed and responsivity of the detector. The interdigitated electrode pattern leads to a physical speed limit of about 150 GHz for a 0.1 μm monolithic detector. The frequency and responsivity decrease with finger separation and the monolithic version has better performance than the discrete version. There is good agreement between simulated results and experimental data for a 0.75 μm detector.