Global simulation of a gallium arsenide metal-semiconductor-metal photodetector for the conversion of optical signals into microwaves

The conversion of light into microwaves by a semiconductor photodetector, a process called photomixing, is studied using simulations. The photomixing process is presently used in the fiber-optic transmission of telephone signals. This study anticipates the use of photomixing in phased-array antennas to generate radar microwaves due to the transmission advantages gained through fiber-optics. Device and global simulators are developed for use as tools in the design of photomixer circuits, and to explore the internal mechanisms of photodetector operation. The photomixer circuit that is modeled consists of two parts: (1) a gallium arsenide metal-semiconductor-metal photodetector with trench electrodes; and (2) an embedding circuit that has a bias tee, a voltage source, and a device parasitic capacitance.;A device simulator is constructed to model electron and hole transport in the photodetector under conditions of illumination with modulated light. The device simulator is based on the physics of the drift-diffusion approximation of the Boltzmann transport equation, and includes six nonlinear first-order partial differential equations. The equations are discretized and solved numerically through a Newton-Raphson technique that calculates six state variables as a function of position and time. The simulation uses a current density boundary condition that is derived in this study from first principles regarding the semiclassical model. The device simulator characterizes photodetector performance, as measured by the optical responsivity and bandwidth. Results indicate that photodetector performance is affected by the mobility model, recombination time constant, voltage, light intensity, and device length.;A global simulator is developed to model the photomixer circuit. The global simulator integrates the device simulator with an efficient convolution that models the embedding circuit. The embedding circuit produces an impulse response that is characterized in the frequency domain as the impedance function. The impedance function is solved, discretized, and inverse fast Fourier transformed into the time domain to generate the discretized impulse response. Global simulations determine the effect of the embedding circuit on device performance, and the results indicate that the parasitic capacitance is significant. The global simulation achieves accuracy through the convolution and the new current density boundary condition. Efficiency is achieved through truncation of the impulse response sequence and through the extrapolation of the current in conjunction with a fixed-point iteration scheme for the discretized convolution.