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UBC Theses and Dissertations

Self-aligned gallium arsenide MESFETs for microwave integrated circuits Sutherland, David B.


A refractory self-aligned gate fabrication process for gallium arsenide MESFETs has been developed and applied to a sample and hold circuit. The process has been shown to reduce the parasitic end resistance of MESFETs which can be a limiting factor in their microwave performance. A mask set was designed to be compatible with Cascade Inc. probes which allowed on chip microwave measurements to be made. Usable gain was measured up to 18GHz on FETs and 5GHz on buffer amplifiers with the microwave probes at the Communications Research Centre in Ottawa Ontario. The microwave probes were also used to test sample and hold operation. The maximum tested sampling rate was limited by the test equipment to 250 MHz. The fabrication process included a plasma etch for producing an undercut 'T' gate structure for self-aligned ion implantation. A method of sputtering a thermally stable alloy of TiW refractory metal was developed to provide suitable Schottky contacts to GaAs. It was found that a rapid thermal anneal following the self-aligned implant maintained suitable TiW/GaAs Schottky characteristics and yielded MESFETs with reduced end resistance when compared to those fabricated by the more conventional selective implant process. A technique was developed to reduce the gate resistance of self-aligned MESFETs using an evaporated metal overlayer. Also, procedures for fabricating airbridges using a single evaporation and Metal-Insulator-Metal (MIM) capacitors using silicon nitride as the dielectric were developed. The effect of gate resistance on the microwave performance of the self-aligned MESFETs was investigated by modeling with the EEsof Inc. microwave software package, Touchstone. The modeling showed that self-aligned MESFETs are capable of giving greater high frequency gain than are selective implant devices with the same design geometry. The operation of the sample and hold circuit was simulated using a version of SPICE that included the Sussman Fort GaAs MESFET model. The simulations showed that the sample and hold could be used for gigahertz sampling.

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