Characterizing the sensitivity of 2DEG-based magnetic field and ultraviolet light sensors in space-simulant environments
Keywords: Aeronautics and Astronautics.

Instruments and vehicles used in space experience a wide range of extreme environments, including high and low pressures, hypervelocity impacts from micro-meteorites, radiation exposure, and high and low temperatures. A major challenge in space exploration is that the electronics used in satellites and space vehicles are often made of silicon; however, silicon-based electronics tend to fail at temperatures above 200°C. In order to operate in the high-temperature environments of outer space, these electronics often require external cooling mechanisms, thus adding further bulk, complexity, and cost to the system. Gallium nitride (GaN) has a much wider temperature range than silicon (up to 1000°C) and has also shown to be more radiation-hardened, making it a viable platform for robust space-grade electronics. In this thesis, I discuss the design, testing, and implementation of two different GaN-based sensors: a Hall-effect (magnetic field) sensor and a UV photodetector. The first part of this thesis focuses on how changing the geometry of the Hall-effect sensor affects its sensitivity, offset, and noise behavior. The experimental results show that the octagonal AlGaN/GaN and InAlN/GaN Hall plates follow the same behavior trends as theorized in the literature for silicon Hall plates: devices with the shortest contacts have the highest current-scaled sensitivity, while devices shaped as regular octagons have the highest voltage-scaled sensitivity. Low frequency noise is shown to increase with contact size, while at high frequency the dominant form of noise is thermal noise, for which an optimization on device shape is also described. After comprehensively characterizing GaN-based Hall-effect sensors in an ambient environment, they are evaluated in various space-simulant environments. The main focus is on high temperature environments; the sensitivity of AlGaN/GaN and InAlN/GaN devices is characterized between room temperature and 576°C. Both devices show decreasing voltage-scaled magnetic sensitivity at high temperatures, but little hysteresis over 2-3 thermal cycles and nearly full recovery of initial sensitivity at room temperature. Additionally, current-scaled sensitivities remain stable over the temperature range, due to the minimal temperature dependence of the electron sheet density on the 2-dimensional electron gas (2DEG). Stability at high temperature is further exhibited through long-term high temperature storage tests as well as a 10-day exposure to a Venus-analogue environment (460°C, 96.5 bar, CO2 atmosphere). The last section of this thesis details the characterization of an AlGaN/GaN photodetector in a high temperature environment (up to 250°C) and discusses the reasons for its dramatic drop in responsivity at this high temperature. The photodetector is then implemented for combustion monitoring in two different hybrid rocket motor applications. In addition to successfully detecting the duration of the combustion, the measurements from the photodetector vary with oxygen-to-fuel ratio for a hybrid rocket motor igniter plume. Additionally, the measurements from the photodetector are used to estimate the flame temperature for the igniter plume as well as in the center of a solid transparent hybrid rocket motor fuel grain. The results from thorough testing of GaN-based Hall-effect sensors and photodetectors suggest that this material platform is a good candidate for use in outer space and terrestrial harsh environments. Further, by demonstrating the functionality of these sensors on space systems, we contributed to increasing the technology readiness level of GaN-based sensors and pushing them one step closer towards industrial use