High-temperature electronic devices and sensors that operate in harsh environments, especially high-temperature environments, have attracted widespread attention. An Al2O3 based a-IGZO (amorphous indium-gallium-zinc-oxide) Schottky diode sensor is proposed. The diodes are tested at 21–400 °C, and the design and fabrication process of the Schottky diodes and the testing methods are introduced. Herein, a series of factors influencing diode performance are studied to obtain the relationship between diode ideal factor n, the barrier height ?B, and temperature. The sensitivity of the diode sensors is 0.81 mV/°C, 1.37 mV/°C, and 1.59 mV/°C when the forward current density of the diode is 1 × 10−5 A/cm2, 1 × 10−4 A/cm2, and 1 × 10−3 A/cm2, respectively.