Semiconductor temperature sensors have been widely used in medical, industrial, aviation and civil fields due to their advantages such as high sensitivity, small size, low power consumption and strong anti-interference ability. However, most Si-based temperature sensors are not suitable for the application in high-temperature environments. The new AlGaN/GaN heterojunction material not only has a wide band gap, but also has a high two-dimensional electron gas concentration and carrier mobility. Therefore, the device made with it not only has good electrical properties, but also can be applied in ultra-high environments. In this paper, a temperature sensor based on gateless AlGaN/GaN high electron mobility transistor structure was fabricated and its temperature-dependent electrical properties were characterized. The temperature dependence of current-voltage characteristics of the device were tested from 50 to 400 °C. The sensitivity of the device was studied as a function of the channel aspect ratio of the device. The stability of electrical properties was characterized after heating in air and nitrogen at 300—500 °C for 1 hour. The theoretical and experimental results show that as the aspect ratio of the device increases, the sensitivity of the device increases. At a fixed current of 0.01 A, the average sensitivity of the device voltage with temperature changes is 44.5 mV/°C. Meanwhile, the good high temperature retention stability is shown during stability experiments.