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We report for the first time on an aluminum nitride/gallium nitride (AlN/GaN) heterostructure as a microscale Hall effect sensor for current sensing applications in extreme environments. The AlN/GaN devices demonstrated high signal linearity as a function of the magnetic field across a temperature range from -193 °C to 407 °C. The measured room temperature (RT) supply voltage-related sensitivity ( ${S}_{\text {svrs}}{)}$ and supply current-related sensitivity ( ${S}_{\text {scrs}}{)}$ are 0.055 T-1 and 32 AV-1T-1, respectively. The supply power-related sensitivity ( ${S}_{\text {sprs}}{)}$ is 1.4 VW-1T-1 above 40-mW input bias, which is higher than that of the Al0.2Ga0.8N/GaN device. The designed AlN/GaN micro-Hall sensor is further determined to have a lower power consumption and higher temperature sensitivity than equivalent Al0.2Ga0.8N/GaN Hall devices. When operated in an ac bias mode, the rise time of the Hall sensor was found to be 102 ns, corresponding to a frequency bandwidth of 9.8 MHz. We also observed a phase shift between an applied magnetic field and the Hall sensor signal, which can potentially be helpful to monitor ac line currents.