Magnetic field sensors based on the Hall-effect have a variety of applications such as current sensing in power electronics and position and velocity sensing in vehicles. Additionally, they have benefits such as easy integration into circuits, low manufacturing cost, and linearity over a wide range of magnetic fields. However, in order to use these devices in an industrial or automotive setting, the effect of high temperatures on the reliability of the Hall-effect sensors needs to be evaluated. This study focuses on the effect of high temperature on the electrical and material properties of novel gallium nitride (GaN)-based Hall-effect sensors and the impacts on the reliability of these devices. Changes in device properties such as resistance and electrical response, as well as on the metallic contacts, are examined, using two sets of devices made with different substrates and contact metals. A probe station is used to characterize electrical responses, while an X-ray photo-electron spectrometer (XPS) and energy-dispersive X-ray (EDX) are used to characterize material interactions. The findings include saturation curves, the presence of gallium on the contacts of the octagonal device, and the activation energy of reaction responsible for resistance increase for the octagonal AlGaN/GaN devices. Additionally, the Greek cross AlGaN/GaN Hall sensors showed excellent thermal stability.