This study investigates the effect of helium-implanted edge termination (ET) on GaN-on-GaN Schottky barrier diode (SBD) temperature sensors. Devices with and without ET of three different diameters are fabricated and characterized from 298 to 473 K. Based on the conventional calculation method, the results show that the sensitivity (Sm) of the device with ET is lower than that of the device without ET at the same given current with the same anode area. Technology Computer Aided Design (TCAD) simulation reveals that ET can restrict the current to a smaller area, resulting in a larger actual current density (JA) for devices with ET. After correcting (JA) in the sensitivity calculation, the two types of devices exhibit the same corrected sensitivity (Sc) values, which also follow the fundamental linearity theory of sensitivity versus the natural logarithm of JA with high sensitivity corresponding to the lower current density. This pseudo-degradation phenomenon of sensitivity after the ET process can serve as a reference for in situ temperature monitoring in the gallium nitride (GaN)-based integrated-circuit design.