A temperature sensor has a first transistor with a gate voltage tied to maintain the first transistor in an off state with leakage current flowing through the transistor, the leakage current varying with temperature. A second transistor is coupled to the first transistor and receives a gate voltage to keep the second transistor in an on state. A current mirror mirrors the leakage current and supplies a mirrored current used to control a frequency of an oscillator signal varies with the mirrored current. The temperature of the first transistor is determined based the frequency of the oscillator signal.