Applying heat to a transistor
A transistor essentially lets through as much current (that's charge per time) as it has charge carriers available in a specially crafted thin zone. 

The number of these carriers can, depending on the type of transistor, be changed by running a current through the base of the transistor, or by applying a potential between the gate and one of its other terminals.

But: Charge carriers also form spontaneously in semiconductors when the temperature is above absolute zero, and the speed at which that happens increases very much with the temperature. Thus, you can make a transistor act very well as a temperature sensor.