Integrated circuit (IC) temperature sensors have been inadvertently part of device design since the beginning of the IC days. IC designers have gone through numerous contortions to minimize temperature effects in their chip systems. But the tables have turned. At one point, an IC designer had a brilliant idea to exploit the temperature behavior of an active circuit’s p-n junction rather than focusing on ways to minimize it. And far more brilliant were the designers who integrated the digital functionality into the same chip. Out of that has come the current class of temperature sensor ICs. The integrated temperature sensor can easily solve most of your temperature sensing woes if your problems live in the −55 to 200ºC temperature range.
At the input
At the temperature sensor IC’s input is the environmental temperature. In other words, the soaked temperature of the package changes the behavior of the internal transistors (Figure 1).

Figure 1 This conceptual circuit shows how matched transistors can sense temperature.