Temperature has a significant effect on electronic products. It impacts both the system performance and the expected life of components. With the increasingly dense circuitry in either the same or often smaller packages, the temperature issues can only increase – even with the offsetting impact of increasingly lower power consumption components. Measuring temperature can allow an electronic system to compensate for its effects and prevent serious overtemperature problems.
Temperature sensors are easily produced with semiconductor processing technology by using the temperature characteristics of the PN junction. As noted in “Semiconductor Junction Thermometers,” in Measurement, Instrumentation, and Sensors Handbook, Second Edition, “The batch processing and well-defined manufacturing processes associated with semiconductor technology provide low cost and consistent quality temperature sensors.” As an integral aspect of the transistor’s defining equations, the temperature sensitivity of the PN junction is quite predictable and very linear over the typical semiconductor operating range of -55 to +150°C.
In addition to a PN junction in a diode, a semiconductor junction temperature sensor can be achieved by short circuiting the collector-base junction of a bipolar transistor to create a diode. When constant current passes through the base-emitter junction it produces a voltage between the base and emitter (Vbe) that is a linear function of the absolute temperature. The overall forward voltage drop has a temperature coefficient of approximately 2 mV/°C.
The semiconductor junction sensor’s voltage versus temperature is much more linear than that of a thermocouple or resistive temperature device (RTD). However, the temperature coefficient of a semiconductor sensor is larger (but still quite small), compared to a thermocouple or an RTD. With integrated circuit design, the compensation as well as amplification and other system interfacing aspects, including diagnostics, digital output and more, can added.
Additional circuitry, such as amplification and compensation can easily be added to the transistor temperature sensor.