Temperature sensors can be fabricated with semiconductor processing technology by employing the temperature characteristics of the pn junction. The batch processing and well-defined manufacturing processes associated with semiconductor technology can provide low cost and consistent quality temperature sensors.
Most semiconductor junction temperature sensors utilize a diode-connected bipolar transistor (short-circuited collector-base junction). A constant current passed through the base-emitter junction produces a junction 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-1.
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Fig: Bipolar transistor configured as a temperature sensor
In the above
figure, the base of the transistor is shorted to the collector. A constant
current flowing in the remaining pn (base to emitter) junction produces a
forward voltage drop VF proportional to temperature.
The temperature
coefficient of a semiconductor sensor is larger but still quite small when compared
to a thermocouple or resistive temperature device/detector (RTD). Furthermore,
the semiconductor sensor’s forward voltage has an offset that varies
significantly from unit to unit. Nonetheless, the semiconductor junction
voltage versus temperature is much more linear than that of a thermocouple or
RTD. Also, the temperature-sensing element, circuitry is easily integrated to
produce a monolithic temperature sensor with an output that can be easily
interfaced to a microcontroller and to provide features that are useful in
particular applications. For instance, by using an embedded temperature sensor
with additional circuitry, protection features can be added to integrated
circuits (ICs). A temperature sensor becomes an embedded item in a
semiconductor product when it has a secondary or supplemental purpose instead
of the primary function.