The Fundamentals of Semiconductor Junction Thermometers

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 (V_be) that is a linear function of the absolute temperature. The overall forward voltage drop has a temperature coefficient of approximately 2 mV °C^-1.

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 V_F 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.

The working Principle of Thermocouples

When two wires with dissimilar electrical properties are joined at both ends and one junction is made hot and the other cold, a small current is produced proportional to the difference in the temperature.

For example, we have the cold end joined at a sensor millivolt meter, and the hot junction forming the sensor end as shown below:

Peltier showed that the heat is absorbed at the hot end and rejected at the cold end. Thompson showed that part of the e.m.f. is due to temperature gradient in the wire as well as the temperature difference between the junctions.  Most of thermocouple metals produce a relationship between the two temperatures and the e.m.f. as follows:

The α and β are constants for the type of thermocouple. The relationship is nearly linear over the operating range. The actual characteristics and suitable operating temperatures depend upon the metals used in the wires. The various types are designated in international and national standards. Typical linear operating ranges are shown for standard types. Note, it is important for thermocouples to be standard so that the e.m.f. will always represent the same temperature.

Thermocouples come in several forms, they may be wires insulated from each other with plastic or glass fibre materials. For high temperature work, the wire pairs are put inside a tube with mineral insulation. For industrial uses, the sensor comes in a metal enclosure such as a stainless steel. 

You can also read: Temperature transducers as applied in Industrial Instrumentation

Example of typical thermocouple industrial probes is shown below:

Temperature Transducers

We have four common temperature sensors used in industrial instrumentation.

Resistance Temperature Detector (RTD)

RTD responds to heat by increasing its resistance to electric current.

Thermistor

This is similar to RTD, except that its resistance decreases as it is heated.

Note that, in both the RTD and Thermistor temperature sensors, the current variation due to temperature change is usually very small. Current through an RTD or Thermistor must be compared to current through another circuit containing identical devices at a reference temperature to detect the change. The freezing temperature of water is used as the reference temperature.

Semiconductor integrated circuit

This type of temperature sensors respond to temperature increases by increasing reverse-bias current across P-N junctions, generating a small but detectable current or voltage proportional to temperature. The integrated circuit may contain its own amplifier.

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Thermocouple

Thermocouple type temperature sensors generate a small voltage proportional to the temperature at the location where dissimilar metals are joined.