Semiconductor Temperature Sensing Diode:

Semiconductor Temperature Sensing Diode is a versatile device and finds use in many applications. Two of its parameter, that is forward voltage drop (V_f) and reverse saturation current (I_s) are temperature sensitive. The sensitivity of I_s with temperature is non-linear, but V_f has a linear temperature coefficient over a wide temperature range. Hence, V_f can serve as the basis for electronic thermometers.

Figure 13.50 shows the characteristics of V_f versus temperature for a typical silicon pn junction diode. The linearity of this characteristics at high values of temperature is affected by the following factors.

1. Dependence of V_f on I_s which is also temperature sensitive.
2. Presence of finite surface leakage component across the pn junction.
3. Finite resistance of the bulk semiconductor used for the temperature sensing diode.

The limitations introduced by point 1 can be overcomes by using a pair of well-matched junction transistors in a differential configuration to serve as the temperature sensors.

If the two transistors are operated at widely different emitter current values, the resulting V_BE, Base emitter differential voltage can serve as an excellent index of temperature.

The association of differential mode voltage gain of this stage with V_BE pro­vides means of adjusting the temperature coefficient to a desired value. This is the basis of IC type temperature transducer.