Transistor

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The silicon temperature sensor's resistance is given by the equation:
R = Rr ( 1 + a.( T - Tr ) + b.( T - Tr )2- c.(T - Ti)d )where Rr is the resistance at temperature Tr and a, b, c and d are constants. Ti is an inflection point temperature such that c = 0 for T < Ti.
The resistance of some of these semiconductor sensorsis dependent on the excitation current (due to current density effects in the semiconductor) and the polarity of the applied voltage. As with other non-passive temperature sensors, self-heating caninduce errors.
There are a number of specialist cryogenic temperature sensors that use resistive semiconductor sensor elements made from silicon and germanium.

5. Diode Temperature Sensors
Theordinary semiconductor diode may be used as a temperature sensor.Cheap and nasty! The diode is the lowest cost temperature sensor and can produce more than satisfactory results if you are prepared toundertake a two point calibration and provide a stable excitation current.   Almost any silicon diode is ok. The forward biased voltage across a diode has a temperature coefficient of about 2.3mV/°C andis reasonably linear. The measuring circuit is simple as shown to the right.
The bias current should be held as constant as possible - using constant current source, or a resistor from a stablevoltage source.
Without calibration the initial error is likely to be too large - in the order of ±30°C - the largest of all the contact type temperature sensors. This initial error is greatly reduced ifsensor grade parts are used.
One advantage of the diode as a temperature sensor is that it can be electrically robust - tolerant to voltage spikes induced by lightning strike. This is particularlytrue if power diodes (e.g. the common 1N4004) are used and a second back to back diode is used to limit power dissipation during high peak currents.
The transistor sensor is used in diode mode by...