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Home Semiconductors Diodes and Rectifiers pnJunction as Temperature Sensor  
See also: Diode Characteristic  
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pnJunction as Temperature SensorA pnjunction (i.e. a diode) can be used as a simple and easy to use temperature sensor. According to the diode equation where,I_{s} ... saturation current e .... Euler's constant (2.71828...) q .... charge of electron (1.610^{19} As) V .... voltage across the diode N .... "nonideality" coefficient (typ. between 1 and 2) k .... Boltzmann's constant (1.3810^{23}) T .... junction temperature in Kelvin the current through the diode depends on the Voltage V_{D} applied to the diode and its temperature. If you keep the current through the diode constant, the voltage decreases with increasing temperature. Assuming a current of 10 mA through the diode we can insert all known values into the diode equation and calculate the change of the voltage if the temperature changes by 1K: Applying the logarithm to both sides and assuming a "nonideality factor" of N=1 we get: for a temperature change of 1° we get a voltage change of This means that the voltage across a forward biased diode decreases by approx. 2 mV/K. Please note that the characteristic curve of the temperature dependence of the diode voltage is strictly linear, so that the calibration of a diode temperature sensor is straightforward and simple.


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