The ebook FEEE - Fundamentals of Electrical Engineering and Electronics is based on material originally written by T.R. Kuphaldt and various co-authors. For more information please read the copyright pages.



pn-Junction as Temperature Sensor

A pn-junction (i.e. a diode) can be used as a simple and easy to use temperature sensor. According to the diode equation

where,
ID ... diode current
Is ... saturation current
e .... Euler's constant (2.71828...)
q .... charge of electron (1.610-19 As)
V .... voltage across the diode
N .... "non-ideality" 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 VD 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 "non-ideality 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.


Last Update: 2010-11-19