We studied about the characteristics of diode by applying Shockley’s equation. Shockley’s equation is as follows. (The details of the terms used in Shockley’s equation are discussed here)
ID = IS(eVD/ηVT – 1)
Consider the term VT given in above equation. This term is called thermal voltage and is dependent on temperature by the relation VT = kT/q, where k is Boltzmann’s constant, q is the charge on an electron and T is the temperature in Kelvin. This term indirectly suggests the dependence of diode characteristics on temperature.
Effect of temperature on forward characteristics :
The characteristics curve of a Si diode shifts to the left at the rate of -2.5 mV per degree centigrade change in temperature in forward bias region.
Refer to the graph shown above. The curves are shown far apart just for illustration purpose and are not to scale. The curve shifts to the left at the rate of -2.5 mV per degree centigrade change in temperature. Hence if the temperature increases from room temperature (25° C) to 80° C, the voltage drop across the diode will be (80-25) x 2.5 mV = 137.5 mV.
Effect of temperature on reverse characteristics :
In the reverse bias region, the reverse saturation current of Si and Ge diodes doubles for every 10° C rise in temperature. Let us take an example to understand how much the reverse saturation current changes with temperature. Consider an increase of temperature from 25 °C to 85 °C, where the reverse saturation current at 25 °C is 100 nA. The temperature increases by 60 °C (25 °C to 85 °C), which is 6 x 10. Hence the reverse saturation current would increase by a factor of 26 = 64. Hence the reverse saturation current at 85 °C will be 100 nA x 64 = 6400 nA.
A graph showing the variation of reverse saturation current with temperature is shown below. The difference between the curve is exaggerated for illustration purpose.
From the above graph it is clear that the reverse saturation current increases with increase in temperature. The graph also shows how the reverse breakdown voltage changes with temperature. It is indicated in the above graph that the reverse breakdown voltage increases with an increase in temperature. However it is only true for avalanche diodes. The reverse breakdown voltage for Zener diodes decreases with an increase in temperature, which is shown in the graph below.