Although diodes are non-ohmic, this doesnt mean that their operation cant be explained mathematically (just as Ohms law or, V = IR, say is a mathematical formula). Diodes, in fact, follow a relationship every bit as mathematical as Ohms law. The relationship is:

where I is the current through the diode, Is is the saturation reverse current, q is the magnitude of an electrons charge, k is Boltzmanns constant, and T is the absolute temperature in degrees Kelvin. As q and k are both constant and at room temperature the absolute temperature is more or less constant, the part of the equation q/kT is also more or less constant at about 40 (work it out yourself if you want: q is 1.6 x 10-19 C; k is 1.38 x 10-23 JK-1 and room temperature, say, 17C is 290 K.

The equation is thus simplified to be approximately:

The exponential factor (e40V), of course confirms what we already knew to be true that the diode characteristic curve is an exponential curve. From this characteristic equation we may calculate the current flowing through a diode for any given voltage across it, just as the formulae associated with Ohms law do the same for resistors.


But even with this simplified approximation of the characteristic equation you can appreciate the value of having a characteristic curve in front of you to look at. If I had the option between having to use the equation or a diode characteristic curve to calculate the current through a diode, I know which Id choose!

<< Diodes II