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Nonlinear Model

  The detailed behaviour of the diode can be explained in terms of semiconductor physics which is beyond the scope of this course. The nonlinear diode characteristic which can be obtained experimentally is shown in Figure 72. However, you should know about a key equation from semiconductor physics:

$$i = I_s [ \exp ( \frac{q v}{kT} ) -1 ]$$ (74)

where

• v and i are the diode voltage and current, as shown in Figure 67.
• I_s is the reverse-bias saturation current (a common value might be 0.07 pA, although I_s is proportional to diode area).
• kT/q = 26 mV at 300 K.

The nonlinear diode characteristic given by (74) is shown in Figure 72. If v increases towards around 0.7 V (silicon), then i increases very rapidly (exponentially). The parameters may not be known exactly, and they may change with temperature, etc., but a typical switch on voltage of 0.6-0.7 V is usually found for Si (other types of diodes may be different, e.g. LEDs might be 1.5 V and Schottky might be 0.3 V).

  

Figure 72: Nonlinear diode characteristic.

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