Large signal models of PN junction diode

Large-signal models replace the non-linear diode with simple linear elements so that dc bias and large-voltage circuits can be solved by hand. Four models are used; they are listed here in order of increasing accuracy.

1. Ideal diode model

The diode is a perfect switch: a short circuit (0 V drop) when forward biased and an open circuit when reverse biased.

• Conducts if VD > 0; blocks if VD < 0.
• Used when the supply voltages are much larger than 0.7 V.

2. Constant voltage drop (CVD) model

A conducting diode is modelled as an ideal diode in series with a battery VD(on) (0.7 V for Si, 0.3 V for Ge).

  A --->|--- (-|+) --- K       ON  : VD = 0.7 V
       ideal   0.7 V           OFF : VD < 0.7 V, I = 0

It is the model most commonly used in hand analysis.

3. Piecewise-linear model

The curve is approximated by two straight-line segments: below the cut-in voltage Vγ the current is zero; above it the current rises linearly with slope 1/rD.

  A --->|--- Vγ --- rD --- K

ID = (VD - Vγ)/rD   for VD ≥ Vγ,     ID = 0   for VD < Vγ

It includes the diode's bulk (forward) resistance rD, so the voltage drop rises slightly with current.

4. Exponential (Shockley) model

ID = IS [exp(VD/(η VT)) - 1]

It is the most accurate model, but it needs iteration or a graphical solution. The CVD model approximates it well, because VD changes by only about 60 mV (η = 1) to 120 mV (η = 2) for every tenfold change in ID.

Comparison

All the models treat the diode as a one-way device; they differ only in how the forward drop is represented.