Diode V-I characteristics

To trace the V-I curve, a battery feeds the diode through a potentiometer (or rheostat) so the voltage can be varied. A milliammeter reads the large forward current and a microammeter the tiny reverse current.

Forward bias: the current stays almost zero until the voltage passes a threshold, then rises steeply (exponentially) for small further increases.

This threshold or cut-in voltage is about 0.2 V for a germanium diode and about 0.7 V for a silicon diode.

Reverse bias: the current is very small (µA) and almost constant as the voltage changes. This is the reverse saturation current.

At a critical reverse voltage, the breakdown voltage Vbr, the reverse current suddenly shoots up. Unless an external circuit limits it below the rated value, overheating destroys the junction; the same happens in forward bias if the current exceeds its rating.

General-purpose diodes are used only below breakdown, in the reverse saturation region.

So a diode passes current essentially one way: its forward resistance is low and its reverse resistance is high. Rectifiers exploit this.

Dynamic resistance: rd = ΔV/ΔI (Eq. 14.6), the ratio of a small change in voltage to the resulting small change in current.

Example 14.4 (silicon diode): between 10 mA at 0.7 V and 20 mA at 0.8 V, the forward resistance is rfb = 0.1 V / 10 mA = 10 Ω.

At V = −10 V the curve gives I = −1 µA, so the reverse resistance is rrb = 10 V / 1 µA = 1.0 × 10⁷ Ω, a million times the forward value.