How Do We Distinguish the DC Resistance and Dynamic Resistance of a Semiconductor Diode?

In order to understand how the resistance of a semiconductor diode varies with current and voltage, we need to distinguish the two different types of resistance. The two types of resistance are static and dynamic. Dynamic resistance is much more variable than static resistance, so we must distinguish the two with care.

Zener impedance

The Zener impedance of semiconductor diode is a measure of the apparent resistance of a semiconductor diode. It is calculated by measuring the ripple in the input and the change in the source current. For example, if the source current changes from three to five milliamps to seven milliamps, the ripple in the output will be about three-half milliamps. The dynamic resistance of a zener diode is equal to 14 ohms.

The breakdown of the zener impedance of a semiconductor diode occurs when a reverse biased voltage is applied to it. At this voltage, the electric field in the depletion region is strong enough to pull electrons from the valence band. The free electrons then break the bond with their parent atom. This is what causes the flow of electric current through a diode.

When working with a buck circuit, the zener impedance of a semiconductor diode is an important parameter. It can affect the efficiency of a simple buck circuit. If it is too high, the diode may fail to work. If this happens, it is best to reduce the current.

The zener effect is most prominent when the voltage of a diode is below 5.5 volts. At higher voltages, the avalanche breakdown becomes the primary effect. The two phenomena have opposite thermal characteristics, but if the zener diode is nearer to six volts, it can perform very well.