Zener diode characteristic

A Zener diode has two terminals: an anode and a cathode. Its current–voltage characteristic shows how the current through it changes as the potential difference across it changes.

Forward bias

When the anode is at a higher potential than the cathode, the diode is forward biased. It conducts like an ordinary diode: once the forward potential difference is large enough, the current increases rapidly.

Reverse bias and breakdown

When the cathode is at a higher potential than the anode, the diode is reverse biased. At first, only a small reverse current flows. As the reverse voltage increases towards the Zener breakdown voltage, the characteristic reaches a bend called the breakdown knee.

Beyond the knee, the reverse current increases substantially, while the voltage across the diode changes comparatively little. The breakdown region is therefore the part of the characteristic where a wide range of reverse currents corresponds to approximately the same voltage.

Minimum operating current

The minimum operating current is the approximate reverse current needed for the diode to operate reliably in its breakdown region. Below this current, the voltage across the diode may be appreciably less than its specified Zener voltage. Reaching the knee does not mean that the voltage is immediately fixed at exactly that value.

Zener diode voltage reference

A Zener diode and a resistor can provide an approximately constant reference voltage from a supply. The resistor connects the supply to the output. The Zener diode is connected across the output in reverse bias, with its cathode at the higher-potential side.

When the supply is sufficient to drive enough reverse current through the diode, the diode operates in breakdown. In this region, the voltage across it is approximately constant and is called the Zener voltage, $$V_Z$$. Because the output is across the diode, the output voltage is then approximately $$V_Z$$.

Current through the series resistor

The series resistor limits the current and takes up the difference between the supply voltage and the output voltage. While the diode is in breakdown, the resistor current is approximately:

Here, $$I_R$$ is the resistor current and $$R$$ is the resistance. With no load connected, this current passes through the Zener diode. The supply and resistor must allow at least the diode’s minimum operating current to flow; otherwise, the diode cannot maintain the reference voltage in breakdown.

Connecting a load to the Zener reference

A load connected across the output has the same voltage across it as the Zener diode. The current through the resistor is shared between the load and the diode:

Here, $$I_L$$ is the load current and $$I_Z$$ is the Zener current. If the load draws more current, less remains for the diode. The output stays approximately at $$V_Z$$ only while the remaining Zener current is at least its minimum operating current. If it falls below that level, the diode no longer maintains the specified reference voltage.