Question

Explain the forward and the reverse characteristic of a Zener diode.

Answer 1

The forward bias region of a Zener diode is identical to that of a regular diode. There is a forward current only after the barrier potential of the pnjunction is overcome. Beyond this threshold or cut-in voltage, there is an exponential upward swing.
The typical forward voltage at room temperature with a current of around 1 mA is around 0.6 V.

In the reverse bias condition, the Zener diode is an open circuit and only a small reverse saturation current flows as shown with a change of scale. At the reverse breakdown voltage, there is an abrupt rapid increase in the current- the knee is very sharp, followed by an almost vertical increase in current. The voltage across the Zener diode in the breakdown region is very nearly constant with only a small increase in voltage with increasing current. There is a minimum Zener current, I_z (min), that places the operating point in the desired breakdown region. At some high current level, I_ZM, the power dissipation of the diode becomes excessive beyond which the diode can be damaged.

Zener diode characteristics

The I-V characteristics of a Zener diode is not totally vertical in the breakdown region. This means that for slight changes in current, there will be a small change in the voltage across the diode. The voltage change for a given change in current is the resistance R_z of the Zener diode.