Q. Explain about Zener Diodes?

Most diodes are not intended to be operated in the reverse breakdown region. Diodes designed expressly to operate in the breakdown region are called zener diodes. A nearly constant voltage in the breakdown region is obtained for a large range of reverse current through the control of semiconductor processes. The principal operating region for a zener diode is the negative of that for a regular diode in terms of both voltage and current. Zener diodes are employed in circuits for establishing reference voltages and for maintaining a constant voltage for a load in regulator circuits. Figure shows the device symbol along with the linearized i-v curve and the circuit model.

As seen from the i-v characteristic, a zener diode approximates an ideal diode in the forward region. However, when the reverse bias exceeds the zener voltage V_Z, the diode starts to conduct in the reverse direction and acts like a small reverse resistance R_Z in series with a battery V_Z. Zener diodes are available with values of V_Z in the range of 2 to 200 V. The circuit model of Figure (c) incorporates two ideal diodes, Df and Dr, to reflect the forward and reverse characteristics of the zener diode. The i-v curve thus has two breakpoints, one for each ideal diode, and three straight-line segments.