PN junction Reverse Bias

AIM: To study the reverse characteristics of zener diode. APPARATUS NEEDED: Voltmeter, ammeter, voltage source, and resistor. THEORY: A diode is a device that allows the flow of current in only one direction. A zener diode not only allow current to flow from anode to cathode but also in the reverse direction on reaching the zener voltage. Zener diode is a special kind of diode that is operated in a reverse bias condition and this diode doesn’t get damaged when the voltage across this exceeds the reverse breakdown voltage .Once the reverse bias voltage becomes greater than the zener breakdown voltage a significant amount of current starts flowing through the diode due to zener breakdown. The voltage remains at the zener breakdown voltage value but the current through the diode increases when the input voltage gets increased. Due to this the depletion region regains its original position when the reverse voltage gets removed. The zener diode doesn’t get damaged despite this massive amount of current flowing through it. As the voltage remains at the zener breakdown voltage we use the zener diode for voltage regulation. We use them in thr voltage stabilizers and various other protection circuits. We also use them in clipping circuit and clamping circuit. They provide a low-cost solution for voltage regulation. Avalanche Breakdown in Zener Diode Avalanche breakdown occurs both in normal diodes and Zener Diode at high reverse voltage. When a high value of reverse voltage is applied to the PN junction, the free electrons gain sufficient energy and accelerate at high velocities. These free electrons moving at high velocity collides other atoms and knocks off more electrons. Due to this continuous collision, a large number of free electrons are generated as a result of electric current in the diode rapidly increases. This sudden increase in electric current may permanently destroy the normal diode, however, a Zener diode is designed to operate under avalanche breakdown and can sustain the sudden spike of current. Avalanche breakdown occurs in Zener diodes with Zener voltage (VZ) greater than 6V. Zener Breakdown in Zener Diode When the applied reverse bias voltage reaches closer to the Zener voltage, the electric field in the depletion region gets strong enough to pull electrons from their valence band. The valence electrons that gain sufficient energy from the strong electric field of the depletion region break free from the parent atom. At the Zener breakdown region, a small increase in the voltage results in the rapid increase of the electric current