The zener breakdown is occurs in PN junctions which are heavily doped. The heavily doped junctions have a narrow depletion region. The zener breakdown is observed in the zener diodes having breakdown voltage is less than 5V or between 5 to 8 volts. The electric field across the junction increase, when the reverse voltage is increased. A high value of electric field causes a covalent bond to break from the crystal structure. Due to this a large number of minority carriers are generated and a large current flows through the junction. This mechanism of breakdown is known as zener breakdown. A current limiting resistance should be connected in series with the zener diode to protect it against the damage due to excessive heating. The zener diodes have a negative temperature coefficient. In zener breakdown, the breakdown voltage depends on the temperature of P-N junction. The breakdown voltage decreases with increase in the junction temperature.