Circuits in an electronic appliance require that the voltages be regulated. Diodes are the electronic components that satisfy this requirement. A zener diode is used to regulate the voltage in a circuit by reversing breakdown voltage current in a circuit. A diode shows a voltage drop because of its junction built-in voltage and internal resistance. The semiconductor material and its doping concentrations determine how much voltage drop occurs across a diode. A zener diode is usually connected to the positive terminal of an operational amplifier and the other terminal to the power supply.
To illustrate, a zener diode with a zener breakdown voltage of 2.5 V will register a voltage drop of 2.5 V if reverse bias voltage is applied across it and it is more than its zener voltage. Controlling the breakdown voltage is done accurately during the doping process. Like resistors, diodes have also tolerance levels in which 5% and 10% tolerances are the most widely used.
When used as forward bias, the zener diode's anode is connected to the positive terminal of a voltage source, such as a battery, and the negative terminal is connected to its cathode. Under this condition, the positive terminal of the battery repels holes within the p-type material toward the p-n junction. The breaking of electron-pair bond within the p-type and n-type produces a flow of electrons in the circuit to which the diode is connected. When the diode is connected as forward-biased the current will flow in the external circuit in the negative-to-positive direction.
A zener diode can also be used as reversed bias. To use the diode in this manner, its anode should be connected to the negative terminal of the battery and its cathode is connected to the positive terminal. In this condition, holes within the p-type material are attracted toward the negative terminal of the battery and away from the p-n junction. At the same time, free electrons within the n-type material are attracted toward the positive terminal of the battery and away from the junction. At this condition, no current will flow in the external circuit.
When the diode is used in a half-wave-rectifier circuit, it is connected in an alternating voltage source. The zener diode in this condition is alternately forward-biased and then reverse-biased during each cycle. The current passes through the circuit load only during one half of each cycle of the input voltage that is why the circuit is called half-wave-rectifier circuit.
When the diode is used in a full-wave-rectifier circuit, two diodes have to be connected in the voltage source. Full-wave-rectifier circuits are those which rectify the whole cycle of an applied voltage.
The zener diode allows the current in the forward direction but reverses its direction if the voltage is bigger than the zener voltage or breakdown voltage. The zener diode is designed specifically to greatly reduced zener voltage. With its doped p-n junction that allows electrons to transfer to the conduction band of the n-type material from the valence band of the p-type material, the zener diode, which is now in reversed-biased state, will exhibit a controlled breakdown. In this state, the zener diode allows the current to keep the voltage across the zener diode to breakdown voltage.