Operation of Zener Diode:
Zener diodes are normally used only in the reverse bias direction. It means that the anode must be connected to the negative side of the voltage source and the cathode must be connected to the positive side. A main difference between zener diodes and regular silicon diodes is the way they are used in the circuits. It is primarily used to regulate the circuit voltage as it has constant VZ. A large change in IR will cause only a small change in VZ. It means that a zener diode can be used as an alternate current path. The constant VZ developed across the diode can then be applied to a load. Thus the load voltage remains at constant by altering the current flow through the zener diode.
Zener Diode V-I Characteristics:
In the forward bias condition, the zener diode behaves like an ideal diode within specified current and power limits, but it differs in reverse bias condition where the zener diode has very steep avalanche characteristic at the breakdown voltage in reverse bias condition. Zener operates mainly in the reverse bias mode by connecting anode to the negative terminal of the power supply. Zener diodes are categorized and rated by the voltage at which they will turn on or start to conduct the reverse bias current. The maximum power intended for a zener diode is specified as Pz=VzIz max and it is a function of the plan and structure of the diode. The knee of the curve is generally approximated as 10% of Iz max,
Generally these zener diodes are used to regulate the voltage. In reverse bias condition after the break down zener diode provides a constant output voltage even if we increase the input voltage. There are specifically two separate mechanisms that might cause a breakdown in a zener diode:
It is predominant above approximately 5.5 volts. This mechanism is also referred to as impact ionization or avalanche multiplication. For reverse conduction it is necessary to visualize the phenomenon of avalanche breakdown. This process begins when a large negative bias is applied to the PN junction, sufficient energy is imparted to thermally generated minority charge carriers in the semiconductors.
It is predominant below approximately 5.5 volts. This mechanism is also referred to as a high field emission mechanism. The phenomenon of zener breakdown is related to the concept of avalanche breakdown. Zener breakdown is achieved by heavily doped regions in the neighborhood of ohmic contact.