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I-V Characteristics of Zener Diode

Md.Mujahiduzzaman

What is Zener Diode?

Zener diodes are a special kind of diode which permits current to flow in the forward direction. What makes them different from other diodes is that Zener diodes will also allow current to flow in the reverse direction when the voltage is above a certain value. 

Zener Voltage

A Zener diode allows current to flow from its anode to its cathode like a normal semiconductor diode, but it also permits current to flow in the reverse direction when its "Zener voltage" is reached.

Zener Diode as Voltage Regulator

The term regulator means which regulates or controls. Zener diode can work as a voltage regulator if it is introduced in a circuit. The output across the diode will be constant. It is driven by a current source. As we know if the voltage across the diode exceeds a certain value it would draw excessive current from the supply. The basic diagram of zener diode as voltage regulator is given in picture.

Zener Diode As Regulator

Zener Breakdown

The phenomenon of zener breakdown is related to the concept of avalanche breakdown. Zener breakdown is achieved by heavily doped regions in the neighbourhood of ohmic contact. It is the second method of disturbing the covalent bonds of the crystal atoms and increasing the reverse bias zener diode current, to be sustained at a much lower specific voltage than normal diode. The reverse bias voltage known as zener voltage, where this mechanism occurs is determined by the diode doping concentration and it occurs when the depletion layer field width is sufficiently enough to disrupting the covalent bonds and cause number of free charge carriers due to electric field generation to swell.

Curve

Forward Bias

Reverse Bias

Zener Diode V-I Characteristics

Zener Diodes are used in the "REVERSE" bias mode, i.e. the anode connects to the negative supply, and from its I-V characteristics curve above, we can see that the Zener diode has a region in its reverse bias characteristics of almost a constant voltage regardless of the current flowing through the diode. This voltage across the diode (it's Zener Voltage, Vz) remains nearly constant even with large changes in current through the diode caused by variations in the supply voltage or load. This ability to control itself can be used to great effect to regulate or stabilise a voltage source against supply or load variations. The diode will continue to regulate until the diode current falls below the minimum Iz value in the reverse breakdown region.

V-I Curve