Zener Diode Characteristics

Zener Diode

Zener diode is a PN junction diode which is highly doped. So its characteristics are same as the normal PN junction diode in forward as well as as reverse condition except the Zener breakdown mechanism.

Forward characteristics contains exponential curve of current with respect to voltage after cut in voltage following the diode current equation as follows

Reverse characteristics contains reverse saturation current in reverse direction. As we increase voltage, a condition arise, when charge carriers from depletion region are made free due to intense electric field because of very high doping. This voltage is called Zener breakdown voltage VZ and we obtain a large current at this voltage.

In this sense, it is not wrong to say that diode works as conducting state in reverse bias condition after breakdown voltage level i.e. in breakdown region of diode.

So diode conducts in forward condition after knee voltage level and in reverse condition after breakdown voltage level in Zener Diode. 

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So we conclude two facts.

1. Vk voltage is set across the diode in forward condition if diode works after knee voltage.

2. VZ voltage is set across the diode in reverse condition if diode works in breakdown region i.e. voltage crosses the level VZ across the diode.

This characteristic of Zener Diode  is used in voltage regulation to load even when load or input voltage are variable.

Zener Diode Symbol and Equilibrium Circuit

VZ voltage is set across the diode in reverse condition if diode works in breakdown region i.e. voltage crosses the level VZ across the diode.

Zener Diode Rating (Zener Diode Specification)

Zener Voltage (VZ):It the voltage across Zener diode from cathode to anode when it is biased to operate in Zener breakdown region. This is the value of reverse voltage across diode at which breakdown occurs. This voltage depends on the material used and doping level in the diode. Specified values of VZ are 3.1V, 4.7V, 5.1V, 6.8V, 9.1V, 12.1V, 15.1V, 18.1V, etc.

Power dissipation (PZ): The Zener is normally operated in the reverse biased manner. So it needed to calculate power dissipation which is given by     PZM = VZ × IZ    

where VZ is Zener voltage and IZ is Zener current in present state.

Maximum Power dissipation (PZM): The Zener is normally operated in the reverse biased manner. So it needed to calculate power dissipation which is given by         PZM = VZ × IZM   

where VZ is Zener voltage and IZM is maximum Zener current which can be flow in the diode without damaging it because of thermal changes in the diode.

Maximum Zener Current (IZM or IZ,max): IZM is maximum Zener current which can be flow in the diode without damaging it because of thermal changes in the diode. This value is different for different diodes and depends upon the material used and doping level in the diode.

Minimum Zener Current (IZ,min):IZ,min is minimum Zener current which is required to flow in the diode to keep it in breakdown region. It is also defined as the minimum current  required by the diode to bring it in breakdown region. It is also known as Zener knee current Izk or break-over current Ibo. Normally this value is 2mA.

Dynamic Zener Resistance (rZ): It is defined as the ratio of change in voltage across diode to change in Zener current.  Ideally voltage across diode should be constant or change in voltage across diode should be 0V. So ideally rZ should be zero and practically very low (few Ω).

Mathematically        rZ = dVZ/dIZ