Zener diode is a junction formed by combining highly doped PN semiconductors. It works on the principle of Zener breakdown and is operated in reverse breakdown region. In reverse breakdown region high current flow through the diode leading to high power dissipation. Hence the Zener diodes are provided with adequate power dissipation capabilities to operate in reverse breakdown region.
When a reverse bias is applied across the diode electric field is generated by uncovered charges at the depletion region. The electric field intensity across a PN junction diode increases as doping level are increased.
For a step graded junction with NA >> ND the magnitude of electric field across the junction and reverse bias voltage at which breakdown occurs is given by
Ez = q*ND*W/ε and Vz = ND*W2*q/(2*ε)
Where q is electronic charge = 1.6*10-19 coulombs
ND is Donor concentration
W is width of depletion region
ε is permittivity of depletion region
Accordingly the electric filed at the junction formed by heavily doped PN semiconductors exerts a sufficiently strong force on a bound electron enough to tear it out of its covalent bond. The new hole electron pair created increases reverse current and eventually enters breakdown region. This process is called Zener breakdown. Zener breakdown occurs at a voltage much less than the avalanche breakdown voltage.
Symbol of Zener diode
Zener diode as the name suggests is a two terminal device with two terminals being anode (p) and cathode (n). The symbol of Zener diode is shown in the figure below
Operation of Zener diode
The VI characteristics of a zener diode is as shown in the figure below
The forward characteristics of zener diode are similar to that of normal PN junction diode. The reverse breakdown region is the VI characteristic of Zener diode above zener breakdown voltage. In reverse breakdown region the reverse voltage across the diode is almost independent of the current flowing through it except for small variation quantified by dynamic resistance of Zener diode. The dynamic resistance of Zener diode is given by
That means if we connect a load across the zener diode with a resistance and supply voltage it will provide almost constant voltage across the load independent of load and supply voltage variations. This means Zener diode acts as a voltage regulator. The minimum current that should be allowed through zener diode to ensure that it is in breakdown region is called Knee current. For currents lower than the knee current regulation will be poor.
Applications of Zener diode
- Zener diodes are used in Voltage stabilizers (or) shunt regulators
- used in Surge suppression circuitry for device protection
- used in Over voltage protection circuits.
- Zener diodes are used in clipping and clamping circuits especially peak clippers .
- They are used as Reference elements.
- Used in switching applications.