Zener Diode

ZENER DIODE

In ordinary PN diode, the doping is light. So, the breakdown voltage is high. If the P and N regions are heavily doped, then breakdown voltage can be reduced.

When the reverse voltage reaches the breakdown voltage, the current through the junction and the power dissipation at the junction will be high. Due to large amount of current, there is possibility of damaging the diode. Therefore, the diodes are designed with adequate power dissipation capability to operate in the breakdown region. The diode designed with such specification is called Zener diode, which is heavily doped than ordinary diode.

When doping is heavy, the electric field at the junction will be very high even at low reverse voltage. The electrons in covalent bonds break away from the bonds. This effect is called as Zener effect.

A diode which exhibits the Zener effect is called Zener diode. Zener diode is a reverse biased heavily doped diode which operates in the breakdown region. Zener diodes are designed to operate at voltages ranging from a few volts to several hundred volts.

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V-I Characteristics of Diode

When Zener diode is forward biased, the Zener diode operation is similar to that of ordinary PN diode. Zener diode is specially designed to operate in the reverse / bias condition.

When the reverse voltage is less than the reverse breakdown voltage, the diode carries reverse saturation current.

When the reverse voltage is greater than the reverse breakdown voltage, the current through the diode increases rapidly and the voltage across the diode remains constant. Usually Zener diode is operated in this reverse breakdown region.

Due to this property, Zener diode is used for providing constant voltage source in voltage regulators.

When the reverse voltage applied to a Zener diode is increased, initially the current through it is very small, in the order of few μA or less. This is the reverse leakage current of the diode, I_o

At certain reverse voltage, the current through Zener diode increases rapidly. This change from a low value to large current is very sharp. This sharp change is reverse characteristics is called knee of the curve.

At the knee voltage, the breakdown occurs. The reverse bias voltage at which the breakdown occurs is called Zener breakdown voltage, V_z.

The current corresponding to the knee point is called Zener knee current, denoted as I_zmin

As current is increased, the power dissipation also increases (: P_z = V_z I_z). If this power dissipation increases beyond certain value, the diode may get damaged. The maximum current at which a Zener diode can operate safely is called Zener maximum current, I_zmax.

Equivalent Circuit of Zener Diode

When the breakdown occurs, I_z increases from I_zmin to I_zmax and, the voltage across Zener diode remains constant.

The internal Zener impedance decreases as current increases in the Zener region. But this impedance is very small. Ideally the Zener diode is indicated by a battery of voltage V_z which remains fairly constant.

Fig. 1.31 shows the equivalent circuit of Zener diode. In reverse bias, the resistance is called dynamic resistance of the Zener diode, r_z. The ratio of change in Zener voltage to the change in Zener current is called Zener resistance.

Zener resistance r_z range is in the order of few tens of ohms.