Zener Diode

In this article, let us learn the function of Zener diodes along with their construction, operation, characteristics, and application.

Zener Diode Explanation

A diode that is heavily doped (Si or Ge) and which operates in the reverse breakdown region with a sharp breakdown voltage is called a Zener Diode.

• In a simple diode, the doping is light; as a result, the breakdown voltage is high and not sharp. But if doping is made heavy, the depletion layer becomes very narrow and even the breakdown voltage gets reduces to a sharp value.

• The Zener diode is designed to operate in the breakdown region without damage.

Note:

• During heavy doping, Si is preferred to Ge because of its higher temperature and current capability.
• The Zener breakdown voltage of the Zener diode is set by carefully controlling the doping level during manufacturing.

Circuit Symbol of Zener Diode

• The schematic symbol of a Zener diode is shown in the figure above.

• This is similar to a normal diode except that the line representing the cathode is bent at both ends like the letter Z for the Zener diode.

Working Principle of Zener diode

The reverse breakdown of a Zener diode may occur either due to the Zener effect or avalanche effect. But Zener diode primarily depends on the Zener effect for its working.

Zener effect?

• When the electric field across the junction is sufficiently high due to the applied voltage, the Zener breakdown occurs because of covalent bonds. This produces a large number of electrons and holes which constitute a steep rise in the reverse saturation current (also called Zener current Iz). This effect is called as Zener effect. Zener current is independent of the applied voltage and depends only on the external resistance.
• Zener diode with reverse voltage less than 6v operate predominantly in zener breakdown.

Avalanche effect

• When the electric field existing in the depletion layer is sufficiently high, the velocity of carriers crossing the depletion layer increases. here, the majority of carriers accelerated by the field collide with the semiconductor atoms in the depletion region. Due to some violent collision few electrons get knocked off from the crystal atoms, thud creating electron-hole pairs. These new carriers so produced acquire energy from the applied potential voltage and in turn produce additional carriers. this forms a cumulative process called avalanche effect or multiplication effect, which in turn leads to a breakdown called avalanche breakdown.

• Those with breakdown voltage greater than 6V operate predominantly in avalanche breakdown.

Characteristics of Zener diode

• The V-I characteristics of Zener diodes are shown in the figure below. The forward characteristic is simply that of an ordinary forward-biased junction diode.

• Under reverse bias conditions, a breakdown of junction occurs. This breakdown depends on the amount of doping. It can be seen from the figure that as the reverse voltage is increased, the reverse current remains negligibly small up to the ‘knee’ of the curve, point K.

• At point K, the effect of the breakdown process begins. The voltage corresponding to the point K in the figure is called the Zener breakdown voltage or simply Zener voltage (V_Z), which is very sharp compared to a simple p-n junction diode. Beyond this voltage, the reverse current (I_z) increases sharply to a high value.

• The Zener diode is not immediately burnt just because it has entered the breakdown region. As long as the external resistance connected to the diode in the circuit limits the diode current to less than the burn-out value, the diode will not burn out. The Zener voltage V_Z remains constant even when Zener current I_Z, increases greatly. This ability of a diode is called regulating ability and it enables us to use Zener diode for voltage regulation.

• The maximum permissible value of current is denoted by I_zmax and the minimum current to sustain breakdown is denoted by I_zmin. By considering any two points A and B on the reverse V-I characteristic, the Zener resistance is given by the relation,

r_z = \frac{\Delta V_z}{\Delta I_z}

Application of Zener diode

Zener diode has a number of applications in the electronics field. Some of their common applications are listed below:

1. Zener diodes are used as voltage regulators.
2. They are used as a fixed reference voltage in transistor biasing and for comparison purposes.
3. They are used in wave shaping circuits as peak limiters or clippers.
4. They ar used for meter protection against damage from accidental applications of excessive voltage.

Frequently Asked Questions – FAQs

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