BASIC KNOWLEDGE - ZENER DIODES Zener diode: Definition, applications, and more

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Zener diodes are one of the best choices for carrying out reverse bias operations. The article explains Zener diodes in a brief comparison with PN junction diodes. The comprehensive guide takes a deeper dive into Zener diodes along with their symbol, working principle, advantages, applications, and testing.

1. What is a zener diode

In 1934, an American Physicist Clarence Melvin Zener discovered the electrical behavior of insulators. The physicist termed his discovery as “Zener effect”. Later, Bell Labs developed a diode based on the Zener effect and named it “Zener Diode”, honoring the physicist.

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What is a diode and what is it used for?

a. Zener diode definition

A Zener Diode is a special type of heavily doped PN junction diode that safely operates in reverse-biased conditions based on the “Zener Effect”. The high doping level produces a high electric field that “breaks downs” the covalent bond to cause a large current flow against a reverse voltage.

2. Zener diode symbol

The symbol of Zenr diodes is almost similar to PN junction diodes. There is a slight change in the symbol towards the negative cathode.

3. Zener diode working principles

The Zener diode works like a normal diode in forward bias. But when the Zener diode is reverse biased, it works on the principle of the “Zener Effect”. Upon comparison, the PN junction diode is lightly-doped whereas the Zener diode is heavily doped with impurities.

Zener diode explained with PN junction diode

In PN junction diodes at 0 Kelvin (absolute zero temperature), the majority charge carrier electrons from N-side diffuse into the P-side, and holes from P-side diffuse into N-side. The diffusion of electrons and holes forms a depletion region with a potential difference of V. The length of the depletion region is “L”. When a voltage greater than V is applied across the diode, the charge carriers move for current flow. This voltage V is known as the barrier potential of the diode.

For a PN junction diode, reverse biasing enables minority charge carrier flow. The minority charge carrier electrons from the P-side move towards the N-side. These free minority electrons collide with the electrons indulged in covalent bonds on the N side.

The collision produces heat and knocks off the bonded electron to move inside the device. Multiple collisions cause the current flow to be termed as reverse leakage current. Instead of behaving like an insulator, the PN diode starts conducting in the reverse bias. The voltage of semiconductor devices beyond which they start conducting is known as the breakdown voltage. At such high values of reverse current, the PN diode is at risk of burning due to the heating effect of the current.

How does a zener diode work?

The working of the Zener diode is well-explained by the laws of quantum mechanics. The heavy doping of the Zener diode creates a narrower depletion region compared to a normal PN junction diode. The length of the depletion region of the Zener diode is “dl”. The width of the depletion region is narrower enough to offer low resistance in Zener diodes.

The varying electric field is dependent upon the small length “dl”. The electric field would be stronger in the Zener diode than in the normal diode because of the small width of the depletion region.

The maximum voltage beyond which the Zener diode starts conducting in the reversed-biased condition is known as the Zener voltage. Below the Zener voltage, the Zener diode is “OFF”. The Zener diode turns “ON” above the Zener voltage. The current increases drastically for the Zener voltage in reverse-biased operation. The minority carrier electrons from the P-side diffuse into the N-side. The collision probability becomes much lesser due to the narrow width of the depletion region. Instead, the high electric field allows the bonded electron in the valence band on the N-side to enter the conduction band on the P-side. The effect in which bonded electrons from the valence band of the N-side “tunnels” to the conduction band of the P-side is known as Zenner breakdown.

Quantum mechanics explains that the transfer of electrons from the valence band to the conduction band is possible through “Quantum Tunnelling”. The phenomenon is based on the principles of quantum mechanics in which the electron is considered a wave. The depletion region is small enough that the wave function of electrons spreads out. The probability of the electron “tunnel” from one point to the other inside the depletion region increases. As a result, the wave function of the electron helps it to cross the small barrier potential without gaining sufficient kinetic energy.

The transfer of minority carrier electrons ruptures/tears the covalent bonds without producing heat like PN junction diodes. The process of acceleration of free electrons into the N-region is termed internal field emission. The effect in which quantum tunneling allows electrons to enable current flow instead of particle collision is known as the “Zener Effect”.

4. Zener diode Characteristics

For a forward bias operation, the Zener diode behaves much like a PN junction. The reverse characteristics directly showcase that Zener breakdown happens for a voltage much less than avalanche breakdown. It is because the Zener diode is heavily doped and the PN junction diode is lightly doped for avalanche breakdown.

5. Zener diode applications

What is the use of the Zener diode?

The popular application is the Zener diode as a voltage regulator. But Zener diodes are used in a variety of applications:

• Surge Suppressors

• Switching

• Voltage Shifters

• Clipper Circuits

• Protection Circuits

6. Zener diode advantages

The major advantage of Zener diodes over other semiconductor devices is the ability to operate in reverse bias. Other semiconductor devices produce heat and often burn under such conditions. But Zener diodes safely operate in such conditions without generating heat.

Another advantage of the Zener diode is that it has both positive and negative coefficients of temperature. The Zener voltage (or Zener potential) is defined by the temperature coefficient. The Zener voltage below 5 V supports a negative temperature coefficient for which an increase in temperature decreases the final Zener potential. For the Zener voltage greater than 5 V, the positive temperature coefficient increases the final Zener potential with increasing temperature. The avalanche breakdown is predominant in the Zener voltages above 6 V.

Some other advantages include:

• Zener diodes have a small size.

• Zener diodes protect from overvoltage.

• Zener diodes are less expensive.

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7. Zener diode disadvantages

Zener diode has a few limitations:

• Zener diodes are insufficient in operation for high-power devices.

• Zener diodes have a low regulation ratio compared to other semiconductor devices.

• Zener diodes operate only when the supply voltage is equal to or greater than the zener voltage.

• Zener diodes tend to waste electricity.

8. How to test a Zener diode

A digital multimeter can test a Zener diode for possible failures. You can use either an ohmmeter or a multimeter for the test.

Step 1: Set the multimeter to “diode”
Step 2: Touch the Zener diode’s anode with the positive multimeter lead.
Step 3: Touch the Zener diode’s cathode with the negative multimeter lead.
The reading should display 0.5 V to 0.7 V.
Step 4: Reverse the leads. Put the negative lead on the anode and the positive lead on the cathode.
The reading should display 0L or infinite.

The Zener diode must be connected in series with a resistor under reverse bias. The datasheet of the Zener diode states the Zener voltage of the device used. You can again touch multimeter leads for measuring the Zener voltage. The Zener diode is damaged if the obtained voltage does not match the Zener voltage.

Watch this video to see how to test a Zener Diode:
https://www.youtube.com/watch?v=of86B18H4c8&t=8s

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