Zener Diode and VI Characteristics of Zener Diode

Zener Diode 

A Zener diode is a special diode that operates in the reverse breakdown region. Normal diodes are used for rectification amplification, Zener diodes, on the other hand, work at the voltage called zener voltage. Such Diodes operate at Zener breakdown. 

This breakdown voltage dependence allows the zener diode to conduct current in the reverse bias region, exhibiting a constant voltage drop across its terminals. 

For these characteristics, zener Diodes are used in 

 

voltage regulation

voltage reference

protection against voltage surges

Zener diode image from Wikipedia

How Zener Diode is designed and operated? 

A Zener diode is similar in structure to a PN-junction diode, but it’s so designed to operate in the breakdown region. 

The diode is made from a heavily doped p-n junction, where the p-type region is heavily doped with impurities containing trivalent atoms and the n-type region is doped with impurities containing pentavalent atoms. 

Since the Diode Regions are heavily doped, the depletion region becomes thin compared to a normal diode which enhances the impact of the electric field.

When a Zener diode is in forward bias, it behaves as the ordinary Diode. 

The majority charge carriers (electrons in n-region and holes in p-region) flow towards the junction, and create forward current. 

When the diode is connected in the reverse bias and when the applied voltage exceeds a certain threshold called the Zener voltage (Vz), the depletion region widens due to the high electric field strength. 

This causes a sudden increase in the electric field and allows the reverse current to flow, leading to the Zener breakdown phenomenon.

Zener Breakdown Types

The Zener breakdown can occur through two mechanisms: Zener breakdown and avalanche breakdown.

1. Zener breakdown:

The Diodes with the breakdown voltage less than 5.6 undergo a zener breakdown. During such a breakdown, the electric field in the p-n junction enables the valence band electrons to tunnel across the depletion region into the conduction band of the p-type region. 

This generates electron-hole pairs that lead to an increase in the reverse current through the diode.

2. Avalanche breakdown: 

 For voltages above 5.6 volts, zener Diodes undergo avalanche breakdown.  

In such a breakdown, high-energy electrons gain enough energy through the electric field that they can liberate valence band electrons by collision. 

These free electrons, now, under the applied electric field accelerate and collide with other atoms, freeing more electrons. 

This process creates an avalanche effect, and the reverse current is increased vigorously. 

VI Characteristics of Zener Diode

The VI characteristics of a Zener diode is the behavior with Diode voltage vs current through the Diode curve of this Diode when operated in reverse bias. show its behavior in the reverse bias region. 

Zener diode image from Wikipedia

These characteristics tell the breakdown voltage and behavior of a Zener Diode when the threshold voltage is crossed in reverse biased. 

Here there are two regions:

the breakdown or the Zener region 

    and

the leakage or knee region.

1. Breakdown or Zener Region 

This is the Diode operation region when the zener voltage has been crossed. 

Here a small change in voltage changes current significantly. 

The voltage across the diode (Vz) remains relatively constant within a narrow range, regardless of the magnitude of current flowing through it. 

This characteristic of zener Diode is used in voltage regulator applications.

2. Leakage or Knee Region: 

The knee region lies before the Zener breakdown voltage.

Here with the change in voltage, a gradual change in current happens.  

Here the diode behaves like a regular diode with a forward characteristic curve.

The current increases exponentially with the voltage that has been applied until the zener voltage is reached. Then the Diode enters the zener region.

Applications of Zener Diodes

Following are zener Diode Applications,

1. Voltage Regulation:

 When a Zener Diode is connected in reverse parallel to the load resistance, a constant voltage can be maintained across the load with varying input voltages as well. 

This helps in power supplies and also in stabilizing circuits.

2. Zener Diode as a Voltage Reference: 

Zener diodes are utilized as voltage references in precision electronic circuits. 

When a zener Diode with a specific zener voltage is selected, a stable and accurate reference voltage is obtained.

Such a reference ensures consistent performance of associated components.

3.  Protection against overvoltages: 

Im voltage surges and transients, Zener Diodes help protection of the circuit. 

The Zener Diode in parallel with sensitive components provides a path of low resistance for greater magnitude voltage. This limits the voltage range the circuit could reach. 

In Conclusion,

Zener diodes are specialized components with unique electrical characteristics that make them ideal for voltage regulation, voltage reference, and overvoltage protection applications.