Zener Diode – The Ultimate Beginner Guide

What is Zener Diode

A Zener diode is a special type of semiconductor diode designed to operate in the reverse-biased breakdown region. Zener diode maintains a nearly constant dc voltage while operated in proper conditions. Zener diodes are silicon based discrete semiconductor diodes that allow electric current to flow bidirectionally – either reverse or forward. It has a silicon PN junction that conducts in reverse direction once the voltage threshold condition is properly satisfied.

The specific voltage known as Zener voltage, is the level at which it starts conducting current in the reverse direction. This unique feature of Zener diode enables it to regulate voltage in electronic circuits. When the applied voltage exceeds the Zener voltage, the diode conducts and keeps the voltage stable, preventing it from exceeding the Zener voltage. This makes Zener diodes ideal for voltage regulation, surge protection, and reference voltage applications in various electronic devices.

Zener Diode Operation

To understand the zener diode operation, it is important to understand definition of bias, and difference between the forward and reverse bias.

Forward bias: To bias a Zener diode, a dc voltage source is applied across it. Forward bias in the world electronics is the state in which current is allowed to pass through the pn junction. During forward bias negative terminal of battery is connected to n region and positive terminal of battery is connected to p region.

Reverse bias: During reverse bias, the positive terminal of voltage source is connected to n-region of diode and negative terminal of source is connected to p-region. The reverse bias prevents current flow across diode.

When forward biased, Zener diodes are identical to conventional diodes. They have a voltage drop of 0.3-0.7 V.

However when reverse biased, a small leakage current flow exists. As reverse voltage increases to the set breakdown voltage, current starts to flow through the diode. As current reaches to a maximum value it stabilizes and remain constant over a wide range of applied voltage. The voltage remains constant despite of the amperes passing through diode. The same voltage will remain there provided that the current stays between maximum and breakdown value.

As described earlier, zener diodes are designed to operate in reverse breakdown. In this regards. two types of reverse breakdown in a zener diode are avalanche and zener.

Zener breakdown

Zener breakdown refers to situation when reverse bias voltage between 2 V and 5 V exists. For that, electric field intensity is quite sufficient to put on force to the valence electrons, separating them from the nuclei. As a result mobile electron-hole pairs are formed which increase current flow. Zener breakdowns are more evident for highly-doped diodes that have a large electric field and low breakdown voltage. As temperature rises, greater amount of energy is attained by the valence electrons, thereby requiring less outward voltage.

Avalanche Breakdown

Zener diodes with breakdown voltages greater than approximately 5 V operate predominately in avalanche breakdown. Avalanche breakdown is prominent for light-doped zeners that possess large breakdown voltage. Unlike zener breakdown, the avalanche breakdown voltage rises alongside temperature. Electrons moving through the diode hit with electrons in the covalent bond thereby disrupting it. With an increase in voltage, the velocity of electrons also increases, this implies that the covalent bonds can be disrupted more easily.

Zener Diode Symbol

Zener Diode Specifications

Given below are top 10 basic specifications related to Zener diode.

• Zener Voltage (VZ): The desired voltage at which a Zener diode is designed to operate in the breakdown region, providing a stable reference voltage. It ranges between 2 to 200 volts and varies based on the specific type of diode.
• Zener Test Current (IZT): The specified current at which the Zener voltage is measured and guaranteed to be within the specified tolerance.
• Zener Power Dissipation (PD): The maximum power that a Zener diode can safely dissipate without exceeding its specified temperature limits.
• Zener Impedance (Z_Z): The dynamic resistance of the Zener diode at a particular operating point, indicating the change in voltage with respect to the change in current.
• Zener Temperature Coefficient (TCZ): The rate at which the Zener voltage changes with respect to temperature. It indicates the stability of the Zener diode over a range of temperatures.
• Reverse Leakage Current (IR): The maximum current that flows through the Zener diode when it is reverse biased and operating below the breakdown voltage.
• Zener Reverse Voltage (VR): The maximum reverse voltage that a Zener diode can withstand without entering the breakdown region and damaging the device.

Practical Applications of Zener Diode

Given below are top 5 practical applications of Zener Diode:

1. Overvoltage Protection: Zener diodes can be used as voltage clamps to protect sensitive electronic components from excessive voltage spikes. When the voltage exceeds the Zener breakdown voltage, the diode conducts, diverting the excess current and limiting the voltage across the protected circuit.
2. Signal Clipping and Clamping: Zener diodes can be used for signal conditioning in electronic circuits. In clipping applications, they limit the amplitude of a signal by selectively conducting at a certain voltage level. In clamping circuits, they shift the DC level of a signal to a desired voltage, useful in AC coupling and biasing applications.
3. Voltage Reference and Precision Measurement: Zener diodes are utilized as voltage references in precision measurement circuits. Their stable and accurate voltage output provides a known reference for calibration and measurement purposes, ensuring the accuracy of the measurement equipment.
4. Voltage Regulation: Zener diodes are widely used in voltage regulation circuits to provide a stable reference voltage. By connecting a Zener diode in parallel with a load, it maintains a constant voltage across the load even when the input voltage fluctuates.
5. Voltage Stabilization: Zener diodes are employed in voltage stabilization circuits to maintain a constant voltage across a load despite changes in the input voltage. This is particularly useful in power supply circuits, where a stable voltage is crucial for proper operation.

FAQs about Zener Diode

What are the applications of Zener diodes?

Zener diodes have various applications, including voltage regulation, overvoltage protection, voltage stabilization, signal clipping and clamping, and voltage reference in precision measurement circuits.

What is the difference between a Zener diode and a regular diode?

The main difference lies in their behavior in the reverse-biased region. Regular diodes have a high breakdown voltage and are designed to operate in the forward-biased region, while Zener diodes have a low breakdown voltage and are specifically designed for reliable operation in the reverse-biased region.

Can Zener diodes handle high power?

Zener diodes have power ratings specified by manufacturers, and they can handle a range of power levels depending on their design. However, for high-power applications, additional cooling methods or alternative voltage regulation techniques may be more suitable.

What is the effect of temperature on Zener diodes?

Temperature can affect the Zener breakdown voltage and stability of a Zener diode. Zener diodes have a temperature coefficient specified by manufacturers, indicating the rate at which the Zener voltage changes with temperature. It’s important to consider temperature variations for accurate voltage regulation.

How are Zener diodes identified and categorized?

Zener diodes are identified by their breakdown voltage (VZ) and power ratings. They are categorized into different voltage ranges, such as 3.3V, 5V, 12V, etc., to meet specific application requirements.

Can Zener diodes be used in AC circuits?

Zener diodes can be used in AC circuits, but proper circuit design is necessary. Additional components, such as rectifiers and filtering elements, may be required to convert AC signals to DC before applying them to the Zener diode for voltage regulation or protection purposes.

Can Multiple Zener Diodes be connected in Series configuration?

Yes, it is possible to connect multiple Zener diodes in series. When connected in series, the Zener diodes will share the applied voltage. This configuration can be useful in situations where a higher breakdown voltage is required than what a single Zener diode can provide.

What are some considerations that should be followed while connecting Zeners in Series

Given below are 4 most important considerations for Zener in series:

1. Voltage Distribution: The applied voltage will divide across the Zener diodes based on their breakdown voltages. It is important to ensure that the voltage distribution is appropriate for the desired operation of each Zener diode.
2. Current Matching: The Zener diodes in series should have similar characteristics, particularly in terms of their dynamic impedance and temperature coefficients. This helps ensure that the current is evenly distributed among the diodes.
3. Power Dissipation: The power dissipation across each Zener diode must be considered. The total power dissipation should not exceed the maximum power rating of the individual Zener diodes to avoid overheating and potential failure.
4. Reverse Biasing: The Zener diodes should be connected in reverse bias with the anode of one diode connected to the cathode of the next diode. This ensures that the breakdown voltage is properly utilized.

Can we Connect Multiple Zener Diodes in parallel

Although, theoretically it is possible to connect multiple Zener diodes in parallel. When connected in parallel, each Zener diode will experience the same applied voltage. This configuration can be useful in situations where a higher current handling capability is required than what a single Zener diode can provide. However practically connecting Zener diodes in parallel connection should be avoided. Even Zener models having same product name on sheet have variations in electrical characteristics such as Zener voltage. As a result current might concentrate in one of the Zener diodes, resulting in characteristic deterioration or irreversible damage to diode.