Working Principle and Characteristics of Zener diodes

Catalog

I Principle of Zener Diodes

Zener diodes are specialized semiconductor diodes designed to operate reliably in the reverse breakdown region. By utilizing the reverse breakdown state of the PN junction, Zener diodes can maintain a nearly constant voltage across their terminals while the current varies within a wide range. This unique characteristic makes them indispensable as voltage regulators and voltage reference elements in electronic circuits.

A Zener diode is essentially a semiconductor device that exhibits very high resistance until it reaches a critical reverse breakdown voltage. At this critical point, the reverse resistance drops dramatically to a very low value. In this low-resistance region, current can increase significantly while the voltage across the diode remains remarkably stable. Zener diodes are categorized according to their breakdown voltage, which typically ranges from 2.4V to 200V, with specialized versions reaching up to 1kV.

Because of this voltage-stabilization characteristic, Zener diodes serve primarily as voltage regulators or voltage reference components. Multiple Zener diodes can be connected in series for applications requiring higher stable voltages, enabling precise voltage regulation across a wide range.

The forward characteristic of a Zener diode's volt-ampere characteristic curve is similar to that of an ordinary silicon diode. However, the reverse characteristic is distinctly different: when the reverse voltage is lower than the reverse breakdown voltage (V_Z), the reverse resistance is extremely high and the reverse leakage current is minimal. As the reverse voltage approaches the critical breakdown value, the reverse current increases sharply—a phenomenon called breakdown. At this critical breakdown point, the reverse resistance suddenly drops to a very small value. Although the current may vary over a large range, the voltage across the diode stabilizes near the breakdown voltage, thereby achieving voltage regulation.

The breakdown mechanism in Zener diodes occurs through two primary phenomena: Zener breakdown (dominant in diodes with V_Z < 5V) and Avalanche breakdown (dominant in diodes with V_Z > 5V). In Zener breakdown, intense electric fields cause direct rupture of covalent bonds, releasing charge carriers. In avalanche breakdown, accelerated charge carriers collide with atoms, creating an avalanche effect of additional charge carriers. Both mechanisms result in the characteristic sharp increase in reverse current at the breakdown voltage.

II IV Characteristics of Zener Diodes

Figure 1. Current-Voltage (IV) Characteristics of Zener Diodes

Zener diodes are operated in "reverse bias" or reverse breakdown mode, where the anode of the diode is connected to the negative supply terminal. As can be seen from the IV characteristic curve above, the reverse bias characteristic region of the Zener diode shows an almost constant negative voltage that remains independent of the current flowing through the diode. The voltage remains nearly constant even when the current varies considerably within the operating range.

The Zener current must be maintained between the minimum breakdown current I_Z(min) and the maximum rated current I_Z(max) for proper operation. Operating below I_Z(min) results in poor voltage regulation, while exceeding I_Z(max) can cause thermal damage to the device. Modern Zener diodes feature improved power dissipation characteristics and tighter voltage tolerances (typically ±2% to ±5%) compared to earlier generations.

This ability to maintain self-control over voltage makes Zener diodes invaluable for regulating or stabilizing voltage sources against fluctuations in power supply or load changes. The fact that voltage across the diode in the breakdown region remains almost constant has proven to be an essential feature for the simplest voltage regulator applications.

A voltage regulator should provide a constant output voltage to loads connected in parallel. Despite supply voltage fluctuations or load current variations, the Zener diode will continue to regulate the voltage effectively until the diode current falls below the minimum I_Z(min) value in the reverse breakdown region, at which point regulation is lost.

III Zener Diode Regulator

Zener diodes can be configured to produce a stable voltage output with low ripple under varying load current conditions. By shunting a small current from the voltage source through the diode via a suitable current-limiting resistor (R_S), the Zener diode will conduct sufficient current to maintain the voltage drop V_out at a constant level.

It's important to remember that the DC output voltage of a half-wave or full-wave rectifier contains ripple superimposed on the DC voltage, and the average output voltage varies with load changes. By connecting a simple Zener stabilizer circuit as shown below to the rectifier output, a more stable and regulated output voltage can be achieved.

Figure 2. Basic Zener Stabilizer Circuit

The resistor R_S is connected in series with the Zener diode to limit the current through the diode, and V_S is the input voltage supplied to this combination. The regulated output voltage V_out is taken across the Zener diode terminals. The cathode terminal of the Zener diode is connected to the positive rail of the DC power supply, so it operates in reverse bias and functions in its breakdown state. The series resistor R_S is chosen to limit the maximum current flowing through the circuit.

Without a load connected to the circuit, the load current will be zero (I_L = 0), and all circuit current passes through the Zener diode, which consequently dissipates its maximum power. When a load resistance R_L is connected, part of the current flows through the load. Selecting an appropriate series resistance value ensures that under no-load or high-impedance conditions, the Zener diode does not exceed its maximum rated power dissipation.

The load is connected in parallel with the Zener diode, so the voltage across R_L is always equal to the Zener voltage (V_R = V_Z). There is a minimum Zener current below which voltage stabilization becomes ineffective, and the Zener current must always remain above this value to operate properly in the breakdown region. The upper current limit depends on the device's power rating. The supply voltage V_S must always be greater than V_Z for proper operation.

One consideration with Zener diode stabilizer circuits is that the diode can sometimes generate electrical noise superimposed on the DC supply as it regulates the voltage. While this is typically not problematic for most applications, it may be necessary to add a large-value decoupling capacitor across the Zener output to achieve additional smoothing and noise reduction.

Zener diodes always operate under reverse bias conditions. A Zener diode can be used to design a voltage regulator circuit that maintains a constant DC output voltage across the load despite changes in input voltage or load current. The Zener voltage regulator consists of a current-limiting resistor R_S in series with the input voltage V_S, with the Zener diode connected in parallel with the load R_L under reverse bias conditions. The stable output voltage is always equal to the breakdown voltage V_Z of the diode.

Practical Design Example

A 5.0V stable power supply is needed from a 12V DC input. The Zener diode has a maximum rated power P_Z of 2W. Using the Zener regulator circuit above, calculate:

a) The maximum current flowing through the Zener diode:

b) The minimum value of series resistance R_S:

c) Load current I_L if a 1kΩ load resistor is connected across the Zener diode:

d) Zener current I_Z at full load:

IV Zener Diode Voltage

In addition to generating a single stable voltage output, Zener diodes can be connected in series with ordinary silicon signal diodes to produce various different reference voltage output values, as shown below.

Zener Diodes Connected in Series

Figure 3. Series Connection of Zener Diodes for Voltage References

The value of each Zener diode can be selected to suit the specific application requirements, while silicon diodes in forward bias consistently drop approximately 0.6-0.7V. The supply voltage V_in must, of course, be higher than the maximum output reference voltage required—in the example above, this would be 19V.

Common Zener diode series include the 500mW BZX55 series and the 1.3W BZX85 series. The nomenclature typically indicates the Zener voltage; for example, BZX55C7V5 designates a 7.5V Zener diode in the BZX55 series with ±5% tolerance (indicated by "C").

The 500mW series Zener diodes have voltage ratings ranging from approximately 2.4V to 100V, generally following the same sequence of values as the 5% (E24) resistor series. These compact yet highly useful diodes are available in discrete voltage ratings as shown in the table below.

V Zener Diode Clamp Circuit

So far, we have studied how Zener diodes regulate constant DC power supplies. But how do Zener diodes react to changing signals when the input is not a steady-state DC voltage but an AC waveform?

Diode limiting and clamping circuits are used to shape or modify input AC waveforms (or any sinusoidal signal) and produce different shaped output waveforms according to circuit configuration. Diode limiter circuits are also called clippers because they clip or cut off the positive (or negative) portion of the input AC signal. Since Zener clamp circuits limit or clip part of the waveform, they are primarily used for circuit protection or waveform shaping applications.

For example, if we want to clip the output waveform at +7.5V, we would use a 7.5V Zener diode. If the output waveform attempts to exceed the 7.5V limit, the Zener diode will "clip" the excess voltage from the input, producing a waveform with a flat top while maintaining the output constant at +7.5V. Note that under forward bias conditions, the Zener diode still behaves as a standard diode. When the AC waveform output drops below -0.7V, the Zener diode will "conduct" like any normal silicon diode and limit the output to approximately -0.7V, as illustrated below.

Figure 4. Zener Diode Clamp Circuit Configuration

Back-to-back connected Zener diodes can function as an AC voltage regulator, producing what is sometimes nicknamed a "Poor Man's Square Wave Generator." With this configuration, we can clip the waveform symmetrically between a positive value of +8.2V and a negative value of -8.2V when using two 7.5V Zener diodes (accounting for the 0.7V forward drop).

For applications requiring clipping between two different minimum and maximum values—such as +8V and -6V—we simply use two Zener diodes with different voltage ratings. Note that the output limits the AC waveform to between +8.7V and -6.7V due to the forward-biased diode voltage drop of 0.7V added in each direction.

In other words, the peak-to-peak voltage becomes 15.4V instead of the expected 14V because the forward bias voltage drop across each diode adds 0.7V in both directions.

This type of limiter configuration is quite common for protecting electronic circuits from overvoltage conditions. Two Zener diodes are typically placed across power input terminals. During normal operation, one of the Zener diodes remains "off" and has minimal effect on the circuit. However, if the input voltage waveform exceeds its safe limit, the appropriate Zener diode turns "on" and clamps the input voltage to protect the circuit from damage.

VI Applications of Zener Diodes

1. Typical Series Regulator Circuit

Figure 5. Typical Series Regulator Circuit Using Zener Diode and Transistor

In this circuit, the base of transistor T is stabilized at 13V by Zener diode D. Consequently, its emitter outputs a constant voltage of approximately 13V - 0.7V = 12.3V (accounting for the base-emitter voltage drop). Within a certain operating range, the output voltage remains constant regardless of whether the input voltage increases or decreases, or whether the load resistance changes. This circuit configuration is widely used in numerous applications.

The 78xx series represents integrated voltage regulator circuits that provide various fixed output voltages. For example, the 7805 outputs 5V, while the series ranges from 7805 to 7824, providing output voltages from 5V to 24V. These regulators find applications in countless electronic devices and have been industry standards for decades. However, modern alternatives include Low Dropout (LDO) regulators such as the AMS1117 series, which offer improved efficiency and lower dropout voltages, making them more suitable for battery-powered and energy-conscious applications.

Figure 6. 7805 Series Integrated Voltage Regulator Circuit

2. Overvoltage Protection Circuit in Television Systems

Figure 7. Overvoltage Protection Circuit in Television Applications

In this circuit, 115V represents the main power supply voltage for the television. When the power supply output voltage exceeds safe limits, Zener diode D conducts, turning on transistor T. The collector potential changes from its original high level (5V) to a low level. This voltage change, transmitted through the standby control line, places the television into standby protection mode, preventing damage to sensitive components.

While traditional CRT televisions used such circuits extensively, modern flat-panel displays and smart TVs continue to employ similar Zener-based protection schemes, adapted for their lower-voltage digital circuitry and more sensitive components.

3. Arc Suppression Circuit

Figure 8. Arc Suppression Circuit for Inductive Load Protection

When an appropriately rated Zener diode is connected in parallel with an inductor coil (alternatively, a standard diode can be used with different characteristics), and the coil current is interrupted while in the "on" state, the high voltage generated by the release of electromagnetic energy is absorbed by the diode. Therefore, when the switch is turned off, arcing at the switch contacts is eliminated or significantly reduced.

This application circuit is widely used in industrial settings, particularly in high-power electromagnetic control circuits such as relay drivers, solenoid valve controllers, and motor control systems. The arc suppression not only extends the life of mechanical contacts but also reduces electromagnetic interference (EMI) that could affect nearby sensitive electronic equipment.

VII Modern Applications and Future Trends

IoT and Smart Devices

As of 2024-2025, Zener diodes have found increasing adoption in Internet of Things (IoT) devices and smart sensors. The global Zener diode market was valued at over USD 850 million in 2023 and is projected to grow at a CAGR of approximately 6-7.8% through 2032. This growth is driven primarily by:

• IoT Device Protection: Low-power Zener diodes protect sensitive microcontroller inputs in battery-powered IoT sensors, wearables, and smart home devices from voltage spikes and electrostatic discharge (ESD).

• Energy Harvesting Systems: Zener diodes regulate voltage from solar panels, piezoelectric generators, and RF energy harvesting circuits used in self-sustainable IoT systems.

• Automotive Electronics: Modern vehicles contain numerous electronic control units (ECUs) that rely on Zener-based voltage regulation and transient voltage suppression for reliable operation in harsh automotive environments.

Renewable Energy Systems

Zener diodes play crucial roles in renewable energy applications, particularly in solar photovoltaic systems and wind turbine generators. They regulate output voltages from variable renewable sources, ensuring consistent power delivery and protecting downstream electronics. The integration of Zener-based protection in maximum power point tracking (MPPT) controllers has become standard practice in modern solar installations.

Advanced Packaging Technologies

Modern Zener diodes are increasingly available in surface-mount device (SMD) packages, including ultra-small form factors like SOT-23, SOT-323, and SC-70. The SMD Zener diode market is expected to reach USD 2 billion by 2033, driven by miniaturization trends in consumer electronics, mobile devices, and wearable technology. These compact packages enable higher circuit density and improved high-frequency performance.

Enhanced Specifications

Contemporary Zener diodes feature improved characteristics compared to earlier generations:

• Lower Temperature Coefficients: Modern devices exhibit better voltage stability across temperature ranges, critical for precision reference applications.

• Tighter Tolerances: Availability of ±1% and ±2% tolerance devices alongside standard ±5% options enables more precise voltage regulation without external trimming.

• Reduced Noise: Advanced manufacturing processes have reduced noise generation, making modern Zener diodes more suitable for sensitive analog and measurement applications.

• Higher Power Ratings: New series like the SMBJ (2W) and larger packages provide increased power handling capability in compact form factors.

⚠️ Important Note:

While Zener diodes remain widely used for voltage regulation, designers should also consider modern alternatives such as Low Dropout (LDO) regulators, switching regulators, and voltage reference ICs for applications requiring higher efficiency, lower noise, or more precise voltage regulation. The choice depends on specific application requirements including load current, input-output voltage differential, efficiency needs, and cost constraints.

📚 Recommended Further Reading

→ Introduction to Types of Diodes

→ What are Laser Diodes?

📝 Article Update Information

Original Publication: 2020

Last Updated: October 2025

Update Summary:

• Corrected technical terminology and improved explanations of Zener breakdown mechanisms

• Updated market data and specifications reflecting 2024-2025 industry standards

• Added comprehensive section on modern applications including IoT devices, renewable energy systems, and automotive electronics

• Included information on advanced packaging technologies (SMD, SOT-23, SC-70)

• Referenced contemporary alternatives including LDO regulators and switching regulators

• Enhanced tables with improved formatting and readability

• Corrected grammatical errors and improved technical accuracy throughout

• Added references to current market trends and growth projections (2024-2032)

• All original images and diagrams have been preserved

This article continues to serve as a comprehensive resource for understanding Zener diodes, now updated with the latest technical information, industry trends, and modern applications relevant to today's electronics industry.