Voltage spikes from ESD, switching loads, or nearby lightning can damage circuits. An avalanche diode prevents this by working safely in reverse breakdown and clamping the voltage when it reaches its breakdown level. This article explains avalanche breakdown, internal structure, Zener comparison, specs, main types, uses, selection, and common failures in detail.
Avalanche Diode Basics
An avalanche diode is a PN junction diode designed to operate safely in reverse breakdown mode. When reverse voltage reaches its rated breakdown voltage (VBR), the diode suddenly conducts a large reverse current. Unlike standard diodes that may be damaged in breakdown, avalanche diodes are built to handle this behavior safely if current and power remain within rated limits.
Avalanche diodes are widely used for surge protection and voltage clamping in circuits exposed to transient spikes such as ESD events, inductive switching surges, and lightning-induced disturbances.
Avalanche Breakdown in the Avalanche Diode
Avalanche breakdown occurs when a reverse-biased diode experiences a strong electric field across its depletion region. This field accelerates free carriers until they collide with atoms in the crystal lattice, releasing additional electrons and holes. These new carriers also accelerate and collide, creating a chain reaction known as impact ionization.
As a result, the diode current rises rapidly while the voltage remains nearly constant, allowing the device to clamp excess voltage. Avalanche diodes are designed so this breakdown spreads evenly across the junction to reduce overheating and prevent localized damage.
Internal Structure of the Avalanche Diode
• Built on a silicon chip with a PN junction that is designed to work in reverse voltage.
• The junction is lightly doped, so the empty (depletion) region becomes wide when reverse biased.
• A wide depletion region lets the diode enter avalanche breakdown at higher voltages instead of using Zener breakdown at low voltages.
• The edges of the junction are shaped and treated so the electric field stays even and does not form sharp high-field spots.
• The chip is mounted on a lead frame or pad that carries current and helps remove heat during surge conditions.
• The avalanche diode is sealed in a glass, plastic, or metal package that matches its power level and working environment.
Avalanche Diode and Zener Diode Comparison
| Feature | Avalanche Diode | Zener Diode |
|---|---|---|
| Main breakdown effect | Avalanche effect caused by impact ionization | Zener effect caused by tunneling |
| Doping level | Lightly doped PN junction | Heavily doped PN junction |
| Depletion region | Wide depletion region | Thin depletion region |
| Typical voltage range | Commonly used above about 6–8 V | Used below about 6–8 V |
| Temperature behavior | Breakdown voltage usually increases with temperature | Breakdown voltage often decreases with temperature |
| Main use | Surge and spike protection, voltage clamping | Low-voltage regulation and voltage reference |
| Energy handling | Can handle higher surge energy for short durations | Handles lower energy compared to avalanche types |
Electrical Specifications of the Avalanche Diode
| Parameter | Meaning | Importance |
|---|---|---|
| Breakdown voltage (VBR) | Reverse voltage where avalanche starts | Sets the point where the diode begins strong conduction |
| Clamping voltage (VCL) | Voltage during a surge at a given current | Shows how high the line can rise during a spike |
| Peak pulse current (IPP) | Highest surge current for a stated pulse shape | Must be higher than the worst surge in the circuit |
| Peak pulse power (P) | Highest surge power for a short pulse | Helps choose a diode that can handle surge energy |
| Reverse leakage (IR) | Small reverse current below breakdown | Affects small standby losses and leakage paths |
| Junction capacitance (CJ) | Capacitance when reverse-biased | Important for high-speed and RF signal lines |
| Response time | Time to start clamping a fast transient | Important for ESD and very sharp voltage spikes |
Avalanche Diode Types and Their Uses
TVS (Transient Voltage Suppression) Diodes
TVS diodes are the most common avalanche diodes used for surge and ESD protection. They clamp voltage spikes quickly to protect sensitive components on power and signal lines.
High-Power Avalanche Rectifier Diodes
These are rectifier diodes designed to survive controlled avalanche under reverse stress, helping them withstand switching spikes in power electronics when used correctly.
IMPATT Microwave Avalanche Diodes
IMPATT diodes use avalanche breakdown plus transit-time effects to generate microwave-frequency oscillations in specialized RF systems.
Noise Avalanche Diodes
These are biased intentionally in avalanche breakdown to create stable broadband electrical noise for testing and random signal generation.
Avalanche Photodiodes (APDs)
APDs use avalanche multiplication to amplify light-generated current, improving sensitivity in low-light detection applications.
Avalanche Diode Surge Protection
In surge protection circuits, avalanche diodes are often called TVS (Transient Voltage Suppressor) diodes. They are usually connected in reverse between a line and ground, or between a line and the supply voltage. During normal operation, the line voltage stays below the breakdown level, so the avalanche diode only has a tiny leakage current.
When a surge or spike pushes the line voltage above the breakdown voltage, the avalanche diode goes into breakdown and starts to conduct strongly. This action clamps the voltage and steers the surge current away from sensitive parts and toward ground. Once the spike is over and the voltage drops back below the breakdown level, the avalanche diode stops conducting and returns to its normal, non-conducting state.
Avalanche Diodes in RF and Microwave Signals
Some avalanche diodes are made specially for RF and microwave circuits. In devices like IMPATT diodes, avalanche breakdown, and the time it takes the charge carriers to move through the depletion region create a delay. This delay causes a phase shift that can look like negative resistance at high frequencies.
When this type of avalanche diode is placed in a tuned circuit or resonant cavity, the negative resistance can keep high-frequency oscillations going, even up to microwave ranges. These diodes are used in radar blocks, local oscillator stages, and some test instruments. They can be quite noisy, so they must be biased and cooled carefully to stay stable and within safe limits.
Avalanche Diode as a Noise Source
• When the avalanche diode is biased in the avalanche region, it creates random current pulses from impact ionization.
• These many small pulses combine into a broadband noise signal that covers a wide range of frequencies.
• This noise can be amplified and used as a test signal for receivers, filters, and other circuits.
• It can also act as an entropy source in hardware random number generators.
• The bias voltage and current must be carefully controlled so the diode stays in a stable avalanche region and does not overheat.
Avalanche Photodiodes Using Avalanche Diode Action
An avalanche photodiode (APD) is a light sensor that uses avalanche breakdown to internally amplify the photocurrent. When photons strike the active region, electron–hole pairs are generated. Since the APD is biased near breakdown, these carriers accelerate and trigger impact ionization, multiplying the output current. This internal gain makes APDs useful for detecting weak light signals in:
• Fiber optic communication
• LiDAR and distance sensing
• Medical imaging and photometry
To keep gaining stability, APDs require bias control and temperature compensation, since breakdown voltage shifts with temperature.
Selecting Avalanche Diodes for Different Circuit Needs
| Design Need | Focus | Parameters |
|---|---|---|
| DC power line protection | Clamp surges while keeping normal voltage OK | VBR vs normal voltage, VCL, IPP, PPP |
| High-speed data line ESD | Very fast action and low capacitance | Low CJ, fast response, ESD rating |
| High-energy surge on cables | Handle very large surge energy | High PPP / energy rating, IPP, package |
| RF noise source | Strong and steady noise in an avalanche | Stable breakdown region, bias range |
| APD / SPAD light sensing | High gain with low dark current | Gain vs bias, dark current, temperature behavior |
Avalanche Diode Reliability and Common Failures
Thermal Overload
A single surge above rating can overheat the junction and permanently damage the diode.
Long-Term Cumulative Stress
Repeated smaller transients can gradually shift the breakdown voltage or raise the leakage current.
Current Crowding and Hot Spots
Poor PCB layout or incorrect diode selection may cause uneven conduction, increasing failure risk.
Environmental Stress
Moisture, vibration, and thermal cycling can degrade packaging and lead to integrity issues.
Good practice for long life
To improve reliability, it helps to derate surge current and energy, use enough copper area for heat spreading, and follow the limits and surge standards when placing and choosing the avalanche diode.
Conclusion
Avalanche diodes clamp voltage spikes by entering controlled reverse breakdown at a set breakdown voltage. Basic factors include breakdown voltage, clamping voltage, peak pulse current and power, leakage current, capacitance, and response time. Types include TVS, avalanche rectifiers, IMPATT, noise diodes, and photodiodes. Reliability depends on heat, repeated stress, layout, and environment.
Frequently Asked Questions [FAQ]
What surge waveform rating should I check for an avalanche diode?
Check the diode’s rated pulse waveform (example: 8/20 µs or 10/1000 µs) and make sure it matches your surge source.
What is the difference between unidirectional and bidirectional TVS diodes?
Unidirectional is best for DC lines. Bidirectional is best for AC lines or signals that swing both ways.
What does VRWM mean in a TVS avalanche diode?
VRWM is the maximum voltage the diode can handle continuously without turning on.
Why is low capacitance required for high-speed signal protection?
High capacitance can distort fast signals. Low-capacitance TVS diodes protect the line without slowing it down.
Where should I place an avalanche diode on a PCB?
Place it as close as possible to the connector or surge entry point with a short, direct ground path.
How do I know if an avalanche diode is damaged?
Signs include higher leakage, heating during normal operation, or weaker clamping during surges.