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Zener vs Avalanche Breakdown: Key Differences Explained

Both are breakdown phenomena in which current rises suddenly when a large reverse voltage is applied to a PN junction, but the mechanisms differ. Zener breakdown occurs in heavily doped junctions with a very thin depletion layer, where valence-band electrons tunnel through to the conduction band. Avalanche breakdown occurs when carriers accelerated by a strong electric field create electron-hole pairs by impact ionization, and these multiply in a chain reaction. In silicon, Zener breakdown dominates at breakdown voltages below about 5 V and avalanche breakdown above about 6 to 7 V, and the two have temperature coefficients of opposite sign.

AspectZener breakdownAvalanche breakdown
MechanismTunneling. Valence-band electrons pass through a thin barrier into the conduction bandImpact ionization. Carriers accelerated by the field create new electron-hole pairs, which multiply in a chain
Junction dopingVery high on both sidesLow on at least one side. The lower the doping, the higher the breakdown voltage
Depletion widthVery thin (about 10 nm or less)Wide. Carriers need distance to accelerate
Breakdown field (silicon, typical)A very strong field of about 10⁶ V/cmAbout 3×10⁵ V/cm (depends on doping)
Breakdown voltage (silicon, typical)About 5 V or less. Between about 5 V and about 6 to 7 V, both mechanisms occur togetherAbout 6 to 7 V or more. High-voltage junctions of hundreds to thousands of V are also set by this breakdown
Temperature coefficient of breakdown voltageNegative. As temperature rises the bandgap shrinks and the breakdown voltage fallsPositive. As temperature rises, scattering by lattice vibrations increases and the breakdown voltage rises
Sharpness of breakdownRelatively gradualAbrupt
NoiseRelatively lowHigh, from avalanche multiplication
Main usesVoltage-regulator diodes of about 5 V or less, low-voltage references and clampsVoltage-regulator diodes of about 6 V or more, surge-suppression diodes, avalanche photodiodes, SPADs

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

Zener breakdown
When you need to hold a voltage at a low level of about 5 V or less. Used in low-voltage regulator diodes and in clamps that limit input voltage.
Avalanche breakdown
For regulated voltages of about 6 V or more, or to protect circuits from surges. Avalanche photodiodes and SPADs, which detect light by multiplying it, use this breakdown deliberately. In power devices it is designed in as the phenomenon that sets the breakdown voltage.
Combined
In regulator diodes with a breakdown voltage of about 5 to 6 V, the negative temperature coefficient of Zener breakdown and the positive coefficient of avalanche breakdown cancel out, so the voltage changes little with temperature. When a temperature-stable reference is needed, choose parts in this range or temperature-compensated types.

FAQ

Do all Zener diodes use Zener breakdown?

No. Voltage-regulator diodes are customarily called Zener diodes, but in silicon those of about 6 to 7 V or more mainly use avalanche breakdown. Zener breakdown by tunneling dominates only in low-voltage parts of about 5 V or less.

Why do the temperature coefficients have opposite signs?

In Zener breakdown, a rising temperature shrinks the bandgap and makes tunneling easier, so the breakdown voltage falls. In avalanche breakdown, a rising temperature increases scattering by lattice vibrations, making it harder for carriers to gain the energy needed for impact ionization, so the breakdown voltage rises.

Is a diode destroyed when it breaks down?

Neither type of breakdown damages the junction by itself, and the diode recovers if the current is limited. It fails when the heat from current and voltage during breakdown is too great. In regulator circuits, a resistor is placed in series with the diode to limit the current.

How is avalanche breakdown handled in power devices?

The breakdown voltage of power MOSFETs and diodes is mainly set by the voltage at which avalanche breakdown occurs. Making the drift layer more lightly doped and thicker raises the breakdown voltage but also increases on-resistance. A device can enter the breakdown region, for example when an inductive load is switched off, and the energy it can withstand there is specified as its avalanche ruggedness.

Check your understanding

Which phenomenon does a voltage-regulator (Zener) diode mainly use?

  1. AForward diffusion current
  2. BBreakdown under reverse bias
  3. CElectromotive force from absorbed light
Show answer and explanation

Answer: B. Breakdown under reverse bias

A Zener (voltage-regulator) diode uses the fact that once the reverse voltage reaches breakdown, current rises sharply while the voltage stays nearly constant. The breakdown voltage is set by doping, and commercial parts span roughly 2–200 V. Below about 5 V the mechanism is mainly tunneling (Zener) breakdown; above it, mainly avalanche breakdown. They are used for voltage references and overvoltage protection. Generating a voltage from absorbed light is the photovoltaic effect, used in solar cells and photodiodes.

Which correctly describes the mechanism of avalanche breakdown?

  1. ACarriers accelerated by a high field collide with the lattice and create new electron-hole pairs, which multiply in a chain reaction
  2. BElectrons tunnel quantum-mechanically through a thin barrier
  3. CThe junction melts as the temperature rises
Show answer and explanation

Answer: A. Carriers accelerated by a high field collide with the lattice and create new electron-hole pairs, which multiply in a chain reaction

In avalanche breakdown, carriers accelerated by the strong depletion-region field strike atoms and create electron-hole pairs (impact ionization); these are accelerated in turn, and the current multiplies like an avalanche. Silicon's critical field is about 3×10⁵ V/cm, and lightly doped junctions with wide depletion regions break down at higher voltages. SiC's critical field is about ten times higher, so it achieves the same voltage rating with a thinner drift layer, giving lower-loss power devices.

In a very heavily doped PN junction, what is the main mechanism of breakdown at low reverse voltage?

  1. AAvalanche breakdown
  2. BZener breakdown due to quantum-mechanical tunneling
  3. CMelting of the junction due to heat
Show answer and explanation

Answer: B. Zener breakdown due to quantum-mechanical tunneling

In a junction heavily doped on both sides the depletion region is very thin (about 10 nm or less), and under the strong field valence-band electrons tunnel through the barrier into the conduction band. This is Zener breakdown; it occurs at low breakdown voltages, roughly below 5 V, and its breakdown voltage falls as temperature rises (because the gap shrinks). Higher breakdown voltages are mainly avalanche, whose temperature coefficient is positive.

  1. How does the avalanche breakdown voltage generally change as temperature rises?
  2. Zener diodes with a Zener voltage of about 5 to 6 V show little change in voltage with temperature. Why?
  3. How does the breakdown voltage of a PN junction generally change when the impurity concentration on one side is lowered?

Practice PN junctions and diodes questions

Key terms explained

Zener diode
A diode that provides a constant voltage by using the nearly fixed reverse breakdown voltage.
Avalanche breakdown
A phenomenon in which carriers accelerated by a strong reverse field strike atoms and create new carriers, which multiply in a chain reaction so that a large current flows.
Depletion layer
A region near a PN junction boundary that contains almost no carriers. It widens under reverse bias.
SPAD (single-photon avalanche diode)
A photodetector biased above breakdown so that even a single photon triggers a large avalanche pulse. It is used for ranging, such as in LiDAR.

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