Zener Diode: Zener Breakdown and Avalanche Breakdown

Every reverse-biased p-n junction eventually breaks down and conducts hard once the reverse voltage is large enough. Zener diodes exploit this deliberately, using controlled reverse breakdown as the operating mechanism rather than a failure mode. But "breakdown" isn't one phenomenon — depending on doping and voltage range, it's either Zener breakdown or avalanche breakdown, and the distinction matters for temperature compensation, noise, and choosing the right part for a reference or clamp design.

Two Distinct Physical Mechanisms

Both mechanisms produce the same qualitative I-V curve (near-vertical current rise at a threshold reverse voltage), but they arise from different physics and behave differently with temperature.

Zener breakdown (quantum tunneling):

• Occurs in heavily doped junctions, where the depletion region is very narrow (tens of nm or less).
• A narrow depletion region under reverse bias creates an extremely high electric field (can exceed 10⁶ V/cm) across a very short distance.
• This field is strong enough to directly rip electrons out of covalent bonds on the p-side valence band, tunneling them through the forbidden gap into the n-side conduction band — no collision or impact ionization needed.
• Dominant for breakdown voltages below roughly 5–6 V.
• Negative temperature coefficient: as temperature rises, the bandgap narrows slightly and tunneling probability increases, so breakdown voltage decreases with temperature.

Avalanche breakdown (impact ionization):

• Occurs in lightly doped junctions with a wider depletion region.
• Thermally generated carriers crossing the depletion region are accelerated by the field and gain enough kinetic energy to collide with lattice atoms, knocking loose additional electron-hole pairs (impact ionization). Each of those carriers is accelerated and can ionize further atoms — an exponentially multiplying chain reaction.
• Dominant for breakdown voltages above roughly 6–8 V.
• Positive temperature coefficient: higher temperature increases lattice vibration (phonon scattering), which reduces the mean free path between collisions, so carriers need a higher field (higher voltage) to gain enough energy to ionize — breakdown voltage increases with temperature.

Between roughly 5 V and 7 V, both mechanisms are active simultaneously and partially cancel each other's temperature dependence — which is exactly why 5.6 V and 6.2 V "Zener" diodes are popular as low-tempco voltage references: the negative-tempco tunneling component and positive-tempco avalanche component largely offset.

Why the Commercial Name "Zener Diode" Is Slightly Misleading

Nearly all diodes sold as "Zener diodes" above about 6 V actually break down predominantly by the avalanche mechanism, not Zener tunneling. The name has stuck as a catch-all for any diode engineered to operate in controlled reverse breakdown, regardless of which physical mechanism dominates at its particular voltage. Datasheets don't usually distinguish — they just specify VZ, tolerance, and temperature coefficient, which already implicitly reflects the mechanism mix.

The I-V Characteristic and Small-Signal Model

In the breakdown region, the diode current rises steeply for a small increase in reverse voltage. The dynamic (differential) resistance in that region is:

rz = ΔVZ / ΔIZ

For a good reference-grade Zener, rz might be 1–10 Ω around the rated test current; it rises sharply as current approaches the knee (near IZK, the knee current) and is a key nonideality — it means the "constant" VZ actually varies slightly with load current.

A practical breakdown-region model:

VZ(actual) = VZ0 + IZ·rz

where VZ0 is the extrapolated breakdown voltage and rz is the incremental resistance.

Worked Example: Zener Shunt Regulator

Design a simple shunt regulator: VZ = 5.1 V, rz = 5 Ω at test current IZT = 20 mA, supply Vs = 12 V (±10%), load current IL = 10 mA.

Step 1 — choose series resistor R for nominal Vs = 12 V: Target IZ ≈ IZT = 20 mA through the Zener, with IL = 10 mA drawn by the load, so total current through R is IZ + IL = 30 mA.

R = (Vs − VZ) / (IZ + IL) = (12 − 5.1) / 0.030 = 6.9/0.030 ≈ 230 Ω

Pick R = 220 Ω (nearest standard value).

Step 2 — check current at Vs_min = 10.8 V (−10%):

I_total = (10.8 − 5.1) / 220 = 5.7/220 ≈ 25.9 mA
IZ = I_total − IL = 25.9 − 10 = 15.9 mA

Still comfortably above IZK (typically ~1 mA) — diode stays in regulation.

Step 3 — check current at Vs_max = 13.2 V (+10%):

I_total = (13.2 − 5.1) / 220 = 8.1/220 ≈ 36.8 mA
IZ = 36.8 − 10 = 26.8 mA

Check this doesn't exceed the diode's rated power: P = VZ·IZ = 5.1 × 0.0268 ≈ 137 mW — fine for a typical 500 mW-rated part.

Step 4 — output voltage variation due to rz: IZ swings from 15.9 mA to 26.8 mA, a ΔIZ ≈ 10.9 mA.

ΔVZ = ΔIZ · rz = 0.0109 × 5 ≈ 54 mV

So VZ output shifts about 54 mV across the full supply range — a load/line regulation figure of roughly 1% at 5.1 V, consistent with a simple shunt regulator (not as tight as an active regulator, but adequate for biasing or reference use).

Check: at Vs_min, IZ = 15.9 mA > 0 and above IZK — regulation holds at both extremes. Power dissipation at max is well under the package rating. Design is valid.

Design and Selection Implications

• Voltage reference design: pick VZ near 5.6–6.2 V where tempco cancellation is best if temperature stability matters; below that, expect negative tempco, above that, expect positive tempco — compensate with a series diode (forward-biased, negative tempco) if needed.
• Noise-sensitive references: Zener (tunneling) noise is generally lower than avalanche noise; for ultra-low-noise references, dedicated bandgap references usually beat either breakdown mechanism.
• Transient/ESD clamping (TVS): avalanche-dominated higher-voltage Zeners are widely used as transient voltage suppressors; the same impact-ionization mechanism gives fast turn-on, useful for surge clamping.
• Power derating: breakdown current must be limited externally (series R or active current source) — the negative slope resistance-like I-V curve in breakdown means a Zener has no inherent current limiting and will thermally run away if unprotected.
• Minimum current (IZK): always bias well above the knee current where the curve is soft and regulation is poor.

Key Takeaways

• Zener breakdown is quantum tunneling in heavily-doped, narrow-depletion junctions, dominant below ~5–6 V, with a negative temperature coefficient.
• Avalanche breakdown is impact-ionization carrier multiplication in lightly-doped, wider-depletion junctions, dominant above ~6–8 V, with a positive temperature coefficient.
• Around 5.6–6.2 V, the two mechanisms coexist and their opposing tempcos largely cancel — the basis for low-drift reference diodes.
• Most commercial "Zener diodes" above ~6 V actually operate via avalanche breakdown; the name is a legacy umbrella term.
• The breakdown region has finite dynamic resistance rz, so VZ shifts with current — always model VZ = VZ0 + IZ·rz for accurate regulation analysis.
• External current limiting is mandatory; the diode itself has no series resistance to prevent thermal runaway in breakdown.