Breakdown Mechanisms

A PN junction under reverse bias conducts negligible current until the applied voltage reaches a critical value. Beyond this point, the current rises sharply in a phenomenon called junction breakdown. Two distinct physical mechanisms cause this behavior: Zener breakdown and avalanche breakdown. Identifying which mechanism is active depends on the doping level and the breakdown voltage, and this distinction is frequently tested in GATE.

Core Concept Explanation

When reverse voltage is applied to a PN junction, the depletion region widens and the internal electric field intensifies. As long as the reverse voltage is below the breakdown voltage (VBR), only a tiny reverse saturation current flows due to minority carriers. Once VBR is reached, one of two mechanisms causes a sudden and dramatic increase in reverse current.

In Zener breakdown, the mechanism is quantum mechanical tunneling. When both P and N sides are heavily doped, the depletion region becomes extremely thin, sometimes only a few nanometers wide. At sufficient reverse voltage, the electric field across this thin region becomes so intense (above approximately 10^6 V/cm for silicon) that electrons in the valence band of the P-side can quantum mechanically tunnel directly into the conduction band of the N-side without needing additional energy. This process is called band-to-band tunneling or the Zener effect, and it produces a large current without any carrier multiplication.

In avalanche breakdown, the mechanism is impact ionization. When the reverse bias is high but the doping is moderate to low, the depletion region is wide and the field is more distributed. A thermally generated electron-hole pair in the depletion region gets accelerated by the field. If it gains enough kinetic energy before colliding with the lattice, it can knock a bound electron free, creating a new electron-hole pair. These new carriers are again accelerated and generate more pairs. This self-sustaining multiplication process, analogous to an avalanche, causes current to grow rapidly.

Mathematical Expression

Avalanche breakdown can be characterized using the ionization coefficient alpha (number of electron-hole pairs created per unit path length). Breakdown occurs when the integral of alpha over the depletion width equals unity: integral(alpha dx) from 0 to W = 1. In practice, empirical formulas relate VBR to doping. For a one-sided abrupt junction, VBR is proportional to (Eg^(3/2)) / (Na or Nd)^(1/2), meaning lightly doped junctions break down at higher voltages.

The temperature behavior provides a practical distinguishing test. Zener breakdown shows a negative temperature coefficient: as temperature rises, the band gap narrows slightly and tunneling becomes easier, so VBR decreases. Avalanche breakdown shows a positive temperature coefficient: higher temperature increases lattice vibrations, which scatter carriers and reduce their mean free path, requiring a higher field to sustain multiplication, so VBR increases with temperature. A diode with VBR below 6 V is predominantly Zener; above 6 V is predominantly avalanche; near 6 V both coexist.

Practical Understanding

Both mechanisms are exploited in Zener diodes, which are designed to operate in the breakdown region for voltage regulation. Even though the device is called a Zener diode, diodes with VBR above about 6 V actually use avalanche breakdown as the dominant mechanism. The name Zener is used loosely for all breakdown-voltage regulation diodes.

In power electronics, unintended avalanche breakdown can destroy a device if current is not limited. However, avalanche-rated devices are specifically designed to absorb a defined amount of energy during breakdown events, protecting circuits from voltage spikes. Zener diodes with predictable breakdown are used in precision voltage references, clamp circuits, and electrostatic discharge protection.

Given:
Silicon abrupt one-sided P+N junction, Nd = 1e16 cm⁻³
Semiconductor permittivity ε = 11.7 × 8.85e-14 = 1.035e-12 F/cm
Critical electric field for silicon Ecrit ≈ 3e5 V/cm
Built-in voltage Vbi ≈ 0.7 V (approximated as negligible vs VBR)

Why this formula applies:
For abrupt one-sided junction, the peak electric field determines avalanche breakdown.

Formula:
VBR ≈ ε × Ecrit² / (2 × q × Nd)
where q = 1.6e-19 C

Substitution:
VBR = (1.035e-12 × (3e5)²) / (2 × 1.6e-19 × 1e16)

Calculation:
Numerator = 1.035e-12 × 9e10 = 9.315e-2
Denominator = 2 × 1.6e-19 × 1e16 = 3.2e-3
VBR = 9.315e-2 / 3.2e-3

Final Answer:
VBR ≈ 29.1 V  (avalanche mechanism dominant since VBR > 6V)

Exam Tip: VBR below 6 V means Zener mechanism (negative temp coeff); VBR above 6 V means avalanche (positive temp coeff). Also remember: increasing doping reduces VBR for both mechanisms because the depletion region narrows.

• Zener mechanism: quantum tunneling through a thin depletion region in heavily doped junctions at low reverse voltages.
• Avalanche mechanism: impact ionization chain reaction in lightly doped, wide depletion regions at higher reverse voltages.
• Temperature coefficient differentiates the two: Zener has negative (VBR falls with temperature), avalanche has positive (VBR rises with temperature).
• Increasing doping reduces VBR because narrower depletion region reaches critical field at lower voltage.
• Most commercial Zener diodes with VBR above 6 V actually undergo avalanche breakdown despite the name.

Quick Revision

• Zener breakdown: heavy doping, thin depletion region, band-to-band tunneling, VBR below 6 V, negative temp coefficient.
• Avalanche breakdown: light doping, wide depletion region, impact ionization, VBR above 6 V, positive temp coefficient.
• Critical field for Si avalanche: approximately 3x10^5 V/cm; for Zener: above 10^6 V/cm.
• VBR formula for abrupt junction: VBR proportional to ε × Ecrit² / (2qNd).
• GATE trap: A Zener diode labeled with VBR = 10 V actually uses avalanche mechanism, not Zener tunneling.
• Both mechanisms are reversible; the diode is not damaged if current is externally limited.