breakdown mechanisms in diodes

breakdown mechanisms in diodes

1. What Breakdown Is

Breakdown = abrupt large increase in reverse current at critical reverse voltage $V_{BR}$ after which diode loses blocking (but if current limited, can be non-destructive, especially Zener). Two distinct mechanisms:

2. Zener Breakdown (Quantum Tunneling)

• When: Highly doped p-n (narrow depletion, $W$ small, $E$ field high), $V_{BR} <6V$ (typically $2$–$5$ V), sharp breakdown.
• Mechanism: Strong electric field ($∼10^6$ V/cm) thins barrier so electrons tunnel directly from valence band (p-side) to conduction band (n-side) across junction (quantum tunneling, like earlier physics). No avalanche needed.
• Temperature coefficient: negative ($V_Z$ decreases with $T$ by ~$-2$ mV/°C) — band gap shrinks, tunneling easier.

3. Avalanche Breakdown (Impact Ionization)

• When: Lightly doped p-n (wide depletion), $V_{BR} >6V$ (e.g., $10$–$1000$ V), gradual breakdown.
• Mechanism: Minority carriers accelerated by high field gain kinetic energy ($≥E_g$) and collide with lattice, knocking out $e-h$ pairs (impact ionization), which further accelerate and multiply (chain reaction) ⇒ avalanche of carriers ⇒ current multiplies $M =1/[1-(V/V_{BR})^n]$.
• Temperature coefficient: positive ($V_{BR}$ increases with $T$ by ~$+2$ mV/°C) — higher $T$ ⇒ more phonon scattering ⇒ carriers need higher field to gain enough energy.

4. Comparison Table (exam must-draw)

Feature	Zener (tunneling)	Avalanche (impact)
Doping	Heavy (narrow $W$)	Light (wide $W$)
$V_{BR}$	<6V (e.g., 3.3V,5.1V)	>6V (e.g., 12V, 100V)
Mechanism	Quantum tunneling $e$ across band gap	Impact ionization, carrier multiplication
$V_{BR}$ vs $T$	Decreases with $T$ (negative TC)	Increases with $T$ (positive TC)
Knee	Sharp (well-defined)	Softer (gradual)
Use	Voltage reference/regulator (5.1V Zener)	Unintentional breakdown to avoid, or avalanche diodes

At ~6V, both occur, TC ~0 (temperature-compensated Zener ~5.6V).

5. Worked Illustration

Zener diode BZX55C5V1: V_Z=5.1V, I_ZT=5mA, dynamic r_z≈10Ω, TC≈+2mV/°C? Actually 5.1V near crossover, small TC.

Avalanche diode 1N4007: V_BR≈1000V, positive TC.

Need 5V regulator? Choose Zener 5.1V (Zener) not avalanche.

6. Exam Points

• Distinguish Zener vs avalanche (doping, $V_{BR}<6$ vs $>6$, tunneling vs impact, TC sign) — 7-10 marks, very frequent (the most common breakdown comparison question).
• State $V_{BR}$ TC signs — 5 marks, frequent.
• Mention Zener for voltage regulation (next topic) — 3 marks.
• Sketch I-V with breakdown at $V_{BR}$ — 3-5 marks, frequent.