When a PN junction is reverse-biased, only a tiny leakage current flows until the reverse voltage reaches a critical value. There the current suddenly rises sharply; this is junction breakdown. There are two physical mechanisms, Zener breakdown and avalanche breakdown, and the difference between them (doping, depletion width, temperature coefficient, breakdown voltage) is a favourite question in BCS (EEE), PGCB, BPDB, polytechnic and university exams. This post explains both with an animation, a comparison table, a Zener-regulator design example and MCQs.
- Zener breakdown: heavily doped junction → very thin depletion layer → very strong field → electrons tunnel directly from the valence band to the conduction band. It occurs at below about 5 V.
- Avalanche breakdown: lightly doped junction → wide depletion layer → carriers gain enough energy to knock out new electron–hole pairs by impact ionization, which multiplies in a chain. It occurs at above about 7 V.
- Between about 5 V and 7 V, both mechanisms contribute.
- Temperature coefficient: Zener is negative (VZ falls as temperature rises); avalanche is positive.
- Breakdown itself is not destructive if the current is limited. Excess power causes thermal breakdown, which destroys the device.
1. Reverse-Biased PN Junction
Under reverse bias, the external voltage adds to the built-in potential, so the depletion region widens and the electric field across it grows. Only minority carriers cross, giving a small, nearly constant reverse saturation current IS (nA for silicon, µA for germanium) that roughly doubles for every 10 °C rise. At the breakdown voltage VBR the field becomes strong enough to generate a large number of new carriers, and the reverse current increases very steeply while the voltage stays almost constant. This nearly vertical I–V region is what makes voltage regulation possible.
Depletion width: W ∝ √(Vbi + VR) / √N (heavier doping N → thinner W → stronger field)2. Animation: Both Mechanisms
3. Zener Breakdown
- Occurs in heavily doped junctions, where the depletion layer is very thin (around 10 nm or less).
- Even a few volts across such a thin layer produce an extremely strong field, of the order of 107–108 V/m (≈ 3 × 107 V/m is often quoted).
- The field bends the energy bands so much that the filled valence band on the P side lines up with the empty conduction band on the N side. Electrons then tunnel through the thin barrier (a quantum-mechanical effect, also called field emission), breaking covalent bonds without any collision.
- Breakdown voltage is low (typically below 5 V) and the knee is relatively soft.
- Negative temperature coefficient: at higher temperature the band gap shrinks and electrons have more energy, so tunnelling happens at a lower voltage.
4. Avalanche Breakdown
- Occurs in lightly doped junctions with a wide depletion layer.
- Minority carriers crossing the depletion region are accelerated by the field. When a carrier gains enough kinetic energy between collisions, it breaks a covalent bond on impact (impact ionization) and creates a new electron–hole pair.
- The new carriers are accelerated too and create more pairs: 1 → 2 → 4 → 8 … This chain reaction is the avalanche, and the current is multiplied by the factor
- Breakdown voltage is higher (above about 7 V, up to thousands of volts in power devices) and the knee is very sharp.
- Positive temperature coefficient: at higher temperature, lattice vibrations increase, carriers collide more often and lose energy, so a higher voltage is needed to reach ionizing energy.
- The same mechanism is used deliberately in avalanche photodiodes (APDs), IMPATT diodes and TVS surge suppressors. It also sets the maximum blocking voltage of transistors and thyristors.
5. Zener vs Avalanche: Comparison Table
| Feature | Zener breakdown | Avalanche breakdown |
|---|---|---|
| Doping | Heavy | Light |
| Depletion width | Very thin | Wide |
| Mechanism | Quantum tunnelling (field emission) | Impact ionization (carrier multiplication) |
| Breakdown voltage | Low (< ≈ 5 V) | High (> ≈ 7 V) |
| Electric field | Very strong, ~107–108 V/m | Lower, but acting over a wider region |
| Temperature coefficient | Negative | Positive |
| I–V knee | Softer | Very sharp |
| Collisions needed? | No | Yes |
| Typical device | Low-voltage Zener / reference diode | Higher-voltage “Zener”, TVS, APD, rectifier rating |
6. Thermal Breakdown
If the reverse current is not limited, the power P = VBR × IR heats the junction. Higher temperature generates more carriers, which raises the current further, and the result can be thermal runaway, which permanently damages the junction. Zener and avalanche breakdowns are reversible; thermal breakdown is not. A series resistor and a correctly rated diode (PZ(max)) prevent it.
7. Application: Zener Voltage Regulator
A Zener diode in reverse breakdown keeps an almost constant voltage across a load, even when the input voltage or load current changes. The series resistor RS absorbs the difference.
RS = (Vin − VZ) / (IL + IZ(min))Example: Vin = 12 V, VZ = 5.1 V, IL = 20 mA, IZ(min) = 5 mA.
RS = (12 − 5.1)/(25 mA) = 276 Ω → choose 270 Ω; I = 6.9/270 ≈ 25.6 mAIf the load is removed, all 25.6 mA flows through the Zener: PZ = 5.1 × 25.6 mA ≈ 130 mW, so a 400 mW or 500 mW Zener diode is adequate. RS dissipates 6.9 × 25.6 mA ≈ 177 mW, so use a ¼ W resistor or larger.
8. Practice MCQs
1. Zener breakdown occurs in: (a) Lightly doped junctions (b) Heavily doped junctions (c) Intrinsic semiconductor (d) Forward-biased junctions
2. Avalanche breakdown is caused by: (a) Tunnelling (b) Impact ionization (c) Thermal emission only (d) Recombination
3. Temperature coefficient of Zener breakdown voltage is: (a) Positive (b) Negative (c) Zero always (d) Infinite
4. A diode with breakdown voltage of 30 V breaks down mainly by: (a) Zener effect (b) Avalanche effect (c) Tunnelling (d) Photoelectric effect
5. Depletion layer in a Zener-breakdown diode is: (a) Very thin (b) Very wide (c) Absent (d) Same as in a rectifier diode
6. Zener diodes with near-zero temperature coefficient have VZ around: (a) 1 V (b) 5–6 V (c) 15 V (d) 100 V
7. A Zener diode is used in a regulator in: (a) Forward bias (b) Reverse breakdown region (c) Cut-off (d) Saturation
8. Which breakdown is permanently destructive? (a) Zener (b) Avalanche (c) Thermal (d) None
9. Reverse saturation current of a silicon diode approximately doubles for every: (a) 1 °C (b) 10 °C (c) 25 °C (d) 100 °C
10. Vin = 15 V, VZ = 10 V, total current 50 mA. RS is: (a) 50 Ω (b) 100 Ω (c) 200 Ω (d) 300 Ω
9. বাংলায় দ্রুত রিভিশন
- জেনার ব্রেকডাউন: উচ্চ ডোপিং, পাতলা ডিপ্লেশন লেয়ার, প্রবল বৈদ্যুতিক ক্ষেত্র → ইলেকট্রন টানেলিং; সাধারণত ৫ V-এর নিচে।
- অ্যাভালাঞ্চ ব্রেকডাউন: কম ডোপিং, প্রশস্ত ডিপ্লেশন লেয়ার → সংঘর্ষজনিত আয়নীকরণে চেইন রিঅ্যাকশন; সাধারণত ৭ V-এর ওপরে।
- তাপমাত্রা সহগ: জেনার ঋণাত্মক, অ্যাভালাঞ্চ ধনাত্মক; ৫–৬ V-এ প্রায় শূন্য।
- কারেন্ট সীমিত থাকলে ব্রেকডাউন ক্ষতিকর নয়; অতিরিক্ত তাপে থার্মাল ব্রেকডাউন ডিভাইস নষ্ট করে।
- জেনার রেগুলেটরে RS = (Vin − VZ)/(IL + IZ(min))।
Related Posts
- Semiconductor Diode: Working, V-I Characteristics & Types
- PN Junction Without Bias
- What Is Negative Resistance?
- FET vs BJT: Key Differences
- Bandgap Tailoring
References: S. M. Sze, Physics of Semiconductor Devices; Robert L. Boylestad, Electronic Devices and Circuit Theory; Ben G. Streetman, Solid State Electronic Devices. Images and animations: original work by Totalecer.
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