--- title: "Zener breakdown and Zener regulator" subject: "Electronics Devices and Circuit" chapter: 6 key: "semester_3/electronics_devices_and_circuit:zener-breakdown-regulator" source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/zener-breakdown-regulator site: BCT Engineering --- # Zener breakdown and Zener regulator Electronics Devices and Circuit · Chapter 6: Power Supplies, Breakdown Diodes, and Voltage Regulators - Asked in: 2080 Bhadra, 5 marks; 2080 Baisakh, 5 marks (asked 2 times) - Video: [Everything You Need To Know About Zener Diodes | Zener vs Avalanche Breakdown & I V Curve Guide](https://www.youtube.com/watch?v=AhMHOu4ZIkw) — Engineeringness - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/zener-breakdown-regulator - Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-6 ## Question What is Zener breakdown? Describe the Zener diode as a voltage regulator with circuit diagram and IV characteristics curve. ## Short answer Zener breakdown occurs in heavily doped PN junctions when a high electric field pulls electrons from covalent bonds, causing a sharp current rise at nearly constant voltage. A Zener diode shunt regulator maintains output close to Vz by diverting excess current through the diode when input or load changes, with series resistor Rs limiting current. ## Answer #### Zener breakdown When a heavily doped p-n junction is reverse biased, the depletion region is very narrow (below about 10 nm). At a reverse voltage Vz (below about 5 V) the electric field across it (about 3 × 10⁷ V/m) is strong enough to pull valence electrons directly out of their covalent bonds (field emission). This produces a large number of carriers, and the reverse current rises sharply while the voltage across the diode stays almost constant at Vz. This is **Zener breakdown**. (Above about 6 V the avalanche effect takes over, but diodes using either effect are called "Zener diodes".) #### V-I characteristic ```text I (forward) ^ | / | / forward region | / (conducts above about 0.7 V) -Vz | / ----+---------------+---+-----------> V | 0 0.7 V | reverse: only a tiny leakage | current until -Vz is reached | | breakdown region: the current | rises steeply while the | voltage stays at Vz v I (reverse) ``` - Forward bias: it behaves like a normal diode (conducts above about 0.7 V). - Reverse bias (up to Vz): only a very small leakage current flows. - At Vz the current rises sharply along an almost vertical line (the dynamic resistance rz is a few ohms). The current must be limited by a series resistor and must not exceed Iz(max) = Pz(max)/Vz. #### Zener as a voltage regulator (shunt regulator) ```text Vin o----[ Rs ]-----+-------+----o Vo = Vz | | (Dz) [RL] | | GND GND ``` The Zener is reverse biased and connected in parallel with the load, with a series resistance Rs. - **Line regulation:** If Vin rises, the current through Rs increases. The extra current is taken by the Zener (Iz rises), and the drop across Rs increases by the same amount as the increase in Vin, so Vo = Vz stays almost constant. - **Load regulation:** If the load current IL increases, the Zener current Iz decreases by the same amount, so Is = Iz + IL stays constant and Vo remains nearly constant. Design condition: Rs = (Vin(min) − Vz)/(Iz(min) + IL(max)); Iz(min) must be at least the knee current Izk, and the Zener power (Vz Iz(max)) must not exceed its rating. Limits: it is only suitable for small currents (tens of mA), its efficiency is low, and the output changes slightly because of rz (ΔVo ≈ rz ΔIz). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What causes Zener breakdown? A. Fast carriers colliding with atoms in a wide depletion region B. In a heavily doped junction with a very thin depletion region, a strong field pulls electrons directly from covalent bonds C. Forward bias above 0.7 V D. Thermal heating of a lightly doped junction Answer: B. Field emission in a narrow depletion region (Vz below about 5 V) is the Zener effect; collision multiplication is avalanche. (Key point: Zener breakdown) 2. What does the reverse region of the Zener V-I curve look like at Vz? A. Current rises almost vertically while voltage stays nearly constant, so a series resistor must limit it B. Current stays near zero for all reverse voltages C. Voltage rises linearly with current D. Current falls as voltage reaches Vz Answer: A. The small dynamic resistance rz makes the curve steep, so only an external resistor limits the current. (Key point: Characteristic) 3. In the shunt regulator, how does the output stay at Vz when Vin rises? A. The Zener current falls, lowering the drop in Rs B. The load current rises by the same amount C. Rs decreases automatically D. The extra current in Rs flows through the Zener, so the extra drop appears across Rs and Vo stays near Vz Answer: D. The increase in Vin is absorbed by Rs while Zener voltage is nearly constant. (Key point: Line regulation) 4. If load current IL increases, what happens? A. Iz increases to supply the load B. Rs drops and Vo falls to zero C. Iz decreases by the same amount so Is = Iz + IL stays constant D. Is increases by IL Answer: C. The current through Rs is fixed by Vin - Vz, so the load takes current that the Zener was carrying. 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