--- title: "Zener reverse breakdown as voltage reference" subject: "Electronics Devices and Circuit" chapter: 6 key: "semester_3/electronics_devices_and_circuit:zener-reference-justify" source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/zener-reference-justify site: BCT Engineering --- # Zener reverse breakdown as voltage reference Electronics Devices and Circuit · Chapter 6: Power Supplies, Breakdown Diodes, and Voltage Regulators - Asked in: 2078 Bhadra, 5 marks - Video: [ECE3400 Lecture 4: Zener Diodes: Voltage References & Regulators (Analog Electronics, Georgia Tech)](https://www.youtube.com/watch?v=RbKWp-VOdgo) — Lantertronics - 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-reference-justify - Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-6 ## Question Explain the reverse break down region in zener diode. "Zener diode acts as a voltage reference element" Justify the statement from IV characteristic curve. ## Short answer In reverse breakdown, the I-V curve is nearly vertical; a large current change yields only a tiny voltage change (ΔV = rz·ΔI, rz small). Hence the diode holds a constant voltage Vz, acting as a voltage reference. ## Answer #### Reverse breakdown region of a Zener diode A Zener diode is a heavily doped p-n junction designed to operate in the reverse breakdown region. For a small reverse voltage, only a tiny leakage current flows. When the reverse voltage reaches the Zener voltage Vz, the strong electric field in the thin depletion region breaks covalent bonds (the Zener effect, with avalanche multiplication at higher Vz). The reverse current then increases sharply. The diode stays safe as long as a series resistor limits the current so that Iz ≤ Iz(max) = Pz(max)/Vz. #### V-I characteristic ```text I ^ | / | / forward -Vz | / ------+---------+-------/-------> V | leakage 0.7 V +-- Izk (knee) | | breakdown region: almost | vertical, V stays ≈ Vz v Iz(max) ``` (In the reverse breakdown region the curve is almost vertical.) #### Why the Zener acts as a voltage reference 1. In the breakdown region, the I-V curve is almost vertical. A large change in current ΔIz (from Izk to Iz(max)) causes only a very small change in voltage ΔVz = rz ΔIz, where the dynamic resistance rz = ΔV/ΔI is small (a few ohms to tens of ohms). 2. Hence the voltage across the diode stays almost constant at Vz, whatever the current through it, and so whatever the input voltage or the load. 3. Vz is fixed by the doping level (Zeners are available from 2.4 V to 200 V), so the diode provides a fixed reference voltage to compare against or to supply to a circuit. Example: Vz = 6.8 V, rz = 20 Ω. If the current changes by 10 mA, the voltage changes by only 20 Ω × 10 mA = 0.2 V (3 %). Therefore, when it is reverse biased with a series resistor, it holds its terminal voltage constant at Vz and acts as a voltage reference element (in shunt regulators, series regulators and clippers). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What happens when reverse voltage reaches Vz? A. Reverse current falls to zero B. The reverse current rises sharply with nearly constant voltage C. The diode conducts only forward current D. Voltage rises linearly with current Answer: B. The breakdown curve is almost vertical, with rz as small slope resistance. (Key point: Breakdown region) 2. Why does this make a voltage reference? A. A large current change ΔIz produces only a small ΔVz = rz ΔIz B. Voltage changes in proportion to current C. Current stays constant while voltage changes D. The diode resistance is infinite Answer: A. Small dynamic resistance keeps the voltage near Vz. (Key point: Reference reason) 3. With Vz = 6.8 V and rz = 20 Ω, a 10 mA current change changes voltage by? A. 2 V B. 20 V C. 0.02 V D. 0.2 V (about 3 %) Answer: D. ΔV = 20 Ω x 10 mA = 0.2 V. (Key point: Example) 4. How is the safe maximum Zener current found and kept? A. Iz(max) = Pz(max) x Vz B. Iz(max) = Vz/rz C. Iz(max) = Pz(max)/Vz, kept by a series resistor D. By connecting a capacitor Answer: C. Power rating divided by Vz gives max current, and Rs limits it. 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