Zener diode: characteristic and voltage regulation

What is a Zener diode?

In plain words: an ordinary diode used the wrong way round blocks current until the voltage is so large that it breaks down, and then it usually burns out. A Zener diode is built to break down at a chosen voltage and survive it. Within its specified current and power range it carries the extra current supplied through a current limiter, keeping its voltage approximately constant, which makes it a simple voltage regulator.

Reverse breakdown in a heavily doped junction

• In reverse bias, the current of any p-n junction diode stays tiny and nearly constant up to a critical voltage, the breakdown voltage, where it rises sharply. Unless the external circuit keeps the current below its rated value, the diode is destroyed by overheating (NCERT Section 14.6, page 336). So general-purpose diodes are not used beyond the reverse saturation region (NCERT page 337).
• NEETS puts it this way: any diode can be reverse biased to breakdown, but not every diode can safely get rid of the heat produced there (NEETS Module 7, page 3-2). A Zener diode is designed to.
• In a Zener diode both the p and n regions are more heavily doped than in a rectifier diode (Laboratory Manual, page 125). Heavier doping gives a thinner depletion region, less than 10⁻⁷ m across (Laboratory Manual, page 125), so even a few volts across it make a very strong electric field.
• Breakdown happens in one of two ways (NEETS Module 7, pages 3-3 to 3-5; Laboratory Manual, pages 125–126):
  • Zener breakdown (the Zener effect): in a very heavily doped junction, reverse bias brings the valence band of the p-side level with the conduction band of the n-side. Electrons in the valence band of the p-side then cross the very thin junction into the n-side by quantum-mechanical tunnelling between available states at the same total energy; they need not thermally climb over the barrier. The Laboratory Manual calls this internal field emission. The effect is named after Clarence Zener, who described it in 1934 (Wikipedia, “Zener diode”).
  • Avalanche breakdown: in a junction doped less heavily than a Zener-effect diode, with a doping profile chosen for the desired breakdown voltage, the depletion region is thicker. Minority carriers accelerated by the field gain enough energy to break covalent bonds when they collide with atoms of the crystal. The electrons and holes freed are accelerated in turn and free more: a chain, or avalanche, of carriers.
• The crossover is gradual, not an exact dividing voltage. NEETS gives a useful guide: the Zener effect accounts for breakdown below about 5 V, and the avalanche effect for breakdown above it (NEETS Module 7, page 3-3; Wikipedia, “Zener diode”, gives about 5.6 V for silicon). Diodes of both kinds are called Zener diodes. By controlling the doping, makers produce breakdown voltages from about 2 V to 200 V (NEETS Module 7, pages 3-3 and 3-5).
• Breakdown itself does no harm. Working in the breakdown region does not damage the diode provided current, power, junction temperature and pulse ratings are respected. The external circuit must limit current, often with a series resistor (NEETS Module 7, page 3-5).
• Symbol: a diode symbol whose bar has bent ends; in use, the cathode (n-side) is connected to the higher potential (NEETS Module 7, Figure 3-4).

I-V characteristic and the reverse breakdown voltage

• Forward bias: a Zener diode behaves like an ordinary diode: little current until the threshold voltage, about 0.7 V for silicon, then a steep rise (reasoned from NCERT page 337).
• Reverse bias, below breakdown: the Manual gives an illustrative leakage of about 10⁻⁸ A to 10⁻¹⁰ A, too small for its meter. Actual leakage depends strongly on device, temperature and proximity to breakdown (Laboratory Manual, page 127).
• At breakdown: when the reverse voltage reaches V_Z, the reverse current rises sharply. The voltage at which this happens is the Zener voltage, and the reverse current at that voltage is the Zener current (Laboratory Manual, page 126).
• Beyond breakdown: the curve is almost parallel to the current axis, so a very small change in voltage goes with a very large change in current (Laboratory Manual, page 126). Read the other way round, within the useful regulation range a large current change produces a small voltage change. Datasheets specify nominal V_Z at a particular test current and temperature, not at every point of the knee. That is the property a regulator uses.
• The steepness is measured by the dynamic resistance r_Z = ΔV/ΔI in the breakdown region, which is small (see Example 5). For an ideal Zener diode the line is vertical and r_Z = 0 (reasoned from NCERT Eq. (14.6), page 337).
• The upper limit: the diode turns electrical energy into heat at the rate P = V_ZI_Z, so its power rating P_Z gives a nominal steady-state current limit at the stated temperature; apply any thermal derating and other ratings (Laboratory Manual, page 127):

• NCERT Figure 14.16(c) (page 336), the Laboratory Manual’s Fig. E 17.1 (page 126) and the figure below draw reverse values as negative, in the third quadrant. In the experiment the meters show V_r and I_r as positive numbers, and the Manual plots V_r along the x-axis and I_r along the y-axis (page 128).

Circuit for the characteristic curve

• Aim (NCERT Laboratory Manual, Experiment 17, page 125): to draw the characteristic curve of a Zener diode and find its reverse breakdown voltage.
• Apparatus (page 125): a Zener diode, a variable dc supply (0–15 V), a microammeter (0–100 μA), a voltmeter (0–15 V), a protective resistance of 125 Ω, a high-resistance rheostat and connecting wires.
• Circuit (page 127): the Zener diode is connected in reverse bias, its n-side towards the positive terminal, in series with the protective resistance R_P and the microammeter; the voltmeter goes across the diode. The rheostat is used as a potential divider across the supply, so that the reverse voltage can be raised from zero.
• Where the voltmeter goes (beyond the NCERT text): before breakdown the diode’s current is far smaller than the current a voltmeter draws, so a microammeter that also carries the voltmeter’s current would read the voltmeter, not the diode. Connect the voltmeter across the diode and microammeter together, as the figure shows; account for the microammeter’s burden voltage in that reading. The same connection tradeoff is discussed for an ordinary diode on the semiconductor diode page.
• Procedure (pages 127–128): check the zero of both meters. Raise the reverse voltage in steps of 0.1 V and note each voltmeter reading V_r and microammeter reading I_r. At first the microammeter reads zero; near breakdown the current starts to rise quickly. Connect the + terminal of each meter towards the + side of the supply (page 127). For the actual apparatus, choose a resistor or supply current limit that protects the meter as well as the diode. For a 15 V supply, a nominal 10 V diode and a 100 μA meter, even an ideal calculation needs at least (15 − 10)/100 μA = 50 kΩ, with extra margin for tolerances; use an appropriate mA range for mA data. Past the knee, raise the voltage slowly and stop before full scale: R_P protects the diode, but in the Manual’s example it still allows (15 − 10)/125 = 40 mA, far beyond a 100 μA meter (reasoned).
• Graph and result (pages 128–129): plot I_r against V_r, with V_r along the x-axis, and read the breakdown voltage V_Z from the graph, where the current shoots up. Small voltage steps near the knee locate it well (reasoned).
• Protective resistance (pages 126–127): after breakdown the diode alone would let the current grow without limit, so R_P is put in series. With the full supply voltage V across the pair, V = V_Z + I_ZR_P; setting I_Z to its largest safe value P_Z/V_Z gives the nominal minimum R_P from the diode power alone (meter limits and component tolerances may require more):

• The Laboratory Manual’s example: a diode with V_Z = 10 V and a power rating of 0.4 W, on a 15 V supply, needs R_P = (15 − 10) × 10/0.4 = 125 Ω, the resistance in its apparatus list (page 127).
• Laboratory Manual slip (page 126): the Manual puts the diode and R_P across a potential divider “with maximum potential V_Z across it”, then writes V = V_Z + I_ZR_P. The maximum potential across the pair is the supply voltage V: with V = V_Z, R_P would be zero. Its own example uses V = 15 V.
• Precautions (page 129): clean the ends of the connecting wires with sandpaper, and check the zero readings of the voltmeter and microammeter.

Zener diode as a voltage regulator

• A dc supply made with a rectifier and capacitor filter is not perfectly steady: its voltage changes as the mains voltage and the load change. A voltage regulator holds the output steady. Zener diodes are most widely used as voltage regulators, because once breakdown is reached the voltage across the diode stays almost constant over the input range for which enough Zener current remains and its ratings are met (NEETS Module 7, page 3-7).
• The circuit: the unregulated input V_in feeds a series resistor R_S. The Zener diode, reverse biased, is connected across the load R_L, so the output voltage is the Zener voltage. NEETS calls this kind a shunt regulator, “because the regulating device is connected in parallel with the load resistance” (NEETS Module 7, page 4-38).
• The currents: the series resistor takes the voltage V_in − V_Z, so its current is fixed by it; the load takes V_Z/R_L; and by the junction rule the Zener diode carries the rest:

• First check that the diode is in breakdown. Take the diode out: the load would then get V_inR_L/(R_S + R_L). In the ideal constant-voltage model, only if this exceeds V_Z can the diode regulate; a real device also needs its specified minimum regulation current. Otherwise the diode carries no current and the circuit is a potential divider (Example 3). In Example 1, 15 × 1000/1300 ≈ 11.5 V, well above 6.0 V (reasoned).
• If the input rises, I_S rises. The extra current flows through the Zener diode, the output stays at V_Z, and the extra voltage appears across R_S. If the load draws more current, I_Z falls by the same amount and the output again stays at V_Z (reasoned; see Examples 2 and 3).
• Two limits (Wikipedia, “Zener diode”; the worst cases are reasoned):
  • R_S must be small enough that some current still flows through the diode and keeps it in breakdown when the input is lowest and the load current largest. With too little current, the output is no longer regulated and falls below V_Z (Example 3).
  • R_S must be large enough that V_ZI_Z never exceeds the diode’s power rating, the worst case being the highest input with the load disconnected.
• Zener diodes are also used to give fixed reference voltages in circuits and to protect circuits from overvoltage (Wikipedia, “Zener diode”).

For the ideal regulator examples, assume adequate Zener and resistor ratings unless a rating is explicitly tested. Real designs also check the minimum regulation current, test-current voltage, tolerances and thermal derating. Toshiba’s Zener explanation describes the two breakdown mechanisms.

Solved examples

Common mistakes to avoid

Quick revision summary

• A Zener diode is a purpose-designed p-n junction diode made to work in reverse breakdown; its symbol has a bent bar, and its cathode goes to the higher potential.
• Zener breakdown (tunnelling) usually dominates at low voltages, avalanche at higher voltages, with both contributing near the roughly 5–6 V crossover; both kinds are called Zener diodes.
• I-V curve: like an ordinary diode in forward bias; in reverse a tiny current until V_Z, then an almost vertical rise in current at nearly constant voltage.
• The characteristic is taken with the diode in reverse bias, a protective resistor and a microammeter in series and a voltmeter across, raising the voltage in 0.1 V steps.
• As a regulator: series resistor R_S, Zener diode reverse biased across the load; the output stays at V_Z while the diode stays in breakdown and within its power rating.

Questions students ask

Is the Zener diode in the JEE Main and NEET syllabus?

Yes, the Zener diode is in both syllabuses: Unit 19, Electronic Devices, of the JEE Main syllabus and of the NEET (UG) 2026 syllabus lists the I-V characteristic of the Zener diode and the “Zener diode as a voltage regulator”, and Unit 20 of both has the experiment “Characteristic curves of a Zener diode and finding reverse breakdown voltage” (JEE wording). The current NCERT Class 12 Physics Part II, still the 2025–26 reprint, has no section on it: page 337 says this special breakdown action “is discussed later in Section 14.8”, a section the reprint lacks. The experiment is Experiment 17 of the NCERT Laboratory Manual (Physics, Class 12), pages 125–129.

How is a Zener diode different from an ordinary p-n junction diode?

A Zener diode differs from an ordinary p-n junction diode in being more heavily doped on both sides and in being made to dissipate enough power to work in the reverse breakdown region (NCERT Laboratory Manual, Physics, Class 12, page 125). Beyond the NCERT text, its doping sets its breakdown voltage, anywhere from about 2 V to 200 V (US Navy, NEETS Module 7, page 3-5); in breakdown its voltage stays nearly constant, so it is used there as a regulator, whereas NCERT says general-purpose diodes are not used beyond the reverse saturation region (page 337). In forward bias the two behave alike (reasoned), conducting beyond the silicon threshold of about 0.7 V that NCERT gives on page 337.

What is the difference between Zener breakdown and avalanche breakdown?

Zener breakdown and avalanche breakdown differ, beyond the NCERT text, in how carriers are freed: in Zener breakdown the field of a very thin, heavily doped junction lets valence electrons tunnel across, while in avalanche breakdown accelerated carriers break bonds in collisions and free more in a chain (US Navy, NEETS Module 7, pages 3-3 to 3-5). In silicon the Zener effect tends to dominate below roughly 5 V and avalanche above roughly 6 V, with both contributing between them, and heat acts on them oppositely: a Zener-effect diode’s breakdown voltage falls as the temperature rises, an avalanche diode’s rises (Wikipedia, “Zener diode”). Close to 5.6 V both act and offset each other, so a suitably chosen device and operating current can have a small temperature coefficient; cancellation is not exact for every nominal 5.6 V diode.

Is the Zener voltage the same as the breakdown voltage?

Yes, for a Zener diode the Zener voltage V_Z is its reverse breakdown voltage: the NCERT Laboratory Manual (Physics, Class 12) gives the name Zener voltage to the voltage at which breakdown occurs (page 126). Beyond the NCERT text, makers set V_Z by the doping, with a tolerance commonly of 5% or 10% (Wikipedia, “Zener diode”), so a diode sold as 7.5 V with a 5% tolerance has a specified voltage range 7.125 V to 7.875 V at the datasheet’s test current and temperature. The onset of the knee and voltages at other currents are not identical to that tolerance range. For an ordinary diode the breakdown voltage is instead a limit that the reverse voltage must stay below.

How does a Zener diode protect a circuit from overvoltage?

A Zener diode protects a circuit from overvoltage, a use beyond the NCERT text that Wikipedia’s “Zener diode” lists, when it is connected reverse biased across the circuit’s supply or input with a Zener voltage a little above the normal working voltage (reasoned). At normal voltages it stays below breakdown and draws only a tiny reverse current, but a spike that would push the voltage past V_Z drives it into breakdown, so it conducts heavily and holds the voltage near V_Z (reasoned). A series resistance must take up the excess voltage and limit the diode’s current (reasoned), and two Zener diodes joined back to back clip spikes of either sign (Wikipedia).

Why is a Zener diode not used as a rectifier?

A Zener diode is not used as a rectifier because, beyond the NCERT text, it is built to break down and conduct in reverse at its Zener voltage: in the half-cycles it should block, any reverse voltage above V_Z drives it into breakdown and current flows backwards through the load (reasoned). A rectifier diode must withstand the whole reverse voltage across it, in a centre-tap rectifier twice the peak of each half of the secondary (rectifier Example 4). As long as that reverse voltage stays below V_Z, a Zener diode would rectify like an ordinary diode (reasoned).

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Why is the Zener diode in a voltage regulator connected in reverse bias?

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   Only in reverse breakdown does the voltage across it stay almost constant at V_Z as the current changes. Forward biased, it would hold only about 0.7 V, like an ordinary silicon diode.

2. Q2. A 5.1 V Zener diode has a power rating of 0.50 W. What is the largest current it can safely carry?

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   I_Z(max) = P_Z/V_Z = 0.50/5.1 ≈ 0.098 A, about 98 mA (NCERT Laboratory Manual, page 127).

3. Q3. A regulator has V_in = 20 V, V_Z = 9.0 V, R_S = 220 Ω and R_L = 1.5 kΩ. Find the current through the Zener diode.

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   I_S = (20 − 9.0)/220 = 50 mA; I_L = 9.0/1500 = 6.0 mA; so I_Z = 50 − 6.0 = 44 mA.