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Zener characteristic curve and reverse breakdown voltage: NEET MCQs with answers

50 questions on this topic of Experimental Skills (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. In the circuit to study the reverse V-I characteristic curve of a Zener diode, the role of the series protective resistor (Rs) connected with the DC source and the diode is to:

  1. Set the breakdown voltage of the Zener diode to a desired reference level
  2. Increase the reverse saturation current before reaching the knee voltage
  3. Prevent the reverse current from exceeding the maximum safe limit after breakdown
  4. Convert the alternating ripple component of the input into direct current
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Answer: (C) Prevent the reverse current from exceeding the maximum safe limit after breakdown

Beyond the Zener breakdown voltage, current increases very rapidly with negligible change in voltage. Without a series protective resistor (Rs), excessively large reverse current would flow and permanently damage the diode due to overheating.

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2. In a laboratory test of a Zener diode with Vz = 5.0 V, the diode is placed in series with a 200 Ω resistor and a variable DC source. When the source voltage is 9.0 V, what is the current through the series resistor, assuming ideal breakdown behavior?

  1. 45 mA
  2. 25 mA
  3. 70 mA
  4. 20 mA
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Answer: (D) 20 mA

In the breakdown region, the voltage across the Zener diode is maintained at Vz = 5.0 V. The voltage across the series resistor is VR = Vin - Vz = 9.0 V - 5.0 V = 4.0 V. The current is I = VR / R = 4.0 V / 200 Ω = 0.02 A = 20 mA.

hard

3. A silicon Zener diode has a breakdown voltage of 7.5 V at 25 °C. The temperature coefficient of its breakdown voltage is +4.0 mV/°C. What will be the breakdown voltage of the diode if its junction temperature rises to 75 °C during operation?

  1. 7.30 V
  2. 7.50 V
  3. 7.90 V
  4. 7.70 V
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Answer: (D) 7.70 V

The change in temperature is ΔT = 75 °C - 25 °C = 50 °C. The change in breakdown voltage is ΔVz = (+4.0 × 10⁻³ V/°C) × 50 °C = +0.20 V. Therefore, the new breakdown voltage is Vz' = 7.5 V + 0.20 V = 7.70 V.

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4. Consider the following statements regarding the experimental determination of Zener characteristics: Statement I: A Zener diode is fabricated with heavily doped p and n regions, resulting in an extremely thin depletion region. Statement II: The electric field produced across the junction at breakdown voltage is of the order of 10³ V/m, which easily frees valence electrons. In light of the above statements, choose the correct answer:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (B) Statement I is correct but Statement II is incorrect

Statement I is correct because heavy doping leads to a very narrow depletion width (typically ~10⁻⁸ to 10⁻⁷ m). Statement II is incorrect because the internal electric field required for field emission (Zener breakdown) is extremely high, on the order of 10⁶ V/m to 10⁷ V/m, not 10³ V/m.

easy

5. While plotting the reverse V-I curve of a Zener diode, a student records the operating point moving from (V₁ = 5.60 V, I₁ = 10 mA) to (V₂ = 5.65 V, I₂ = 35 mA) deep inside the breakdown region. The dynamic resistance of the diode in this region is:

  1. 0.5 Ω
  2. 12.5 Ω
  3. 5.0 Ω
  4. 2.0 Ω
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Answer: (D) 2.0 Ω

Dynamic resistance rz is defined as ΔV / ΔI. Here, ΔV = 5.65 V - 5.60 V = 0.05 V and ΔI = 35 mA - 10 mA = 25 mA = 0.025 A. Therefore, rz = 0.05 / 0.025 = 2.0 Ω.

hard

6. A student performs the Zener diode reverse characteristic experiment at room temperature (25 °C) and then repeats it at an elevated temperature (75 °C) using a Zener diode that exhibits true Zener breakdown (Vz = 3.3 V). How will the characteristic curve shift at the higher temperature?

  1. The breakdown voltage increases, but the reverse saturation current decreases
  2. The knee point shifts to a higher reverse voltage, and the breakdown voltage increases
  3. The breakdown voltage remains exactly identical because it only depends on the doping level
  4. The knee point shifts to a lower reverse voltage, and the breakdown voltage decreases
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Answer: (D) The knee point shifts to a lower reverse voltage, and the breakdown voltage decreases

For breakdown voltages below about 5 V, the primary mechanism is Zener breakdown (field emission). In Zener breakdown, increased temperature narrows the band gap, making valence electrons tunnel more easily into the conduction band at a lower electric field; thus, the temperature coefficient is negative and Vz decreases.

hard

7. During the experimental plotting of the reverse characteristic curve of a Zener diode, if the variable input voltage is increased continuously after reaching breakdown, which of the following quantities increases most rapidly?

  1. Power dissipated across the series current-limiting resistor
  2. Voltage across the Zener diode
  3. Dynamic resistance of the Zener diode
  4. Width of the depletion region of the Zener diode
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Answer: (A) Power dissipated across the series current-limiting resistor

Once in breakdown, the voltage across the Zener diode remains virtually constant. Therefore, any extra input voltage appears entirely across the series current-limiting resistor, causing the current I to rise linearly, and the power in the resistor P = I²R to increase quadratically and rapidly.

easy

8. During the determination of the dynamic reverse resistance of a Zener diode in its breakdown region, an increase in reverse voltage from 5.58 V to 5.62 V causes the reverse current to surge from 10 mA to 30 mA. The dynamic resistance of the Zener diode is:

  1. 1.2 Ω
  2. 0.5 Ω
  3. 4.0 Ω
  4. 2.0 Ω
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Answer: (D) 2.0 Ω

The dynamic reverse resistance rz is defined as ΔVz / ΔIz. Here, ΔVz = 5.62 V - 5.58 V = 0.04 V, and ΔIz = 30 mA - 10 mA = 20 mA = 0.02 A. Thus, rz = 0.04 / 0.02 = 2.0 Ω.

hard

9. In a Zener breakdown voltage test circuit, the unregulated input DC voltage varies from 12 V to 16 V. The Zener voltage is 8.0 V, and the load resistance is R_L = 400 Ω. If the minimum required Zener current to stay well inside the breakdown region is 5 mA, what is the maximum permissible value of the series resistance R_s?

  1. 160 Ω
  2. 200 Ω
  3. 250 Ω
  4. 400 Ω
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Answer: (A) 160 Ω

The load current is constant at I_L = Vz / R_L = 8.0 V / 400 Ω = 20 mA. To maintain regulation even at minimum input voltage Vin_min = 12 V, the total current must be at least I_min = Iz_min + I_L = 5 mA + 20 mA = 25 mA. The voltage drop across R_s at this condition is Vin_min - Vz = 12 V - 8 V = 4 V. Thus, R_s_max = 4 V / 25 mA = 160 Ω.

easy

10. In an experimental setup to plot the reverse V-I characteristics of a Zener diode, which of the following arrangements correctly describes the connection of measuring meters?

  1. A milliammeter connected in parallel and a voltmeter connected in series with the diode
  2. A microammeter connected in series and a voltmeter connected in series with the diode
  3. A milliammeter connected in series and a voltmeter connected in parallel with the diode
  4. A voltmeter connected in series and an ammeter connected in parallel with the diode
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Answer: (C) A milliammeter connected in series and a voltmeter connected in parallel with the diode

To study reverse characteristics across breakdown, currents rapidly rise to several tens of milliamperes (mA). Hence, a milliammeter is connected in series with the diode to measure reverse current, and a voltmeter is connected in parallel to measure reverse voltage across the diode.

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11. Consider the following statements regarding the experimental determination of Zener breakdown voltage: Statement I: The reverse breakdown voltage Vz is practically determined by extrapolating the linear, steeply rising portion of the post-breakdown V-I curve back to the voltage axis. Statement II: Below the breakdown voltage, the reverse current is independent of the applied reverse bias because it is dominated by thermally generated minority carriers. Which of the following is correct?

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (A) Both Statement I and Statement II are correct

In practical laboratory experiments, the breakdown voltage is obtained at the intersection of the extrapolated steep linear current region with the voltage axis (or near the knee point). Statement II is also correct because prior to breakdown, the tiny reverse saturation current is due to minority carriers generated thermally and remains nearly constant with applied voltage.

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12. A Zener diode is connected across a variable DC voltage supply in series with a 500 Ω current-limiting resistor. If the breakdown voltage of the diode is 8.0 V, what is the current through the circuit when the supply voltage is set to 14.0 V?

  1. 8 mA
  2. 12 mA
  3. 16 mA
  4. 28 mA
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Answer: (B) 12 mA

Once the supply voltage (14.0 V) exceeds the breakdown voltage (8.0 V), the diode maintains 8.0 V across its terminals. The potential drop across the series resistor is Vs - Vz = 14.0 V - 8.0 V = 6.0 V. Thus, the circuit current is I = 6.0 V / 500 Ω = 0.012 A = 12 mA.

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