MPPSC 2022 Prelims Paper-I · Question 48 of 100

Correct — C, Tunnel diode. Because of quantum tunneling across a very narrow, heavily-doped p-n junction, a tunnel diode (Esaki diode) responds in picoseconds and shows negative differential resistance, making it well suited to ultra-high-speed switching and microwave-frequency oscillator circuits.

• (a)Zener diode — Designed to operate in reverse breakdown for stable voltage regulation/reference — not a switching device.
• (b)Varactor diode — Used as a voltage-controlled variable capacitor in tuning circuits and oscillators, not for switching.
• (d)Schottky diode — Also switches quickly (it is a majority-carrier device with no minority-carrier storage delay) and is prized for low forward-voltage-drop rectification and RF applications — but the 'ultra-high-speed' (picosecond-range) switching credited here comes from the tunnel diode's quantum-tunneling mechanism.

A tunnel diode (Esaki diode) is a heavily-doped p-n junction diode, made so narrow that electrons can quantum-mechanically 'tunnel' straight across the depletion region without needing extra activation energy. This produces a region of negative differential resistance and an extremely fast response, making it useful in ultra-high-speed switching and microwave oscillator circuits.

The exam trap is matching each diode type to its one signature application: Zener → voltage regulation, Varactor → tuning/variable capacitance, Tunnel → ultra-high-speed switching/oscillation, Schottky → low-drop rectification. Several of these diodes 'switch fast' in some sense, so precision about which one earns the 'ultra-high-speed' label matters.

• Tunnel diode (Esaki diode) — heavily-doped p-n junction exhibiting negative resistance via quantum tunneling
• Used in ultra-high-speed switching and microwave oscillator circuits (response time in picoseconds)
• Invented by Leo Esaki, who shared the 1973 Nobel Prize in Physics for tunneling research
• Zener diode → voltage regulation; Varactor diode → variable capacitance/tuning

• Confusing Schottky (fast switching, low forward drop) with Tunnel (ultra-high-speed switching via quantum tunneling) — both are 'fast' but for different physical reasons
• Assuming the Zener diode has a switching application — it is built for voltage regulation

A classic 'diode type → principal application' matching MCQ; MPPSC/UPPSC electronics questions frequently test Zener vs Varactor vs Tunnel vs Schottky roles.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which diode is primarily used for voltage regulation, operating in its reverse breakdown region?

  • (a)Tunnel diode
  • (b)Zener diode
  • (c)Varactor diode
  • (d)Schottky diode

  Answer(b) Zener diode — designed to operate safely in reverse breakdown for a stable voltage reference.

• practice — not a real PYQ

  The tunnel diode was invented by which scientist, who later shared a Nobel Prize in Physics for related work?

  • (a)William Shockley
  • (b)Leo Esaki
  • (c)John Bardeen
  • (d)Walter Brattain

  Answer(b) Leo Esaki — invented the tunnel diode (Esaki diode) and shared the 1973 Nobel Prize in Physics.