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Which semiconductor device is designed to act as a voltage-controlled capacitor?

The varactor diode is a voltage-controlled capacitor.

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  1. Tunnel diode
  2. Varactor diode ✔
  3. Silicon-controlled rectifier

Why: A reverse-biased varactor changes its junction capacitance as the voltage changes. Radios use that to tune oscillators and filters without moving parts.

Huh, didn't know that: A Schottky barrier diode is a metal-semiconductor junction (E6B08).

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If you patch a broken model train bridge with spare diodes, what do you call it? A bridge rectifier.
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References

Answer: Varactor diode. A reverse-biased varactor changes its junction capacitance as the voltage changes. Radios use that to tune oscillators and filters without moving parts.

  1. NCVEC 2024-2028 Amateur Extra Question Pool, E6B04 (4th errata, Feb. 4, 2026) www.ncvec.org/downloads/2024-2028%20Extra%20Class%20Question%20Pool%20and%20Syllabus%20Public%20Release%20with%204th%20Errata%20Feb%204%202026.pdf
    A. Varactor diode
    Checked 2026-10-11.
  2. NCVEC 2024-2028 Amateur Extra Question Pool, E6B08 (4th errata, Feb. 4, 2026) www.ncvec.org/downloads/2024-2028%20Extra%20Class%20Question%20Pool%20and%20Syllabus%20Public%20Release%20with%204th%20Errata%20Feb%204%202026.pdf
    A. Metal-semiconductor junction
    Checked 2026-10-11.

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