Which one of the following is NOT a negative-resistance microwave device?

Introduction:
Negative-resistance devices can sustain oscillations or provide gain by exhibiting a region where current decreases as voltage increases. Identifying which devices exhibit negative resistance is foundational in microwave electronics.

Given Data / Assumptions:

• Candidate devices: Gunn, Tunnel, IMPATT, Varactor diodes
• Context: microwave generation/amplification vs reactive tuning

Concept / Approach:
Gunn and IMPATT diodes are classic negative-resistance sources for microwave oscillators and amplifiers. Tunnel diodes exhibit negative differential resistance due to quantum tunneling in their I–V curve. Varactors, by contrast, are voltage-dependent capacitors used for tuning and frequency multiplication; they do not operate via negative resistance in normal use.

Step-by-Step Reasoning:

Verification / Alternative check:
Small-signal models: varactor shows C(V) with series R but no negative-resistance region; others have negative differential resistance ranges.

Why Other Options Are Wrong:

• A/B/C: Each is a well-known negative-resistance device used for oscillation/amplification.

Common Pitfalls:
Equating nonlinearity with negative resistance; assuming all “special diodes” share the same property.

Final Answer:
Varactor diode