For a varactor (varicap) diode used as a voltage-controlled capacitor, how should the applied reverse bias relate to the device’s avalanche breakdown voltage during normal operation?

Introduction / Context:
Varactor diodes provide a voltage-dependent junction capacitance used in tunable filters, VCOs, and phase shifters. Safe biasing is essential to achieve wide tuning ranges without damaging the device or injecting excessive noise.

Given Data / Assumptions:

• Operation in reverse bias to exploit depletion-region capacitance.
• Breakdown voltage V_BR is specified by the manufacturer.
• Goal: maintain reliable, low-noise capacitance control.

Concept / Approach:

The depletion width increases with reverse bias, reducing junction capacitance approximately as C_j ∝ (V_R + V_bi)^−n (with n depending on doping profile). However, exceeding V_BR causes avalanche breakdown with large currents, potential thermal damage, and severe noise, rendering the varactor unsuitable for linear tuning.

Step-by-Step Solution:

Verification / Alternative check:

Datasheets show recommended reverse-bias ranges safely below V_BR, with capacitance–voltage curves specified only in that region. Circuits violating V_BR suffer breakdown and device degradation.

Why Other Options Are Wrong:

• Exceed breakdown: Destroys the device and increases noise.
• Above/below without effect: Reliability is strongly affected by breakdown.
• Much greater than breakdown: Unsafe and unnecessary.
• Alternating with forward conduction: Not how varactors are used; they are not switch-mode capacitors.

Common Pitfalls:

Ignoring RF swing on the junction, which adds to DC bias; failing to account for temperature variation of V_BR; omitting ESD protection.

Final Answer:

It must remain less than the avalanche breakdown voltage