Semiconductor device identification: A diode designed so that changing the reverse-bias voltage varies its junction capacitance is called a ______.

Introduction / Context:
Voltage-controlled capacitance is widely used in RF tuning, voltage-controlled oscillators (VCOs), and phase-locked loops (PLLs). The semiconductor device that provides a controllable junction capacitance is the varactor (also called varicap or tuning diode).

Given Data / Assumptions:

• Reverse-biased diode junction is operated well below breakdown.
• Capacitance depends on depletion region width, which varies with reverse voltage.
• Small-signal RF operation around the bias point.

Concept / Approach:
In a varactor, the junction is engineered so the depletion region changes predictably with reverse bias, producing a capacitance C_j(V_R) that decreases as reverse voltage increases. This effect enables electronic tuning without mechanical variable capacitors. Other diode types (zener, tunnel) have different operating principles and intended uses (breakdown regulation and negative resistance respectively).

Step-by-Step Solution:

Verification / Alternative check:
Datasheets for varactors provide C–V curves and quality factor (Q) at RF; tuning ranges in VCOs directly correspond to the chosen varactor’s C_j(V_R) characteristics.

Why Other Options Are Wrong:

• Tunnel diode: Used for high-speed switching/oscillation via quantum tunneling and negative resistance.
• Zener diode: Operated in reverse breakdown for voltage regulation, not for variable capacitance.
• Switching diode: General-purpose fast diode for logic-level switching; capacitance is not the primary feature.

Common Pitfalls:
Biasing a varactor forward by mistake (destroying the tuning function) or neglecting series resistance and Q in RF designs.

Final Answer:
varactor diode