In microwave and RF switching applications, which semiconductor diode type is most commonly used as a high-isolation, low-loss switch (controlled by bias current) over a broad GHz range?

Introduction / Context:
Pulsed radar front ends, T/R modules, and microwave test equipment often require electronically controlled RF switches with low insertion loss, high isolation, and wide bandwidth. The device most widely used for this function is the PIN diode, thanks to its current-controlled RF resistance behavior.

Given Data / Assumptions:

• Frequency of interest: microwave (GHz) range.
• Need: reliable switching (on/off) of RF signals.
• Bias control available (forward and reverse bias currents/voltages).

Concept / Approach:

A PIN diode has a wide intrinsic region between the p and n layers. Under forward bias, injected carriers flood the I-region and dramatically lower the RF resistance (on-state). Under reverse bias (or zero bias), carrier density drops and the RF resistance rises to a very large value (off-state). Its junction capacitance is small and nearly voltage independent over useful ranges, giving broadband performance.

Step-by-Step Solution:

Verification / Alternative check:

Measured S-parameters of PIN-diode SPST/SPDT switches show insertion loss often < 1 dB and isolation > 30–50 dB over octave bands when correctly biased and matched.

Why Other Options Are Wrong:

• Varactor diode: Used for voltage-controlled capacitance (tuning/phase), not primary switching.
• Tunnel diode: Negative-resistance device used in oscillators/amplifiers, not typical RF switch.
• Gunn diode: Microwave oscillator in transferred-electron materials, not a switch.
• Schottky diode: Excellent detector/mixer/limiter, but not the standard broadband RF switch.

Common Pitfalls:

Confusing PIN with varactor; forgetting bias networks that isolate DC while passing RF; overlooking thermal limits at high RF power.

Final Answer:

PIN diode