Assertion (A): A PIN diode is used as a fast RF/microwave switch. Reason (R): A PIN diode presents very high resistance when reverse biased and very low resistance when forward biased, enabling effective on/off control.

Introduction / Context:
RF front ends frequently require rapidly controllable switches for routing, protection, and T/R operation. PIN diodes are preferred because they behave like current-controlled resistors at RF, toggling between low-loss conduction and high-isolation states.

Given Data / Assumptions:

• Operation in the RF/microwave domain.
• Bias networks can forward or reverse bias the diode.
• Switching refers to RF path control, not logic-level digital gates.

Concept / Approach:

The intrinsic region of a PIN diode stores charge under forward bias, reducing RF resistance strongly; under reverse bias, the stored charge is removed and RF resistance becomes very high. With proper bias drivers, switching times from microseconds down to nanoseconds are achievable depending on device and power level.

Step-by-Step Solution:

Verification / Alternative check:

Datasheets for PIN-diode switches show low insertion loss and high isolation over wide bands, with specified switching speeds meeting typical RF system needs.

Why Other Options Are Wrong:

• R not explanatory: It directly explains why the device works as a switch.
• A correct but R wrong / A wrong but R correct / Both wrong: Contradict practical RF switch design and measurements.

Common Pitfalls:

Equating “fast” with digital nanosecond logic without considering RF power; forgetting bias network design which limits practical speed.

Final Answer:

Both A and R are correct and R is the correct explanation of A