Assertion–Reason on switching speed: Assertion (A) Power transistors have lower switching time than SCRs. Reason (R) Power transistors have greater switching time than SCRs.

Introduction / Context:
Switching speed determines the feasible operating frequency and losses in power converters. Comparing devices is common in selecting semiconductors for choppers and inverters.

Given Data / Assumptions:

• Power transistors (BJTs, MOSFETs, IGBTs) versus SCRs (thyristors).
• General, typical device behavior is intended, not niche specialty parts.
• Switching time includes turn-on and turn-off transitions.

Concept / Approach:

SCRs are latching devices with relatively long turn-off (device recovery) times and require current to be forced below holding current. Power transistors are minority/majority carrier devices designed for faster switching. Thus, power transistors typically switch significantly faster than SCRs, enabling higher switching frequencies.

Step-by-Step Evaluation:

Verification / Alternative check:

Application practice: SCR choppers and inverters are limited to comparatively low frequencies, while MOSFETs and IGBTs operate efficiently at tens of kHz and beyond, reflecting faster switching dynamics.

Why Other Options Are Wrong:

• Any option treating R as correct is inconsistent with established device behavior.
• Declaring both wrong would dismiss the accurate assertion about relative speed.

Common Pitfalls:

• Confusing di/dt and dv/dt limits with switching time; they are related but not identical metrics.
• Overlooking that some specialized inverter-grade SCRs are faster but still not as fast as modern transistors for high-frequency PWM.

Final Answer:

A is correct but R is wrong