SCR basics — layers and switching behavior: “SCRs are four-layer devices that are either fully conducting or not conducting.” Determine whether this statement correctly characterizes a silicon-controlled rectifier.

Introduction / Context:
Silicon-controlled rectifiers (SCRs) are cornerstone devices in power electronics. Recognizing their internal structure and bistable conduction behavior is key to understanding controlled rectification and DC choppers.

Given Data / Assumptions:

• SCR structure: four-layer PNPN device with three terminals (anode, cathode, gate).
• Latching behavior: once turned on by gate current and adequate anode current, it remains on until current falls below the holding current.
• Idealized behavior between off (blocking) and on (conducting) states.

Concept / Approach:
An SCR behaves like a latching switch. In the off state, it blocks forward voltage with very small leakage. A gate pulse that raises anode current above latching current turns it on; it then remains on without further gate drive as long as the current stays above the holding threshold. This binary-like operation justifies the phrase “either fully conducting or not conducting” within normal operating regions.

Step-by-Step Solution:

Verification / Alternative check:
V–I characteristics show distinct blocking and conduction regions; application notes define latching and holding currents that delimit the switching behavior.

Why Other Options Are Wrong:

• Incorrect: Contradicts the canonical SCR model.
• True only at low temperature / only DC loads / depends on gate resistance: These factors affect thresholds and triggering ease, not the fundamental bistable nature.

Common Pitfalls:
Assuming gate drive is needed to keep the SCR on; confusing SCRs with transistors that modulate conduction level via continuous base/gate drive.

Final Answer:
Correct