Silicon-controlled rectifier (SCR) structure and terminals: evaluate the claim—“An SCR is a three-layer device consisting of a gate, an anode, and a cathode.”

Introduction / Context:
SCRs are foundational thyristor devices used in power control, rectification, and switching. Accurately recalling their internal structure and external terminals is essential for proper application. This question checks whether you can distinguish between the number of semiconductor layers and the number of terminals in an SCR.

Given Data / Assumptions:

• SCR refers to a standard silicon-controlled rectifier.
• Terminals: gate (G), anode (A), cathode (K).
• Focus is on physical semiconductor layer count vs. external terminal count.

Concept / Approach:
An SCR is a four-layer PNPN semiconductor device with three junctions. It has three terminals: anode, cathode, and gate. The phrasing in the claim confuses layers with terminals. Saying it is a “three-layer device consisting of a gate, anode, and cathode” is incorrect because the layers are PNPN (four layers), and the listed items (gate, anode, cathode) are terminals, not layers. The gate terminal controls turn-on by injecting current into the appropriate junction region, allowing the device to latch in conduction until current falls below the holding current.

Step-by-Step Solution:

Verification / Alternative check:
Textbook cross-sections depict the SCR as a series of P-N-P-N layers. Its equivalent model can be represented by a PNP and an NPN transistor interconnected, reinforcing the presence of four layers and explaining latching behavior.

Why Other Options Are Wrong:
“Correct”: contradicts standard definitions.
Conditional variants referencing low current, germanium devices, or unused gates do not change the internal layer count; they are irrelevant distractions.

Common Pitfalls:
Confusing the number of terminals with the number of layers; thinking that a terminal count equals layer count; overlooking the PNPN depiction.

Final Answer:
Incorrect