Structure of an SCR (thyristor) How many doped semiconductor regions are present in a conventional silicon-controlled rectifier (SCR)?

Introduction / Context:
The SCR (thyristor) is a four-layer semiconductor device fundamental to line-commutated converters and controlled rectifiers. Knowing its layer structure is critical for understanding its two-transistor analogy, triggering behavior, and turn-off requirements.

Given Data / Assumptions:

• Conventional SCR with anode (A), cathode (K), and gate (G) terminals.
• Silicon technology assumed; doping alternates by layer.

Concept / Approach:

An SCR has a PNPN structure, i.e., four doped layers forming three junctions (J1, J2, J3). The gate terminal modulates carrier injection near J2/J3 to initiate regenerative action that latches the device on, provided the anode-cathode circuit supplies sufficient current above latching/holding thresholds.

Step-by-Step Solution:

Verification / Alternative check:

The two-transistor model (PNP + NPN) constructed from the PNPN stack reinforces the four-region structure.

Why Other Options Are Wrong:

2 or 3 layers cannot realize the characteristic thyristor latching behavior; 5 or 6 layers describe other multi-junction devices (e.g., LASCR variants) not the basic SCR.

Common Pitfalls:

Confusing number of layers (4) with number of junctions (3); mixing SCR with triac (which is effectively two SCRs in inverse parallel within one package).

Final Answer:

4