What is an SCR (silicon-controlled rectifier) in terms of structure and terminals?

Introduction / Context:
The SCR is the foundational member of the thyristor family used in controlled rectifiers, AC regulators, and motor drives. Understanding its internal structure and external terminals is basic to applying it correctly.

Given Data / Assumptions:

• SCR is a four-layer (PNPN) device.
• Three accessible terminals: anode (A), cathode (K), and gate (G).
• Gate provides turn-ON control; turn-OFF requires current reduction.

Concept / Approach:
The PNPN structure can be modeled as a pair of coupled transistors (PNP and NPN). A small gate current initiates regenerative feedback, latching the device into conduction from anode to cathode. Hence, the concise and correct description is “PNPN thyristor with 3 terminals.”

Step-by-Step Solution:
Identify layers: P–N–P–N forms the thyristor core.Identify terminals: Anode on outer P, cathode on outer N, gate into inner P layer region.Gate trigger → conduction → device latches until current < holding current.Therefore, SCR = PNPN, three terminals (A, K, G).

Verification / Alternative check:
Equivalent two-transistor model shows positive feedback loop gain > 1 after gate injection, validating the latch behavior and the structural description.

Why Other Options Are Wrong:

• 4 terminals: No such standard SCR terminal set.
• PNP or NPN thyristor: Thyristors are four-layer, not simple three-layer BJTs.
• Zener device: Two-terminal breakdown diode, unrelated to SCR operation.

Common Pitfalls:

• Expecting the gate to turn the SCR OFF; it cannot.
• Confusing SCRs with TRIACs (which are bidirectional and have MT1, MT2, G).

Final Answer:
a PNPN thyristor with 3 terminals