Two-transistor equivalent model of an SCR The well-known two-transistor model of a thyristor (SCR) consists of which pair of transistors?

Introduction / Context:
The regenerative action in an SCR can be visualized using an equivalent of two coupled bipolar transistors. This model provides intuition for triggering, latching, and holding behavior, and explains why gate injection can rapidly turn the device on.

Given Data / Assumptions:

• SCR layer sequence: P–N–P–N from anode to cathode.
• Model uses two interlinked BJTs sharing internal junctions.

Concept / Approach:

The inner junctions of a PNPN structure can be mapped to a p–n–p transistor and an n–p–n transistor connected in positive feedback. When gate current is applied, it increases the current gain of one transistor, which in turn drives the other, rapidly pushing the sum of current gains toward unity and beyond, leading to latching.

Step-by-Step Solution:

Verification / Alternative check:

Derivations show I_A = I_CPNP + I_CNPN with α_sum approaching 1; holding and latching currents follow naturally from the model.

Why Other Options Are Wrong:

Two identical polarities (both PNP or both NPN) cannot represent PNPN feedback correctly. A UJT is a different device and not part of the SCR model.

Common Pitfalls:

Forgetting that both carrier types contribute in the on-state (conductivity modulation), consistent with the complementary transistor pair.

Final Answer:

One n–p–n and one p–n–p