Junction biasing in a reverse-biased SCR When a standard PNPN thyristor (SCR) is reverse biased (anode negative with respect to cathode), which junctions are reverse/forward biased?

Introduction / Context:
An SCR consists of a PNPN structure with three junctions (J1, J2, J3). Understanding which junctions are forward or reverse biased in forward-blocking and reverse-blocking states explains why the device blocks voltage until turned on.

Given Data / Assumptions:

• Conventional SCR with anode at outer P layer and cathode at outer N layer.
• Reverse bias means anode potential is lower than cathode.

Concept / Approach:
In forward blocking (anode positive), J1 and J3 are forward biased, J2 is reverse biased. In reverse blocking (anode negative), the biasing swaps such that J1 and J3 become reverse biased and J2 becomes forward biased. The two reversed junctions enforce blocking.

Step-by-Step Solution:
Identify layers: P1–N1–P2–N2 with junctions J1 (P1–N1), J2 (N1–P2), J3 (P2–N2).Apply reverse bias (anode negative): junctions at outer terminals oppose current flow.Outcome: J1 and J3 reverse biased; J2 forward biased → overall reverse blocking.

Verification / Alternative check:
Two-transistor model (PNP–NPN) under reverse bias also indicates both outer junctions block, preventing conduction.

Why Other Options Are Wrong:
All three reverse/forward: Not possible with PNPN under simple reverse polarity.
One reverse and two forward (option c): Describes forward-blocking state, not reverse-blocking.

Common Pitfalls:
Forgetting the swap between forward and reverse blocking arrangements or mislabeling junction order.

Final Answer:
two junctions are reverse biased and one junction is forward biased.