SCR in forward-blocking (off-state) When a silicon-controlled rectifier (SCR) is forward-biased but still blocking (no gate trigger applied), how is the externally applied forward voltage distributed across its internal PN junctions?

Introduction / Context:
An SCR (PNPN device) in forward-blocking condition has its anode positive with respect to cathode, but no gate current is applied. Understanding which junction blocks the forward voltage is foundational for device physics and protection design.

Given Data / Assumptions:

• Device: SCR with junctions J1 (P–N), J2 (N–P), J3 (P–N) from anode to cathode.
• Forward-blocking: anode positive, gate open (no trigger), device non-conducting.
• Quasi-static conditions, neglecting leakage and transient effects.

Concept / Approach:

Under forward bias without gate triggering, junctions J1 and J3 are forward biased, whereas the central junction J2 is reverse biased. The reverse-biased junction sustains almost the entire applied forward voltage, keeping the device in the blocking state until breakover or gate triggering occurs.

Step-by-Step Solution:

Verification / Alternative check:

Two-transistor model equivalence shows insufficient regeneration without gate injection; thus central junction remains reverse-biased and holds the voltage.

Why Other Options Are Wrong:

Equal sharing among two or three junctions (options b, c) contradicts the biasing state (only J2 is reverse-biased). Option d is vague and not representative of steady forward-blocking physics.

Common Pitfalls:

Confusing forward-blocking with reverse-blocking, and assuming uniform voltage sharing irrespective of bias polarity.

Final Answer:

Appears across only one junction