Location of the gate connection in an SCR structure In a conventional thyristor, the external gate lead is bonded to which internal layer?

Introduction / Context:
An SCR is a four-layer PNPN device with terminals anode, cathode, and gate. Understanding the internal structure clarifies why the gate effectively controls turn-on by injecting carriers into a specific junction region.

Given Data / Assumptions:

• Layer sequence (from anode to cathode): P1–N1–P2–N2.
• Anode is connected to P1, cathode to N2.
• Gate terminal must effectively modulate the device near the cathode junction.

Concept / Approach:
The gate is bonded to the inner p-base region (P2), which is adjacent to the cathode (N2). A positive gate current (with respect to cathode) injects holes into this region, lowering the required anode-to-cathode voltage for turn-on and triggering regenerative action of the PNPN structure.

Step-by-Step Solution:
Identify controllable junction: J2 (between N1 and P2) is influenced by gate injection.Thus, the gate must contact P2, the p-layer nearest the cathode.Therefore, the correct internal layer for the gate bond is the p layer nearest to the cathode.

Verification / Alternative check:
Cross-sectional drawings of SCRs consistently show the gate metallization on the P2 region adjacent to the cathode emitter fingers.

Why Other Options Are Wrong:

• Anode p-layer (P1) is not used for gate control.
• N-layer near anode or “outside n layer” does not yield the desired minority carrier injection into the correct junction.
• “Metal substrate only” is irrelevant to junction control.

Common Pitfalls:

• Mistaking the external package pin locations for the internal diffused layer contacts.

Final Answer:
p layer nearest to the cathode terminal