LASCR internal structure A light-activated SCR (LASCR) contains how many semiconductor layers and junctions?

Introduction / Context:
The LASCR is an SCR that can be triggered by light impinging on its gate region. Despite the optical triggering method, the internal thyristor structure remains that of a conventional SCR.

Given Data / Assumptions:

• LASCR is a PNPN device (thyristor).
• Light generates carriers near the gate area to initiate conduction.

Concept / Approach:

The classic SCR comprises four layers (PNPN) and three junctions (J1, J2, J3). The LASCR leverages the same layered stack; the difference lies only in how turn-on is initiated (photons instead of, or in addition to, electrical gate current).

Step-by-Step Solution:

Verification / Alternative check:

Datasheets specify LASCR gate sensitivity to light but list the same blocking ratings and junction structure as standard SCRs.

Why Other Options Are Wrong:

• 2- or 3-layer structures cannot provide thyristor-like latch characteristics.
• “Any of these depending on rating” is incorrect; ratings change geometry, not the fundamental layer count.

Common Pitfalls:

Confusing photodiodes or phototransistors (different layer counts) with LASCRs.

Final Answer:

4 semiconductor layers and 3 junctions