Assertion–Reason on DIAC structure and conduction Assertion (A): A DIAC has four semiconductor layers but only two external terminals. Reason (R): A DIAC conducts in both directions (it is a bilateral trigger device).

Introduction / Context:
DIACs (diode for alternating current) are two-terminal trigger devices used to symmetrize TRIAC firing and to generate consistent breakover behavior in either polarity. Their internal structure explains both their two-terminal nature and their bidirectional conduction.

Given Data / Assumptions:

• DIAC: four-layer, two-terminal device (often PNPN symmetrical).
• Breakover occurs at approximately the same magnitude in either polarity.
• No gate terminal exists; conduction is controlled by applied voltage magnitude.

Concept / Approach:
The DIAC’s symmetrical four-layer construction produces similar breakover characteristics for positive and negative voltages. Because it is bilateral, it uses only two terminals, and its behavior does not depend on polarity (apart from small asymmetries). This bilateral conduction is the essential reason that only two terminals are needed despite four layers.

Step-by-Step Solution:
Assess A: “Four layers but two terminals” is a factual description of DIAC construction—true.Assess R: “Conducts in both directions” is also true.Does R explain A? Yes; bilateral conduction arises from a symmetrical multilayer structure that does not require separate anode/cathode designation, hence only two terminals.

Verification / Alternative check:
Component datasheets and application notes show DIACs as two-terminal symmetrical PNPN devices with essentially equal positive/negative breakover voltages.

Why Other Options Are Wrong:

• B: Splits the truths but denies the explanatory link; however, symmetry and bilateral action are exactly why the device is two-terminal.
• C/D/E: contradict established device structure and operation.

Common Pitfalls:

• Confusing DIAC (no gate) with TRIAC (three terminals, bidirectional controlled device).
• Assuming four layers always imply three terminals like SCRs; symmetry changes terminal count.

Final Answer:
Both A and R are correct and R is correct explanation of A