Within thyristor families, what is a DIAC best characterized as?

Introduction / Context:
DIACs are commonly used as trigger devices in AC control circuits, especially for firing TRIACs in lamp dimmers and motor speed controllers. Recognizing their directional behavior is essential for correct application in alternating-current waveforms.

Given Data / Assumptions:

• Device type: DIAC (diode for AC).
• Two terminals, no gate.
• Used symmetrically on AC lines.

Concept / Approach:
A DIAC conducts only after its breakover voltage is reached; importantly, this breakover and conduction are symmetrical in both polarities. That makes it a bidirectional trigger device. It is not gated and does not behave like a unidirectional diode or a transistor.

Step-by-Step Solution:
Consider polarity: DIAC’s V–I curve is symmetric about the origin.Below breakover (both polarities), current is negligible.At breakover, the DIAC snaps into conduction and then resets when current falls.This behavior in both directions identifies it as bidirectional.

Verification / Alternative check:
Practical lamp dimmers place a DIAC in series with a TRIAC gate to produce symmetrical triggering on positive and negative half-cycles, reducing waveform distortion—clear evidence of bidirectional action.

Why Other Options Are Wrong:

• Transistor: Has three terminals and different physics.
• Unidirectional: Would conduct in one polarity only (like a rectifier diode or zener in breakdown direction).
• Three-layer device: DIAC structures vary; the key characteristic is bidirectionality.
• A gated triode device: Describes a TRIAC (with a gate), not a DIAC.

Common Pitfalls:

• Confusing DIAC (2-terminal) with TRIAC (3-terminal).
• Assuming all trigger diodes are unidirectional zener-like devices.

Final Answer:
bidirectional device