In power electronics, what is an SUS (Silicon Unilateral Switch)? Choose the description that correctly identifies its internal construction and how it achieves unilateral triggering, while keeping in mind how it differs from an SCR, UJT, and PUT.

Introduction / Context:
In power electronics and triggering circuits, the Silicon Unilateral Switch (SUS) is a small-signal thyristor-like device used to generate a precise pulse when its breakover condition is met. Understanding its internal construction helps distinguish it from SCRs, UJTs, and PUTs.

Given Data / Assumptions:

• SUS provides unilateral triggering at a preset breakover voltage.
• Comparison is with SCR, UJT, and PUT.
• Focus is on internal structure and functional equivalence.

Concept / Approach:
The SUS can be modeled functionally as a PUT combined with a built-in avalanche reference path. This arrangement establishes a repeatable trigger point without external zeners or elaborate biasing.

Step-by-Step Solution:
1) SUS goal: produce a sharp trigger when V reaches a designed breakover.2) PUT alone needs external biasing; adding an avalanche diode between gate–cathode sets a reference.3) When anode–cathode voltage and gate conditions satisfy breakover, the device snaps on unilaterally.4) Hence, a practical equivalence is “PUT + avalanche diode (anti-parallel across gate–cathode)”.
Verification / Alternative check:
Datasheets often state SUS as a PUT derivative with an integral avalanche junction to establish V_BO precisely. That differentiates it from an SCR (power switch) or UJT (two-layer device with emitter injection behavior).

Why Other Options Are Wrong:

• Exactly similar to an SCR: incorrect; SCR is a power device with different structure and bilateral gate physics.
• Similar to a UJT: incorrect; SUS behaves closer to PUT than UJT.
• SCR + avalanche diode in parallel: not the way SUS is realized; parallel across anode–cathode is not the SUS reference path.
• None of the above: incorrect because the correct construction is known.

Common Pitfalls:

• Confusing SCR family devices; SUS is low-power triggering, not a high-power controller.
• Thinking the avalanche diode is across anode–cathode instead of gate–cathode.

Final Answer:
A PUT with an avalanche diode in anti-parallel between gate–cathode (Option C).