The functional distinction is the device's switching behavior: thyristors (SCRs), TRIACs, and DIACs are bistable devices that latch into conduction and require a commutation event to turn off, whereas power transistors in 77633 are continuously controllable. This operational difference — not the power rating alone — is the basis for separate classification. Importers of power modules that integrate both thyristors and transistors in a single package should seek a binding ruling to confirm the correct heading.

GTOs and IGCTs are thyristor-family devices and generally fall within the thyristor heading, though their specific HTS 10-digit subheading placement depends on voltage and current ratings. Because these are high-power devices used in traction and grid-scale power conversion, importers should verify the applicable subheading carefully and retain manufacturer specifications, as the duty treatment may differ from standard phase-control thyristors.