Self-heating
Beginner
A transistor warming itself up as it works, because the heat can’t escape fast enough from such a tiny space.
Novice
The temperature rise of a transistor from its own current. Fins and nanosheets are thin and surrounded by insulators that conduct heat poorly, so they warm up more than flat transistors did, which slows them and ages them faster.
Expert
Local channel heating set by power density and the thermal resistance to ambient. Confined fins and oxide-wrapped sheets raise that resistance; heat leaves mainly through the substrate, contacts and wiring. It shifts delay and accelerates BTI, HCI and electromigration.
Explained in Fins and nanosheets (Transistors).
See also: FinFET, Gate-all-around (GAA) nanosheet.