SOI (silicon on insulator)
Beginner
A chip base where the transistors sit on a very thin layer of silicon above a layer of glass. The glass blocks leak paths underneath.
Novice
A wafer with a thin layer of crystalline silicon on top of a buried oxide (insulator). Transistors built in a thin enough top layer have no deep silicon underneath for current to leak through.
Expert
Ultra-thin-body SOI and double-gate SOI were the first answers to short-channel effects: making small shrinks . Fully depleted SOI (FD-SOI) is still used for low-power planar processes; FinFETs moved the same idea into a vertical fin on bulk wafers.
Explained in Shrinking (Transistors).
See also: Natural length (scale length, λ), FinFET.