Bulk and SOI (Silicon-on-Insulator) are not different transistor types, but two different wafer technologies – the question is not which MOSFET is built, but what it sits on. Both approaches can be combined with the same basic principles from the MOSFET chapter; the difference lies solely in the substrate underneath and how the active region is electrically separated from it.
In classic bulk technology, the transistor sits directly in a solid, homogeneous silicon wafer. Electrical separation from neighbouring devices and from the rest of the substrate is achieved through doped wells and the reverse-biased p-n junctions they form – an electrical, but not a physical, isolation.
SOI takes a different approach: beneath the thin, active silicon layer in which the transistors are fabricated lies a buried oxide layer (BOX) of silicon dioxide, with the actual carrier substrate only below that. This oxide layer physically and completely isolates the active region from the substrate – not just electrically via a junction, but through a genuine insulator.
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