Silicon On Insulator (SOI) refers to a semiconductor fabrication technique that uses a layered structure consisting of a silicon layer on top of an insulator, typically silicon dioxide (SiO2). This structure enhances device performance by reducing parasitic capacitance and improving control over the fabrication process. SOI wafers are generally more expensive than traditional silicon wafers, costing about three times more. The discussion also touches on the impact of tunneling effects on insulation properties as device dimensions shrink, although specific scaling thresholds were not detailed. Overall, SOI technology is crucial for advanced semiconductor applications, including transistors and micromachines.