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It is transistors in which the 'bulk' or 'body' terminal is an isolating material (for example, SiO2 or Saphire) instead of semiconductor (for example silicon). The bulk terminal in SOI transistors is mainly left floating. The power savings with SOI mainly comes about from savings in dynamic power since parasitic capacitances for an SOI transistor are usually smaller then in bulk devices. For the same reason, SOI devices are usually faster, however there are some penalties to pay:
- The potential at which the floating body sits makes the transistor dependent on previous activity (how long ago was it switched last, what state it was kept in before the current switch). This is referred to as SOI history effect. It makes timing analysis for digital designs a little trickier than for bulk and makes analog design A LOT harder.
- Floating body effects impose need for larger cross-coupled inverters in SRAM bit cells and flip flops/latches so area-wise SOI sometimes looses in comparison to same size geometry bulk process.
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