SOI wafer,which has a structure that SiO₂ inserted between Si substrate and surface Si layer.Parasitic capacitance of transistor can be reduced therefore it's effective in improving operation speed and reducing electricity consumption.

In a conventional integrated-circuit, isolation between elements are formed by reversing PN connection, however floating capacitance will be resulted between parasitic diode or substrate,as a result,signal delay or electric leakage to substrate will be occurred.

In order to reduce this floating capacitance, an isolation film is formed beneath MOSFET channel, SOI is used to reduce floating capacitance.

Moreover, a wafer including such an isolation film is called SOI wafer. On the other hand, a conventional wafer sometimes is called bulk silicon (or bulk wafer) to distinguish from SOI wafer.