Using the advantages of reduced junction capacitance and completely isolated structure, we are studying how to apply and expand into high-frequency devices used in communication LSIs. Next, we would like to discuss SOI-CMOS device performance as an RF (Radio Frequency) circuit component that is required to operate at high frequencies. Figure 4 illustrates a fundamental RF circuit block. The SPDT (Single Pole Double Throw) switch functions to separate transmission from reception and is required to reduce signal transmission loss as much as possible. Figure 5 illustrates the frequency dependency of the SPDT switch transfer coefficient in SOI/ bulk MOS transistors. Since the junction capacity is small with SOI, it is possible to minimize signal transmission loss even in high-frequency areas. Also, in the case of RF circuits, many passive devices such as inductors and capacitors are used for impedance matching and frequency selection. There is much signal loss with normal Si substrates due to the high conductivity. The Q value of passive devices manufactured on this substrate decreases. Figure 6 illustrates the Q value frequency dependency when a spiral inductor is manufactured on a Si substrate with a different resistance via an equivalent insulating film. It has been confirmed that a high Q value can be obtained in the high-frequency area for inductors manufactured on high-resistance substrates. Highresistance substrates cannot be used with bulk CMOS devices since effects such as reduced latch-up tolerance would result. On the other hand, since SOI-CMOS devices employ a complete isolation structure, it is possible to use a highresistance substrate as the supporting substrate. It is also possible to combine passive devices that have superior highfrequency characteristics.