Consequences of quantum-mechanical phenomena in SOI devices with very thin semiconductor layers and ultrathin gate oxides are theoretically considered. The effect of quantization of electron energy in the very thin semiconductor layer in a double-gate SOI transistor on the drain current and phonon mobility is discussed. The applications of tunneling through the ultrathin gate oxide in the MOS SOI tunnel diode, MOS(n(+)p)/SOI tunnel diode and MOS/SOI transistor are considered.