The Pt/YMnO3/Y2O3/Si structure for a metal/ferroelectric/insulator/semiconductor (MFIS) field effect transistors was fabricated and the effect of a Y2O3 layer on the properties of MFIS structure was investigated. The Y2O3 thin films on p-type Si(111) substrate growth by pulsed laser deposition were crystallized along (111) orientation irrespective of the deposition temperatures. Ferroelectric YMnO3 thin films deposited directly on p-type Si (111) by metalorganic chemical vapor deposition resulted in a Mn deficient layer between Si and YMnO3. However, YMnO3 thin films having good quality and stoichiometric composition can be obtained by adopting a Y2O3 buffer layer. The memory window of the YMnO3 thin films with a Y2O3 film is greater than that of the YMnO3 thin films without a Y2O3 film after the annealing at 850 degreesC in vacuum ambient (100 mTorr). The memory window is 1.3 V at the applied voltage of 5 V.