Fundamental electronic properties of methyl-modified n-Si(111) interfaces created by the displacement of Cl on Si(111) surface by the methyl group from Grignard reagents (RMgX; X=Br or Cl, R = methyl) were measured in the presence and absence of water. The presence of water played a significant role in determining the behavior of the devices. The structure of Ga-In/methyl monolayer/n-Si(111) was not rectifying but only consisted of series resistances. On the other hand, a diode-like rectifying property was observed with the water/methyl layers/n-Si(111) heterojunction structure. Functionalization of Si(111) surfaces with various alkyl moieties such as methyl, ethyl, propyl, butyl, and mixed methyl/propyl, was studied by electrochemical measurements performed in a mixture of 3 mM K3Fe(CN)(6)/3 mM K4Fe(CN)(6)/1 MKCl/H2O and by surface characterization with synchrotron radiation X-ray photoelectron spectroscopy. The transistor characteristics of field effect transistor-like devices with water/alkyl/n-Si(111) gate structures were investigated. The device properties clearly depended on the alkyl moiety: transistor-like behavior was observed only for devices with methyl moiety, and for those with a mixed methyl-propyl monolayer prepared by using Grignard reagents with mixed methyl and propyl (CH3MgBr:CH3CH2CH2MgBr=1:9) moieties, whereas no transistor-like behavior was observed for devices with butyl or ethyl moiety. (c) 2007 The Electrochemical Society.