Nanometer-size Si quantum dots (Si-QDs) with and without Ge core were prepared on thermally-grown SiO2 in a self-assembling manner by controlling the early stages of low pressure chemical vapor deposition (LPCVD). The surface potential changes in individual dots caused by charging or discharging of one electron or a few as were measured by using a Kelvin probe technique in an atomic force microscope (AFM). For Si-QDs larger than 20nm in dot height, surface potential images with a characteristic potential profile with a dimple around the center of the charged dots are observable after electron or hole injection, indicating Coulomb repulsion among the charges retained in the dot. For Si-QDs with a Ge core, electrons are retained stably in Si clad while holes in Ge core, reflecting the energy band discontinuity at the interface between the Si clad and the Ge core. The influence of phosphorous doping to Si-QDs on their electron charging and discharging characteristics was also been studied. Electrical characteristics of metal-oxide-semiconductor (MOS) capacitors and n-channel MOS field-effect-transistors (nMOSFETs) with Si-QDs floating gates confirm multiple-step charging to and discharging from the Si-QDs floating gate at room temperature. From the temporal changes in the drain current with gate voltage switching, it is suggested that the change in the electron distribution in the Si-QDs floating gate play an important role to trigger the transition from a metastable charged state to the next charged state.