  We wish to report in this paper a study of the effective mass (m^*) in
thin-oxide Si-metal-oxide-semiconductor field-effect-transistors, using the
temperature dependence of the Shubnikov-de Haas (SdH) effect and following the
methodology developed by J.L. Smith and P.J. Stiles, Phys. Rev. Lett. {\bf 29},
102 (1972). We find that in the thin oxide limit, when the oxide thickness
$d_{ox}$ is smaller than the average two-dimensional electron-electron
separation r, m^* is still enhanced and the enhancement can be described by
$m^*/m_B = 0.815 + 0.23(r/d_{ox})$, where $m_B = 0.195 m_e$ is the bulk
electron mass, $m_e$ the free electron mass. At $n_s = 6 \times 10^{11}/cm^2$,
for example, $m^* \simeq 0.25 m_e$, an enhancement doubles that previously
reported by Smith and Stiles. Our result shows that the interaction between
electrons in the semiconductor and the neutralizing positive charges on the
metallic gate electrode is important for mass enhancement. We also studied the
magnetic-field orientation dependence of the SdH effect and deduced a value of
$3.0 \pm 0.5$ for the effective $g$ factor in our thin oxide samples.
