Some many-body effects on the electrons in the n inversion layer on the Si (100) surface of the metal-oxide-semiconductor structure have been calculated. Screening was treated in the Lundqvist-Overhauser approximation. In this paper we report calculations on the exchange and correlation energies and effective mass of the electrons in the lowest subband both in the limit of a two-dimensional interacting electron gas and for a finite thickness of the layer. Corrections due to finite oxide thickness and dispersion in the insulator have been investigated and found to have a very small influence on the effective mass. Finally, contributions from the electron-phonon interaction are estimated by deformation-potential theory and found to be negligible. © 1976 The American Physical Society.