The minimum energy of the first conduction subband varies with gate voltage in trigate silicon-on-insulator metal-oxide-silicon field-effect transistors (MOSFETs) in subthreshold operation. In an inversion-mode trigate device, the energy level of the lowest subband increases with electron concentration, while it decreases under the same conditions in some accumulation-mode devices. As a result of this quantum effect, the subthreshold swing of accumulation-mode trigate FETs is smaller than predicted by classical theory. This effect is not observed in fin-shaped FETs and gate-all-around MOSFETs and can be amplified by modifying the device cross section. (C) 2008 American Institute of Physics.