Multicarrier transport planar fully-depleted silicon-on-insulator MOSFETs has been investigated employing magnetic-field dependent geometrical magnetoresistance measurements and high-resolution mobility spectrum analysis. The results indicate that electronic transport in the 10 nm thick Si channel layer is due to two distinct and well-defined electron species. According to self-consistent Poisson-Schrödinger calculations, the two distinct electron species detected correspond to carriers in distinct energy sub-bands arising from strong carrier confinement and volume inversion. The mobility peak of the dominant carrier was found to occur under gate bias conditions that result in a minimum perpendicular effective electric field.