Low field mobility in double- and single-gate structures is analyzed for (100) and (110) SOI substrate orientation. A Monte Carlo algorithm for vanishing driving fields allows the calculation of the mobility for arbitrary scattering rates and band structure without further approximations. Due to volume inversion, mobility in double-gate ultra-thin body (110) SOI FETs is enhanced in comparison with the mobility of single-gate structures in the whole effective field range. In double-gate (100) structures the mobility decreases below the single-gate value for high effective fields. It is argued that the twice as high carrier concentration in double-gate FETs causes significant occupation of higher subbands, where mobility is low, and that additional inter-subband scattering channels for the lowest subband are opened. These effects partly compensate the mobility enhancement due to volume inversion and lead to a mobility decrease in double-gate (100) structures. (c) 2007 Elsevier Ltd. All rights reserved.