Subthreshold Swings, SSs, smaller than theoretical 'In(10)kT/q' are observed in nanowire junctionless (JL) and inversion mode (IM) MuGFETs. The low SS is attributed to the floating body effect in the channel, which is initiated by impact ionization. Comparing with IM devices, decreased drain voltage (V-d) is required for the generation of steep subthreshold slopes in JL devices and the floating body is relatively dynamic. In both JL and IM MuGFETs the floating body effect is suppressed for narrow devices, which indicates this effect is geometry dependent. Moreover, it shows that the short channel JL devices with enhanced electric field can exhibit steep subthreshold slopes at lower V-d. It gives another option to decrease the supply voltage (V-dd) for low power applications of JL devices. (C) 2013 Elsevier Ltd. All rights reserved.