The performance of C-60-based organic vertical field-effect transistors (VFETs) is investigated as a function of key geometrical parameters to attain a better understanding of their operation mechanism and eventually to enhance their output current for maximal driving capability. To this end, a 2D device simulation is performed and compared with experimental results. The results reveal that the output current scales mostly with the width of its drain electrode, which is in essence equivalent to the channel width in conventional lateral-channel transistors, but that of the source electrode and the thickness of C-60 layers underneath the source electrode also play subtle but important roles mainly due to the source contact-limited behavior of the organic VFETs under study. With design strategies acquired from this study, a VFET with an on/off ratio of 5.5 x 10(5) and on-current corresponding to a channel length of near 1 mu m in a conventional lateral-channel organic field-effect transistor (FET) is demonstrated, while the drain width of the VFET and the channel width of the lateral-channel organic FET are the same.