Summary form only given. The authors describe three novel UMOSFET structures, called the accumulation, inversion, and extended trench field-effect transistors (ACCUFET, INVFET, EXTFET). The principal difference between these structures and the conventional UMOSFET is that the trench (UMOS) gate extends all the way down to the n/sup +/ substrate. A detailed comparison of all the UMOSFET structures has been performed. Two-dimensional numerical simulations using PISCES have demonstrated that a specific on-resistance approaching 100 mu Omega -cm/sup 2/ can be obtained for devices capable of supporting 25 V. For experimental verification, devices were made using a six-mask process and SF/sub 6//O/sub 2/ RIE (reactive ion etching) to form trenches with gate oxide thickness ranging from 260 to 900 AA. The measured specific on-resistances of the experimentally fabricated devices with a gate oxide thickness of 720 AA were found to be 125, 195, and 245 mu Omega -cm/sup 2/ for the ACCUFET, EXTFET, and INVFET structures. >