Trech MIS device with active trench coreners and thick bottom oxide and method of making same

A trench MOSFET (40) includes active corner regions (25) and a thick insulative layer (33) centrally located at the bottom of the trench (19). A thin gate insulative layer (15) lines the sidewall and peripheral portion of the bottom surface of the trench (19). A gate (14) fills the trench, adjacent to the thin insulative layer (15). The gate (14) is adjacent to the sides and top of the thick insulative layer (33). The thick insulative layer (33) separates the gate (14) from the drain conductive region (13) at the bottom of the trench (19) yielding a reduced gate-to-drain capacitance and making the MOSFET (40) particularly suitable for high frequency applications.