Trench mis device having implanted drain-drift region and thick bottom oxide

Abstract

A trench MIS device is formed in a P-epitaxial layer that overlies an N-epitaxial layer and an N+ substrate. In one embodiment, the device includes a thick oxide layer at the bottom of the trench and an N-type drain-drift region that extends from the bottom of the trench to the N-epitaxial layer. The thick insulating layer reduces the capacitance between the gate and the drain and therefore improves the ability of the device to operate at high frequencies. Preferably, the drain-drift region is formed at least in part by fabricating spacers on the sidewalls of the trench and implanting an N-type dopant between the sidewall spacers and through the bottom of the trench. The thick bottom oxide layer is formed on the bottom of the trench while the sidewall spacers are still in place. The drain-drift region can be doped more heavily than the conventional 'drift region' that is formed in an N-epitaxial layer. Thus, the device has a low on-resistance. The N-epitaxial layer increases the breakdown voltage of the MIS device. In alternative embodiments, the thick bottom oxide layer can be omitted.