A semiconductor device has a channel termination region for using a trench (30) filled with field oxide (32) and a channel stopper ring (18) which extends from the first major surface (8) through p-well (6) along the outer edge (36) of the trench (30), under the trench and extends passed the inner edge (34) of the trench. This asymmetric channel stopper ring provides an effective termination to the channel (10) which can extend as far as the trench (30).