A multiphase transformer rectifier unit for converting a three-phase alternating current supplied from a power distribution system to direct current supplied to at least one load. The semiconductor device includes: a substrate; a gate structure disposed on the substrate, wherein the gate structure comprises a high-k dielectric layer; and a first seal layer disposed on a sidewall of the gate structure, wherein the first seal layer is an oxygen-free seal layer. The inner vessel can have an inside diameter of at least 50 inches.