A semiconductor device and a method for forming the same are provided. The semiconductor device comprises: a substrate (1); an insulating layer (2), formed on the substrate (1) and having a trench (21) to expose an upper surface of the substrate (1); a first buffer layer (3), formed on the substrate (1) and in the trench (21); and a compound semiconductor layer (4), formed on the first buffer layer (3), wherein an aspect ratio of the trench (21) is larger than 1 and smaller than 10, wherein the first buffer layer (3) is formed by a low-temperature reduced pressure chemical vapor deposition process at a temperature between 200° C. and 500° C., and wherein the compound semiconductor layer (4) is formed by a low-temperature metal organic chemical vapor deposition process at a temperature between 200° C. and 600° C.