Still referring to FIGS. 6A and 6B, next, the isolation structure 23 and the drift oxide region 24 are formed on the top surface 21a and in contact with the top surface 21a. The isolation structure 23 is for defining the device region 23a (as indicated by a dashed frame shown in FIG. 6A). The isolation structure 23 is not limited to the local oxidation of silicon (LOCOS) structure as shown in the figure; in other embodiments, it may be a shallow trench isolation (STI) structure instead. The drift oxide region 24 is formed on the top surface 21a and in contact with the top surface 21a. The drift oxide region 24 is located on the drift region 22a (as indicated by the dashed frame shown in FIG. 2) within the device region 23a, and is in contact with the drift region 22a.
