The invention relates to a method of manufacturing a semiconductor device (10) comprising a substrate (11) and a semiconductor body (12) in which at least one semiconductor element (1) is formed, wherein on the substrate (11) a semiconductor layer (2) is formed comprising a mixed crystal of silicon and germanium, further called the silicon-germanium layer (2) and having a lower surface close to the substrate (11) and an upper surface more remote from the substrate (11), and wherein the silicon-germanium layer (2) is subjected to an oxidizing treatment at a surface of the silicon-germanium layer (2) while the other surface of the silicon-germanium layer (2) is protected against the oxidizing treatment by a blocking layer (3). According to the invention, the blocking layer (3) is formed on the upper surface of the silicon-germanium layer (2), a cavity (5) is formed in the semiconductor body below the silicon-germanium layer (2) and the lower surface of the silicon-germanium layer (2) is subjected to the oxidizing treatment through the cavity (2). In this way, a device 10 may be obtained in which the surface of the silicon-germanium layer (2) after the oxidizing treatment does not suffer from roughening and/or germanium pile up. This enables e.g. to manufacture in particular a MOSFET on top of or in the silicon-germanium layer (2) with excellent properties and high yield.