The invention relates to a method of fabricating a CMOS device, comprising providing a semiconductor substrate (101) having therein a layer of insulating material (102), the method comprising providing a layer (106) of a first material over the insulating layer (102), the thickness of the layer (106) of the first material being less in a first region (103) for supporting a first active device than in a second region (104) for supporting a second active device. A layer (107) of a second material is then deposited over the layer (106) of a first material, and the structure is then subjected to a thermal treatment to alloy the first and second materials. The portion of the layers over the first region is entirely alloyed, whereas the portion of the layers over the second region is not, so that a portion (109) of the layer (106) of the first material remains.