The present invention relates to an SOI semiconductor device and a method for fabricating an SOI semiconductor device, in which the portions formed with silicide layers are laterally restricted by spacers to a predetermined range in the diffusion regions to be used for diodes or well resistors. In this manner, it is possible to fix the length of distance between the sides of a silicide layer and a diffusion region, greater than that available in the prior art techniques, thereby minimizing power leakage at the sides of the diffusion regions. In the SOI semiconductor device thus constructed, the diffusion regions to be used for diodes (or well resistors) are constructed with spacers in a double junction structure of different density of impurity layers (for instance, a P− or N− layer respectively surrounds a P+ or N+ layer), in other words, only onto a high density of impurity layer, the P+ or N+ layer, or in a single junction structure in which the spacers restrict a range of space for forming the silicide layer in the diffusion region.