Isolation methods and devices for isolating regions of a semiconductor device. The isolation method and structure include forming an isolating trench in an active area and filling the trench with a doped conductive material containing silicon. Suitable conductive materials containing silicon include polysilicon and silicon-germanium. There is also provided a method and structure for isolating the regions by providing a trench in an active area of a substrate, growing an epitaxial layer in the trench to fill the trench or to partially fill the trench and depositing an insulating material over the epitaxial layer and within the trench to completely fill the trench.