Localized anodic thinning

A protective mask and a method of localized anodic thinning to prevent excessive thinning of an epitaxial layer in the forming of a field effect transistor. A moat exposing the epitaxial layer is provided in the protective mask surrounding a device pattern or group of patterns to be formed in the epitaxial layer by anodic thinning. During the thinning process, the area of the epitaxial layer exposed by the moat becomes fully depleted and cuts off current into the region surrounded by the moat and prevents excessive thinning of the epitaxial layer under the design pattern.