Abstract
A semiconductor structure includes a first III-V compound layer. A second III-V compound layer is disposed on the first III-V compound layer and is different from the first III-V compound layer in composition. The second III-V compound layer has a top surface. A source feature and a drain feature are disposed on the second III-V compound layer. A gate electrode is disposed over the second III-V compound layer between the source feature and the drain feature. A fluorine region is embedded in the second III-V compound layer under the gate electrode. The fluorine region has a top surface lower than the top surface of the second III-V compound layer. A gate dielectric layer is disposed under at least a portion of the gate electrode and over the fluorine region.