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AlGaN/GaN heterostructure field effect transistors with different gate recess depths have been fabricated using an ICP etch process. Subsequently, electrical DC characterization has been performed. The results have been compared with the theoretical predictions according to the simple charge control model, showing that for a given AlGaN/GaN structure there is an optimum thickness with respect to the maximum achievable transconductance. In our case a maximum of 210 mS/mm for the transconductance is found experimentally at approx. 14.5 nm barrier thickness under the gate. Furthermore, we took a detailed look at the measured threshold voltage which has a change in sign for a 9.5 nm gate-channel separation, demonstrating that an enhancement mode HFET has been realized.