Conventional heterostructure field-effect transistors (HFETs) have a high gate leakage current due to the gate electrode being in contact with the exposed channel layer and with the carrier-providing layer on the mesa sidewall. In this study, we use a new step-free (SF) air-bridge gate structure to reduce the gate leakage and improve the breakdown voltage. The proposed structure does not increase any MASKs as compared with the conventional process. In addition, this new structure promises a gate-source capacitance smaller than those of conventional heterostructure FET devices. Consequently, the high-frequency performance of the HFETs using the proposed structure can be improved.