We demonstrate that a new high electron-mobility transistor (HEMT) structure, using an additional n-GaAs cap layer, simultaneously fabricates both the enhancement-mode and depletion-mode of δ-HEMTs on the same chip, thus implementing a direct-coupled field-effect transistor logic circuit. For δ-HEMTs which have a gate dimension of 1×100 µm 2 , the threshold voltage V T for the depletion-mode δ-HEMT is about −1.4 V, while the threshold voltage V T and the maximum applied gate-to-source voltage for the enhancement-mode δ-HEMT are about +0.5 V and +1.6 V, respectively. When V DS = 1.5 V, the output current I D and the g m transconductance are 200 (180) mA mm −1 and 160 (180) mS mm −1 , respectively, for the depletion-mode (enhancement-mode) δ-HEMT. The ac characteristics have also been investigated. Furthermore, it is found that better inverter performances can be obtained by using a narrower gate width of the depletion-mode load with a higher supply voltage.