Selective and multiple ion implantations directly into a semi-insulating GaAs substrate were utilized to fabricate planar integrated circuits with deep-depletion plasma-grown native oxide gate GaAs MOSFET's. 1.2-µm gate 27-stage enhancement/depletion (E/D) type ring oscillators, with the circuit optimized to reduce parasitic capacitance, were fabricated (using conventional photolithography) to assess the speed-power performance in digital applications. A minimum propagation delay of 72 ps with a power-delay product of 139 fJ was obtained, making these devices the fastest among current GaAs and Si logic fabricated by conventional photolithography. A minimum power-delay product of 36 fJ with a propagation delay of 157 ps was obtained. The power-delay product is comparable with that of 1.2-µm gate GaAs E-MESFET logic, and the speed is more than twice as great. This paper includes a comparison of the theoretical cut off frequency of MESFET and MOSFET logic devices operating in depletion mode. Results indicate that MOSFET logic has superior potential for high-speed operation.