By using thin AlN film as a passivation layer between the GaAs and HfO2, GaAs metal-oxide-semiconductor (MOS) devices exhibited well-behaved accumulation and inversion capacitance-voltage characteristics with low leakage current of 6 x 10(-5) A/cm(2) at V-g=1 V and interface state density of similar to 4.9 x 10(10) cm(-2) eV(-1). In order to improve the N+ source/drain activation efficiency, the SUP coimplantation technique was adapted to fabricate GaAs n-type metal-oxide-semiconductor field-effect-transistor (MOSFET). High quality N+/P GaAs junction was achieved with extremely low junction leakage current of 3 x 10(-6) A/cm(2) at reverse bias of 1 V and high forward current to reverse current ratio of I-forward/I-reverse similar to 10(7) at GaAs source/drain junction. GaAs n-MOSFETs integrated with HfO2 gate dielectric and TaN gate electrode were fabricated by using AIN passivation layer and Si/P coimplantation techniques. The results show that our GaAs n-MOSFETs operate in enhancement mode with I-on/I-off ratio of approximately six orders and good transfer and output characteristics. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3025909]