Effects of annealing in oxygen on electrical properties of AlGaN/GaN heterostructures grown on Si

•We examined the effects of O2 annealing on electrical properties of AlGaN/GaN heterostructures.•O2 annealing suppresses tunneling leakage current due to surface states or dislocations.•O2 annealing depletes the two-dimensional electron gas and decreases threshold voltage.•O2 annealing increases the resistance of GaN buffer by annihilating 0.38eV deep electron traps.