Semi-insulating (SI) GaN is grown using N₂ as the nucleation layer (NL) carrier gas combined with an optimized annealing time by metalorganic chemical vapour deposition. Influence of using H₂ and N₂ as the NL carrier gas is investigated in our experiment. It is found that the sheet resistance of unintentionally doped GaN can be increased from 10⁴ Ω/sq to 10¹⁰ Ω/sq by changing the NL carrier gas from H₂ to N₂ while keeping the other growth parameters to be constant, however crystal quality and roughness of the film are degraded unambiguously. This situation can be improved by optimizing the NL annealing time. The high resistance of GaN grown on NL using N₂ as the carrier gas is due to higher density of threading dislocations caused by the higher density of nucleation islands and small statistic diameter grain compared to the one using H₂ as carrier gas. Annealing the NL for an optimized annealing time can decrease the density of threading dislocation and improve the film roughness and interface of AlGaN/GaN without degrading the sheet resistance of as-grown GaN significantly. High-quality SI GaN is grown after optimizing the annealing time, and AlGaN/GaN high electron mobility transistors are also prepared.