The effects of sputtered and room temperature plasma enhanced chemical vapour deposition (RT-PECVD) SiNx passivation on the dc and microwave performance of AlGaN/GaN high electron mobility transistors (HEMTs) are studied. The pulsed I-V characteristics from a class B quiescent bias point and transient measurements indicate that the sputtered SiNx passivation is more efficient in suppressing lag effects in AlGaN/GaN HEMTs. Dispersion-free sputtered SiNx passivated AlGaN/GaN HEMTs were obtained using this technique. Continuous-wave (CW) measurements without active cooling give a maximum output power density of 6.6 W mm-1 at Vgs ≤ -4 V, Vds ≤ 50 V and a maximum power added efficiency of 51.3% at Vgs ≤ -4 V, Vds ≤ 30 V at 3 GHz on 2 × 50 νm AlGaN/GaN HEMTs on the sapphire substrate, with a gate length of 2 νm and without field-plated gates. To the best of our knowledge, this is the highest level power density reported on the sapphire substrate without field-plate design. The extrinsic cut-off frequency (ft) and maximum oscillation frequency (fmax) are 51 GHz and 100 GHz, respectively, on 2 × 50 × 0.15 νm HEMTs. To our knowledge, the sputtered SiN x passivation for AlGaN/GaN HEMTs is a unique technique, which has never been published before.