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In this letter, ultrathin-barrier AlN/GaN high-electron mobility transistors (HEMTs) capped with in situ metal-organic-chemical-vapor-deposition-grown SiN have been successfully fabricated on 100-mm Si substrates. Output current density exceeding 2 A/mm has been reached, which represents, to the best of our knowledge, the highest value ever achieved for GaN-on-Si HEMTs. This results from the high 2DEG density of the optimized AlN/GaN heterostructure. Despite the ultrathin barrier of 6 nm, low gate and drain leakage currents of about 10 μA/mm are obtained without the use of a gate dielectric that generally induces reliability issues. Furthermore, the high aspect ratio (gate length Lg/gate-to-channel distance) and low RF losses (at the buffer/Si substrate interface) are reflected in excellent RF performances. The current gain extrinsic cutoff frequency fT and the maximum oscillation frequency fmax were 52 and 102 GHz with a 0.2- μm gate length, respectively, resulting in an fT·Lg product as high as the reported state-of-the-art GaN-on-Si HEMTs.