Thermal atomic layer deposition (ALD) grown AlN passivation layer is applied on A1GaN/GaN-on-Si HEMT and the impacts on drive current and leakage current are investigated.The thermal ALD grown 30nm amorphous AlN results in a suppressed off-state leakage, however, its drive current is unchanged.It was also observed by nano-beam diffraction method that thermal ALD amorphous AlN layer barely enhanced the polarization.On the other hand, the PECVD deposited SiN layer enhanced the polarization and resulted in an improved drive current.The CV measurement also indicates that thermal ALD passivation results in a better interface quality compared with the SiN passivation.