The fabrication of self-aligned top gate a-IGZO TFT with source-drain doped by hydrogen diffusion was studied. The doping of hydrogen was achieved during the growth of SiNx passivation layer by PECVD. The experimental results show that the performance of the device is greatly affected by the option for the gate dielectric etching following the gate electrode patterning. For the devices with gate dielectric layer etched, it is found that the leakage current is large, which may be caused by the etching residue on the side wall of active layer; The VTH of short channel devices is negative and the mobility decreases after annealing, which is caused by the deep H lateral diffusion. For the devices without gate dielectric layer etched, it is found that the VTH of devices is positive, because of the suppression of H lateral diffusion thanks to the inhibiting effect of SiO2 gate dielectric on H doping.The on-state current and mobility of devices increases after annealing, which may be due to the entrance of H in gate dielectric to the source and drain area by thermal diffusion, reducing resistance of source and drain area.