In this paper, positive bias temperature instability (PBTI) in fully recessed gate GaN MIS-FETs is studied by using an eMSM (extended Measure-Stress-Measure) technique, which consists of a set of stress/recovery tests. By using this technique, VTH shift after a stress and the relaxation information can be collected in one experiment. First of all, a typical forward-reverse gate sweep and frequency-dependent conductance method are used to characterize VTH shift and interface state density (Dit) in fully recessed gate MIS-FETs with two different gate dielectrics (PEALD SiN and ALD AhO3), showing that ALD AhO3 has a smaller VTH shift compared with PEALD SiN although the latter has a smaller Dit. Then, an eMSM technique is used to understand the trapping/de-trapping phenomena under stress and relaxation period. The results show a power law dependency of VTH shift with respect to the stress time. Furthermore, the voltage dependency of Vth shift (7) can be extracted, showing that ALD Al2O3 has a higher 7 compared to PEALD SiN. The physical model is proposed to explain the mechanism for the different voltage dependency. On the other hand, the relaxation data is collected as well, indicating that Al2O3 has a faster relaxation even under a high voltage overdrive stress, which is consistent with physical model since accessibility of defects in Al2O3 are located at energies less favorable for channel carriers, compared to SiN.