In AlGaN/GaN HEMTs, slow current transients are often observed even if the gate voltage or the drain voltage is changed abruptly. This is called gate lag or drain lag, and problematic in circuits applications. The slow transients mean that dc I-V curves and RF I-V curves become quite different, resulting in lower RF power available than that expected from dc operation. This is called current collapse. It is recognized that experimentally and theoretically that the introduction of field plate reduces the lag phenomena and current collapse. Recently, it is reported that introducing a high-k passivation layer can improve the breakdown voltage of AlGaN/GaN HEMTs. Therefore, in this work, we study how the high-k passivation layer affects the lag phenomena and current collapse in field-plate AlGaN/GaN HEMTs. The drain lag, gate lag and current collapse appear as the drain current reduction from the steady state drain current ID. So, we evaluate the current reduction rate as functions of field-plate length LFP, passivation-layer thickness Ti and its relative permittivity er. We plot the current reduction rate due to drain lag, gate lag and current collapse as a function of Ti. Then it is found that the curve takes a minimum value depending on er. When er is 7, the minimum occurs at Ti = 0.03 um. When er is 20, 30 and 50, the minimum occurs at Ti = 0.1 um, 0.1 um, and 0.2 um, respectively. Therefore, at the minimum, the value of er/Ti, which is proportional to the capacitance, becomes nearly constant around 250/um between er = 7 and 50. In this condition for Ti, the lags and current collapse are reduced when LFP becomes long for all er.