GaN HEMTs can realize high-power-density operation with low power loss in power electronic systems due to ultra low specific on-resistance below the Si-limit. The dynamic on-resistance, however, is degraded by the current collapse phenomena. The relation between the dynamic on-resistance and the maximum electric field peak showed universality, which was independent from the field plate (FP) structure and the wafer. The gate-edge electric field strongly affects the increase of the dynamic on-resistance. Yellow luminescence intensity strongly related with current collapse phenomena and can be utilized as a useful index for improving the wafer quality. High speed switching was obtained at the turn-off switching test with an inductive load. The switching speed of the GaN-HEMT can be controlled by the external gate resistance as same manner as the conventional Si-MOSFET. For the compatibility with the same loss and dV/dt, the gate resistance must be set to 10 times higher than that at the Si-MOSFET due to low Qgd.