The physical mechanisms underlying current collapse effects in the passivated GaN high-electron mobility transistors(HEMTs), the gate field-plated GaN HEMTs and the gate-source field-plated GaN HEMTs are investigated in experiments and numerical device simulations. And the intrinsic relationships of the current collapse with the carrier concentration, the probability of traps ionization, and the electric field within the cap layer are established. Results show that the direction of the longitudinal electric field, as well as the intensity distribution of both the transverse and longitudinal electric fields within the cap layer, can be modulated effectively by the field-plates. The electric field intensity near the gate is reduced and that beneath the field-plates increased. Due to the effects of the field-plates on electric field, the transverse movement of electrons near the gate is reduced, and the longitudinal electron movement beneath the field-plates is increased. These affects the electron concentration distribution and the ionization probability of the traps in the cap layer, which makes field-plates effective for the reduction in the current collapse.