In this study, we demonstrate the preparation of P-I-N type GaN ultraviolet (UV) detectors on self-supported substrates, and then investigate their forward current transport mechanisms. The results show that the electron diffusion current starts to dominate only when the forward voltage V_F>2 V. Moreover, the effective forbidden bandwidth E_g~2.21 eV is much lower than the ideal value, which can be attributed to the energy band perturbation introduced by the conductive dislocations. An ideal factor n>2 when 1.35 V<V_F<2 V indicates that the electron defect-assisted tunneling current is the dominant current component. The current has a negative temperature coefficient, which is primarily caused by the increase in the effective forbidden bandwidth of the electron once it is excited to a higher energy conduction band. In the V_F<0.8 V and 0.8 V<V_F<1.35 V regions, the current-voltage curves are power dependent; this behavior is consistent with the electron space charge confinement mechanism. The power factors are eight and four, respectively, and two different effective energy bandwidths, corresponding to two exponentially decaying defect state distributions, are obtained from the characteristic temperature.