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We present a mechanism for the vertical leakage induced current degradation with identification of the properties of the relevant traps in AlGaN/GaN heterostructures on Si. The extent of the current degradation is determined by back-gating sweep measurements in double directions at different sweep rates and temperatures. It is found that the current degradation is only observed at relatively slow sweep rates and high temperatures. Time dependent back-gating measurements further suggest that the current degradation process is related to traps with long time constants. By comparing with the measurement results of samples on sapphire substrates, we confirm that the current degradation is caused by vertical leakage in heterostructures on Si. On the basis of the vertical leakage induced current degradation mechanism and in conjunction with the long-time degradation process, we measure both the trapping and detrapping processes of the relevant trap states to identify their properties. We find that there is a 0.6 eV capture barrier and a 0.67 eV emission barrier for the trap states, indicating that the trap states are of large lattice relaxation. [ABSTRACT FROM AUTHOR]