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The origin of the current collapse which is present in HEMTs on AlGaN/GaN heterostructures is assigned to deep level states in the layer system. Using photoionization spectroscopy we investigated these levels in doped and undoped HEMT structures on sapphire. In both HEMTs we found two different trap energies of about 3.2eV and 2.9eV. By varying the gate voltage we found that a decrease of the gate bias leads to an increased occupation of trap states involved in the current collapse, which indicates that the concentration of trapped donor surface states has a strong influence on the current collapse.