We have grown high-electron mobility transistor structures in the Al0.3Ga0.7As/In0.25Ga0.75As/GaAs material system with In0.25Ga0.75As channel thicknesses in the range 40-200 Angstrom. We have monitored the onset of channel relaxation using Hall mobility measurements, polychromatic cathodoluminescence mapping, time-resolved photoluminescence, transmission electron microscopy, low-frequency noise,and deep-level transient spectroscopy measurements. It appears that the first relaxation symptom, the Stransky-Krastanow growth mode, is observed only by the last three techniques. This shows that the onset of relaxation is not detected by characterization techniques which measure global properties of the material. On the other hand, it is detected by low-frequency noise, deep-level transient spectroscopy, and transmission electron microscopy measurements, which yield an estimation of the defect density in the material. (C) 1996 American Institute of Physics.