Modulation doped GaAs/In0·25Ga0·75As/Al0·3Ga0·7As high electron mobility transistor structures were grown using different molecular beam epitaxy growth temperatures and In0·25Ga0·75 As channel thicknesses. Drain current deep level transient spectroscopy (DLTS) and standard capacitance DLTS measurements were carried out on these devices using a Fourier transform technique. The results show that traps in the barrier as well as those in the buffer have an influence on the drain current of such transistors. In addition, a broadening of one of the DLTS peaks was observed for high channel thicknesses and is shown to be related to a high increase in dislocation concentration, as confirmed by cathodoluminescence measurements.