The relationship between structural defects and device performance of In0.21Ga0.79As/(Al,Ga)As high electron mobility transistors with different In0.21Ga0.79As channel thicknesses (75–300 A˚) was analyzed. Using triple axis x‐ray diffraction and transmission electron microscopy, we determined that the presence of misfit dislocations along only one of the ⟨110⟩ directions did not impair device performance. In fact, the sample with the highest cutoff frequency possessed the misfit dislocations along one ⟨110⟩ direction. However, for thicker samples, with an orthogonal array of misfit dislocations, the device parameters were significantly degraded. We also determined that x‐ray diffuse scattering correlates strongly with device performance, making this nondestructive technique useful for device performance evaluation. ©1995 American Institute of Physics.