A transmission electron microscopy (TEM) study was carried out on a series of AlGaN/GaN high-electron mobility transistor (HEMT) structures grown by plasma assisted molecular beam epitaxy (PA-MBE) on Si (111). Threading dislocation (TD) behavior and density were investigated for three heterostructures using an AlN/GaN superlattice and/or differently strained GaN layers. Threading dislocation densities (TDDs) were measured by TEM (at different depths) and high resolution x-ray diffraction (HRXRD) allowing one of the most complete and few studies so far, presenting separated values on edge, screw and mixed type TDs quantities. (C) 2012 Elsevier B.V. All rights reserved.