We have made a study of GaAs/InGaAs/GaAs (001) strained layer heterostructures using Transmission Electron Microscopy (TEM) as a structural tool to determine the misfit dislocation structure and density as a function of Indium concentration. The average misfit dislocation spacing varies from > 10 µm for x < 0.3, to a few microns at x = 0.3, and drops to a few hundred Angstroms at x = 0.5. We did in-situ annealing experiments in order to study the strain relaxation process, measuring the temperature at which the structure begins to relax, and the dislocation velocities. Dislocation velocities are a few microns per second at the growth temperature of 450 ° C, and tens of microns per second at 690 ° C. In addition to interfacial dislocations in the usual <110> directions, in samples where x ≥ 0.4, we observed dislocations running in <100> directions. A study of the electrical characteristics of the material was made in parallel with the structural measurements: the mobility of the InGaAs layer was measured, the material was processed into Heterojunction Bipolar Transistors (HBT’s) and the gain was measured. The electrical characteristics initially improved with the addition of In, peaking at x = 0.1 and dropping sharply for higher x.