3.6.1 Microstructures at the Substrate/Nucleation Layer Interface-Misfit Dislocations -- 3.6.2 Microstructures in the Nucleation Layer: Stacking Faults, Local Cubic Phases, and Antiphase Boundaries -- 3.6.3 Microstructures in High-Temperature GaN Layer: Low-Angle Grain Boundaries, Threading Dislocations, and Point Defects -- 3.6.4 Cracks and Deposits -- References -- CHAPTER 4: ELECTRICAL PROPERTIES AND RELATED MECHANISMS OF GAN HETEROSTRUCTURES USED IN HIGH ELECTRON MOBILITY TRANSISTORS -- 4.1 2DEGs in GaN-Based Heterostructures -- 4.1.1 Formation Mechanism of 2DEGs in GaN-Based Heterostructures