The research work presented in this book is based on two direct and wide band gap semiconductors: ZnO and GaN. On the first part of the book, the synthesis of ZnO nanorod array via the low temperature solution growth method was discussed. Due to the high surface-to-volume ratio of ZnO nanorod, to alleviate the some of the drawbacks such as carrier mobility and thickness dilemma of organic solar cells, ZnO nanorod array were integrated into organic solar cells. Power conversion efficiency (η) of 1.8% is achieved in our ZnO nanorods integrated bulk heterojunction organic solar cells on flexible In2O3-PET substrates. On the second part of the book, the fabrication and characterization of Aluminum gallium nitride/gallium nitride high electron mobility transistors (AlGaN/GaN HEMTs) were discussed. Device testing and characterization under both room temperature and high temperature up to 300 °C were performed. The results show that the device can operate even at 300 °C with minimal degradation.