Colloidal semiconductor nanocrystals (NCs) have been shown to be promising materials for electronic and optoelectronic device applications because of their unique size-dependent properties and low-cost solution processability. However, the integration of these materials into devices has been challenging due to a lack of available methods to: 1) accurately control charge carrier statistics, such as majority carrier type and concentration, and carrier mobilities, and 2) efficiently passivate surface defects inherent in NC materials arising from their high surface-volume ratio. In this thesis, we study the fundamental physics of charge carriers paramount for device application. Then, we introduce several measurement techniques to characterize the type, concentration, and mobility of charge carriers and the density and energy of surface states. Lastly, we propose a novel, systematic, and rational method to engineer those properties, in order to design high performance electronic and optoelectronic nanostructured devices. We develop stoichiometry control method through thermal evaporation or solution based atomic layer deposition to precisely control the electronic and optoelectronic properties of nanocrystals. We demonstrate that remote doping in nanostructured device is effective and a promising route to realizing high mobility and reducing scattering, in contrast to commonly pursued substitutional doping methods. Thermal diffusion doping process to passivate the trap states and the use of small ligands to enhance the electronic coupling are introduced. In addition, we emphasize the important role of the metal-semiconductor interface and semiconductor-gate dielectric layer, to enhance charge injection and prevent charge trapping, respectively. Through the careful engineering of the interface and junction, as well as the precise charge carrier statistics and trap states controls, we design and fabricate low cost, high performance nanocrystal thin film field-effect transistors, photodetectors, and solar cells. Finally, we introduce novel techniques, correlated scanning photocurrent microscopy and scanning confocal photoluminescence measurement system, that can explore the photoelectric and photophysical properties of semiconductor structures and devices.