Hexagonal boron nitride sheets have been noted especially for their enhanced properties as substrates for sp2 carbon-based nanodevices. To evaluate whether such enhanced properties would remain under various realistic conditions, we investigate the structural and electronic properties of semiconducting carbon nanotubes on perfect and defective hexagonal boron nitride sheets under an external electric field as well as with a metal impurity, using density functional theory. We verify that the use of a perfect hexagonal boron nitride sheet as a substrate indeed improves the device performances of carbon nanotubes, compared with the use of conventional substrates such as SiO2. We further show that the hexagonal boron nitride even with some defects can perform better performance as a substrate. Our calculations, on the other hand, also suggest that some defective boron nitride layers with a monovacancy and a nickel impurity could bring about poor device behaviors since the imperfections impair electrical conductivity due to residual scattering under an applied electric field. ; Comment: 7 pages, 4 figures