|Abstract:||We present the first ab initio study of semiconducting carbon nanotubes adsorbed on the unpassivated Si(100) surface. A dramatic reduction of the semiconducting gap for these hybrid systems as compared with the electronic gaps of their isolated constitutive components was found. This is caused by the changes in the electronic structure as the surface reconstructs due to the tube's proximity, the concomitant electronic charge transfer from the nanotubes, and the band hybridization with silicon and carbon states resulting in the appearance of states within the energy gap of the formerly isolated nanotube. It is determined that semiconducting nanotubes exhibit weaker adsorption energies and remain at a greater distance from the Si(l00) surface as compared to metallic nanotubes of similar diameter; this may be useful for the solid-state separation of metallic and semiconducting nanotubes. The electrostatics and band alignment of carbon nanotubes adsorbed in the fully passivated Si(1OO) surface have also been investigated. Spectral data indicate a relative displacement, depending on the type of doping, of the surface's band edges from those of adsorbed nanotubes, and this is confirmed by the ab initio calculation of semiconducting nanotubes on the Si(l00):H-2xl surface. A markedly different spatial charge redistribution occurs depending on the dopant specie involved. Finally, and in a different direction, we calculate the effects of a longitudinal electrostatic perturbation on a metallic carbon nanotube to demonstrate conductance modulation. The external modulation would be screened in bulk metals but occurs in nanotubes because of their quasi two-dimensional shape allowing electrons to interact with nearby charges. This modulation is determined by the strength of the self-consistent potential and its periodicity over shorter or longer distances. We employ the zero temperature singleparticle Green's function transport approach in the empirical tight-binding approximation to quantify the conductance.