We have performed device simulations on three-dimensional carbon nanotube field effect transistors by solving the Poisson equation self-consistently with the quantum transport equations formulated in the the non-equilibrium Green's function approach. The single-band tight binding Hamiltonian is used for the charge transport on the carbon nanotube surface and detailed numerical aspects for self-consistent calculations, including our simple method to calculate the surface Green's function at the contacts, are presented. Simulation results from our in-house simulator for Schottky-barrier devices, as well as doped source/drain devices, are. also demonstrated.