Ever since the discovery of graphene, the 2D carbon structure material has been attracting a lot interest due its electrical, thermal and mechanical properties. Here, we investigate the carbon nanotubes (CNT), wrapping a 2D graphene sheet to form a 1D carbon structure. CNT has a Dirac-cone energy band, which makes it either a semiconductor or metal. With continuous aggressive scaling, the technology using copper as interconnect is reaching its limit on current density, thermal and conductivity performance, thus finding a new interconnect material is an ongoing quest. CNT with its high conductivity, high electro-migration, and low temperature effect opens a door for on-chip interconnect applications. However, the uncontrollable chirality of pure CNT motivates us to explore doped CNTs. Doping alleviates the need to control chirality and enhances the metallic property of CNTs. In this work, we start first to describe the fundamental physics of doped CNTs and describe how doping changes the electrical property of CNTs. We also provide analytical models and simulation results on CNT conductance variation for N-type doped CNTs. Simulation results show that doping can reduce overall CNT resistance by 89%.