We present experimental studies and simulations on gate-controlled doping in carbon-based field-effect transistors. While the low density of states in carbon-based semiconductors allows e.g. the investigation of the coupling between a metal and graphene, it is detrimental to the functionality of novel device concepts such as band-to-band tunneling FETs. Creating appropriate doping profiles in the source/drain contacts with additional gates, on the other hand, avoids all dopant-related issues. It will be shown that gate-controlled doping allows obtaining optimum transistor performance which enables exploiting the exceptional properties of graphene and carbon nanotubes for future nanoelectronics devices.