State of the art CMOS devices have been scaled to such dimensions that we need take atomistic approach to understand their operation for nano-electronics. From a bottoms-up perspective, the smallest functional element within a nano-device would be a single (dopant) atom itself. Control and understanding of the eigenenergies and wavefunctions of a single dopant in Si is a key ingredient for device technology beyond-CMOS like quantum-information processing. Here, we will discuss the eigenlevels of a single As donor in a three terminal configuration. The donor is incorporated in the channel of prototype transistors called FinFETs. The measured eigenlevels are shown to consist of levels associated with the donors Coulomb potential, levels associated with a triangular well at the gate interface and hybridized combinations of the two. The theoretical framework in which we describe this system (NEMO-3D) is based on a tight-binding approximation.