In this work, the effect of the source doping profile due to possible fabrication aspects has been investigated and discussed for both carbon nanotube tunneling field effect transistors (TFETs) and metal oxide semiconductor field effect transistors (MOSFETs). A quantum 2D simulator, which is based on self-consistent solution of non-equilibrium Green's function and Poisson's equation, is utilized. It has been found that the source doping gradient remarkably affects the drain current, especially in the case of TFET. This raises the need for fabrication precautions and design to retain higher ON-state current. Besides, the high frequency behavior of TFET and MOSFET structures is inspected in terms of cut-off frequency f T. It is found that the source doping profile has a more significant impact on TFETs than on MOSFETs. For better performance, source doping needs to be abrupt. However, with a small source-gate underlapping, degradations due to gradual doping can be reduced. It is also found that a TFET with source/drain low-k oxide spacer can achieve more immunity to these degradations than the single oxide structure.