This chapter discusses the Tunnel Field-Effect Transistors (TFET) current-voltage dependence on several physical parameters is performed considering devices designed with different materials. As TFETs present particular electrical characteristics, this chapter aims to identify key parameters in order to improve the TFET performance in circuit applications that require low and large intrinsic resistance at forward and reverse bias conditions respectively. In TFETs, the doping levels in the source and drain regions must be carefully optimized in order to improve the electrical characteristics of the device, i.e. improve the on-current and decrease the leakage current. In contrast, for a Ge-TFET the increase of the drain-doping concentration is shown to decrease the reverse current of the device. Therefore, in TFETs, the choice of drain doping concentration has to be carefully chosen in order to mitigate the diffusion and excess current, without enabling a reverse BTBT mechanism between the drain and source regions.