Stacked nanosheet field effect transistors are the potential candidate for future CMOS-based design as a successor of the FinFET technology due to their improved short channel effect and higher effective width in the same area footprint. This research investigates the impact of self-heating and substrate effects on the small signal conductance and capacitance of FinFET and nanosheet devices using Synopsys TCAD. The presence of substrate introduces transitions in the conductance–frequency plot due to the substrate and self-heating effects in FinFET examined. Further, nanosheet devices show similar trends, with less pronounced substrate effects which is discussed first time in this work. Analyzing the effect of the type of substrate, voltage, work function, substrate doping, and electrode resistance variation provides further insights into the substrate-induced transition in the Nanosheet FET. These findings contribute to advancing nanoscale device technologies, particularly in RF applications.