Lecture
This lecture discusses the subthreshold operation of NMOS transistors, focusing on the behavior of the device when the gate voltage is below the threshold voltage. The instructor begins by correcting an exercise related to the flatband voltage and the structure of a bipolar transistor. The lecture explains how a small channel is formed when the gate voltage is below the threshold but above the flatband voltage, resulting in a very weak current. The instructor illustrates that the voltage drop occurs mainly at the interface between the channel and the drain, resembling the behavior of a bipolar transistor at low currents. The discussion includes the exponential current characteristics of bipolar transistors and how they relate to the subthreshold region of NMOS devices. Various simulations are presented to show the impact of different gate-source voltages on the current flow, emphasizing the transition from diffusion to drift currents as the gate voltage increases. The lecture concludes with a comparison of the current behavior in strong inversion versus subthreshold operation.