Controlling Threshold Voltage in Semiconductor Devices

This lecture discusses the control of threshold voltage in semiconductor devices, focusing on the effects of oxide thickness and substrate doping. The instructor begins by analyzing a basic scenario involving a lightly doped semiconductor and a gate voltage at threshold. The impact of increasing the oxide thickness is examined, revealing that while the substrate remains unchanged, the voltage required to maintain the threshold increases due to the thicker oxide. Next, the lecture explores the consequences of increasing substrate doping while keeping the structure at threshold. It is shown that higher doping reduces the depletion region but increases the electric field at the interface, resulting in a higher threshold voltage. The relationship between the electric field in the oxide and the semiconductor is also analyzed, demonstrating how variations in dielectric constant affect the threshold voltage. The lecture concludes by addressing the implications of lowering the threshold voltage too close to zero, highlighting the risk of increased leakage current in densely integrated circuits.