MOSFET Unity-Gain Frequency

This lecture by Vincent Chang concludes the series on MOSFET small-signal models by focusing on the unity-gain frequency (fT) of the device. The instructor begins with a simplified high-frequency model that reduces the five internal capacitances to two key ones: Cgs and Cgd. He then introduces the h-parameter model, specifically the forward current gain h21, and applies it to the MOSFET in a common-source configuration with the output short-circuited. Through circuit analysis, he derives the high-frequency current gain transfer function, showing that it behaves like an integrator with a -20 dB/decade roll-off. The unity-gain frequency is identified as ωT = gm/(Cgs + Cgd), where the current gain drops to 0 dB. The lecture emphasizes that below this frequency the device acts as an amplifier, while above it, it exhibits loss. The instructor also relates fT to device parameters such as transconductance, capacitances, and geometry, noting that increasing bias or reducing channel length increases fT. The lecture concludes with a preview of the next topic: bipolar junction transistor small-signal models.