MOSFET Unity-Gain Frequency

MOSFET Unity-Gain Frequency

by Vincent Chang

Formal & Physical Sciences Engineering & Technology TJFElectronics engineeringTJFDElectronic devices and materials
🎙 Vincent Chang 👥 2K 📅 February 24, 2021 ⏱ 17 min 👁 1K 📄 tutorial 🧭 2026-08-17
Available in: English (current) Français

Keywords

MOSFETunity-gain frequencysmall-signal modelhigh-frequency responsecurrent gain

Summary

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.

169 words

Critical Evaluation

Value of the Information & Strength of the Argument

The lecture provides a clear and logical derivation of the MOSFET unity-gain frequency, building on previous lessons. The use of the h-parameter model and the step-by-step circuit analysis makes the argument easy to follow. The instructor explicitly states assumptions, such as gm >> ωCgd, and explains their validity. The comparison to an op-amp integrator helps contextualize the frequency response. The value lies in connecting device physics to circuit-level performance, offering practical insights for IC design.

Scientific Rigor, Source Quality, Title Accuracy

The scientific rigor is high, as the derivation follows standard small-signal analysis and device physics. The instructor’s credentials and experience lend credibility. However, the video does not cite specific sources or references, relying solely on the instructor’s expertise. The title accurately reflects the content, and the presentation is well-structured. No external sources are provided in the description, so the analysis is based solely on the lecture content.

157 words

Title / Content Match

The title accurately reflects the content, which focuses on deriving and explaining the unity-gain frequency of a MOSFET.

Quality & Reliability

8/10

The lecture is based on established semiconductor device physics and small-signal modeling, with clear derivations and assumptions. The instructor has extensive academic and industry experience. However, the video lacks formal citations or references to specific sources, and the presentation is a single instructor's explanation.

Key Moments

Markers derived by PSI from the transcript: the creator did not define chapters.

Contribution & Novelties

The lecture provides a clear and concise derivation of the MOSFET unity-gain frequency, connecting device-level capacitances to circuit-level performance. It emphasizes the physical meaning of fT as the boundary between amplification and loss, and relates it to bias and geometry. This is a fundamental concept for IC designers.

Pour aller plus loin :

80 words

Radar Profile

The radar profile shows high scores in quality, technical level, and reliability, with a slightly lower score in quantity of information, reflecting the focused scope of the lecture.

Reliability 8/10