Distributed substrate resistance noise in fine-line NMOS field-effect transistors

Thermal noise voltage across the distributed substrate resistance induces a fluctuating substrate potential. These random variations couple to the FET channel, giving rise to fluctuations in the channel current. For devices built on epi substrates, this adds 25 percent more noise power to that already existing due to channel thermal noise. More, compact device layouts for high-frequency applications will result in an increase in this noise source. The situation can be partly rectified by using a thinner and less lightly doped epi material.