On the Excess Noise Factor Γ of a FET Driven by a Capacitive Source

The excess noise factorΓ, also known as Ogawa's noise factor, is frequently used in the literature on optical receivers to calculate the noise and sensitivity of FET front-ends. After revisiting its definition and clarifying its applications and limitations, we derive an analytical expression forΓin terms of the channel noise factorγ, the gate noise factorδ, and the correlation coefficientc. We explain the difference betweenΓandγand discuss the dependence ofΓon the source (photodetector) capacitance. The latter dependence, which is weaker than previously thought, is verified by circuit simulations. Additional insight is obtained by complementing van der Ziel's noise model with Pospieszalski's noise model. Finally, we use the derived expression forΓto calculate its value from measured noise data of a 0.18-μ mCMOS technology.