On the Excess Noise Factor $\Gamma$ of a FET Driven by a Capacitive Source

The excess noise factor Γ , 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 for Γ in terms of the channel noise factor γ, the gate noise factor δ , and the correlation coefficient c. We explain the difference between Γ and γ and discuss the dependence of Γ on 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 for Γ to calculate its value from measured noise data of a 0.18- μm CMOS technology.