Simple Expression of the Thermal Noise Excess Factor for LNA Design

This paper proposes a simple model of the thermal noise excess factor in saturation gamma(nsat) for short-channel transistors affected by velocity saturation. It is shown that gamma(nsat) in strong inversion and saturation under velocity saturation increases proportionally to the inversion coefficient IC reaching values as high as 3.5, which is about 4 times higher than the long-channel value. The proposed model is successfully validated against measurements made on various CMOS technologies. It is then used in the expressions of the input-referred thermal noise resistance R-n and minimum noise factor F-min which are compared to measurements made on a 40nm RF transistor. The model perfectly captures the behavior of R-n and NFmin versus IC and particularly the minimum that is reached on the upper side of moderate inversion.