With the development of the semiconductor manufacturing process, the size of the metal-oxide-semiconductor field-effect transistor (MOSFET) device has been on a tens-of-nanometer scale. The shot noise appears in the excess channel noise of the device, and the noise mechanism of the device begins to change gradually. Due to the fact that the electron temperature gradient is neglected in calculation and the significant enhancement of the lateral channel electric field are not taken into consideration, the traditional electron temperature model and the thermal noise model underestimate the effect of hot carrier effects, resulting in the underestimate of the thermal noise. Moreover, the traditional drain-source current model ignores the electron temperature gradient in the calculation and does not include the effect of the electron temperature on the mobility degradation effect either. Therefore, the calculation accuracy of the shot noise and the Fano factor on the basis of the traditional model will be reduced to a certain extent as the size of the device decreases, thus affecting the analysis of the noise mechanism of the device.In this paper, we establish the channel electron temperature model and the electron velocity model by solving the energy balance equation, and develop the drain source current model based on these two models. Moreover, the shot noise model and the thermal noise model suitable for devices below 40 nm are established based on the drain-source current model. Meanwhile, the Fano factor of the shot noise is calculated. The influence of the MOSFET device size on the noise mechanism and the Fano factor of the shot noise are also investigated when the device is under different bias voltages. The results show that the accuracy of the existing thermal noise model and the shot noise model decline as the device size decreases, which eventually leads the Fano factor of the shot noise to be overestimated. When the size of the NMOSFET device is below 20 nm, the shot noise affects the device noise in the strong inversion region. With the size decreasing, the characteristic of the noise mechanism of the NMOSFET device changes from the characteristic of single thermal noise to the common characteristic of both the thermal noise and the shot noise. When the NMOSFET device size is scaled down to 10 nm, the channel noise of the device can no longer be characterized by the thermal noise alone. Instead, the noise mechanism of the device changes and is characterized by both the channel thermal noise and the suppressed shot noise. The shot noise has become an important factor that contributes to the excessive noise in the device.