The aim of the paper is to present a simple noise model using the Delegedeudeauf formulation for the Fukui constant K-F, in order to estimate the minimum noise figure of Gallium nitride (GaN) based MESFET/HEMT devices. The Fukui model is compared with published experimental results of a 0.15 mum gate-length GaN HEMT and shows good agreement with the measured minimum noise figure between 5 and 26 GHz. The work indicates that with an improved F-I, which is dependent on material quality, very low noise figures of 0.4 dB at 12 GHz may be feasible from a GaN HEMT.