Gallium nitride (GaN) high electron mobility transistors (HEMT) promise very high RF output power density per unit gate periphery, which has been experimentally verified in a number of publications, for example 11.2 W/mm at 8 GHz published by the University of Cornell [Compd. Semicond. 7 (2001)]. Research workers [Electron. Lett. 36 (2000) 469, IEEE Trans. Electron Device 48 (2001) 581, IEEE Trans. Electron Device] have also shown that the GaN HEMT will give surprisingly low microwave minimum noise figures (NFmin). The publications also suggest that the minimum noise figure plotted as a function of frequency can behave in quite different ways. This paper will present a simple analytical discussion on the reasons for the different minimum noise versus frequency behavior. The work will also compare the noise performance trends with the Indium Phosphide (10) metaphorphic HEMT, and briefly discuss the performance limitations of the GaN HEMT. Finally, the work indicates the feasibility of a low microwave noise, sub 0.3 dB (NFmin) in I-band (8-12.4 GHz), with a high drain gate breakdown voltage. (C) 2004 Elsevier Ltd. All rights reserved.