AbstractTwo‐dimensional electron gas transport in AlN/GaN and AlGaN/GaN heterostructures has been investigated employing geometrical magnetoresistance measurements and quantitative mobility‐spectrum analysis. The channel magnetoresistance of ungated four‐terminal test structures, with an effective width to length ratio of 10, was measured using pulsed drain‐to‐source voltages equivalent to longitudinal electric fields up to 750 V/cm at magnetic field intensities up to 12 T and sample temperatures from 10 to 300 K. Two distinct electron populations, with significantly different mobilities, are shown to be present in the channel of both AlGaN/AlN/GaN and AlGaN/GaN heterostructures. It is also shown that application of longitudinal electric fields up to 750 V/cm cause a reduction in the mobility of these carrier populations and change the shape of mobility spectrum of the dominant electron population. The origin of these carrier populations is discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)