Experimental as well as theoretical studies have been performed on the velocity-field characteristics of AlGaN/GaN heterostructures. A comparison of these studies shows that the experimental velocities are comparable to those expected from previously published simulations based upon Monte Carlo techniques. Several possible mechanisms for the low value of the velocity previously found are discussed, including nonequilibrium phonons and local inhomogeneities in the field. (C) 2004 American Vacuum Society.