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A general ballistic FET model that was previously used for ballistic MOSFETs is applied to ballistic high electron mobility transistors (HEMTs), and the results are compared with experimental data for a sub-50 nm InAlAs-InGaAs HEMT. The results show that nanoscale HEMTs can be modeled as an intrinsic ballistic transistor with extrinsic source/drain series resistances. We also examine the "ballistic mobility" concept, a technique proposed for extending the drift-diffusion model to the quasi-ballistic regime. Comparison with a rigorous ballistic model shows that under low drain bias the ballistic mobility concept, although nonphysical, can be used to understand the experimental phenomena related to quasi-ballistic transport, such as the degradation of the apparent carrier mobility in short channel devices. We also point out that the ballistic mobility concept loses validity under high drain bias. The conclusions of this paper should be also applicable to other nanoscale transistors with high carrier mobility, such as carbon nanotube FETs and strained silicon MOSFETs.