In this paper a new analytical carrier mobility model of a heterostructure unipolar transistor, High Electron Mobility Transistor (HEMT), is presented. The influence of the two dimensional electron gas confined in a HEMT channel on the device carrier mobility, is considered. The mobility dependence on temperature is also included in the model. Advantages of this model are its simplicity and straightforward implementation. Besides, it promises to be applied to quite different types of HEMTs. The model was tested. The results derived from simulations based on the proposed model are in very good agreement with the already known experimental data and theoretically obtained ones, available in literature.