In this paper, a novel approach to the measurement of mobility of GaAs HEMT devices is presented. The new approach employs high-order derivatives as a means of determining the parameters of the proposed new mobility equation. The new approach is compared to established mobility measurement methods, and shown to offer better accuracy. The results presented also consider the behavior of mobility in the linear and saturation bias regions. The mobility value extracted by this new method has permitted improvements to the MESFET/HEMT model when simulating the behavior of the device in the linear region. This is critical in many applications, such as in low current linear-mixing applications.