This paper presents a study of the different phenomena that define the large signal behaviour of the GaAs-MESFET as trapping effects and operating point dependence (real heating). These phenomena have been included in a CAD-oriented circuital model. The pulsed measurements have been made maintaining the transistor at each quiescent operating point for several seconds in order to make the operation temperature constant. Considering the effects described before, a CAD oriented large signal MESFET model capable of simulating DC, pulsed, RF and the I/V characteristic variations with the operating point has been developed. The Ids current is modelled by two non-linear sources, one of them is a bias point dependent nonlinear equation and the other one represents the differences between DC and Pulsed characteristics at every bias point. Experimental pulsed characteristics and simulations, for a 10*140 µm chip transistor from GEC-Marconi foundry, have been carried out, showing excellent agreement. Furthermore, a complete model of this transistor has been obtained. Successful comparisons between MDS simulations using the extracted model and experimental Scattering and power measurements (loaded by 50 Ohms at the input and output ports) have been done. Also, this method has been used to model a 6000 µm chip transistor from PML foundry showing a very good agreement.