The design and development of computer models to simulate analog devices and their effects on circuit applications has been investigated at length. The focus of this research is the development of theoretical and computer models for discrete devices using the popular simulator PRECISE, PRogram for Evaluating Circuits in an Interactive Simulation Environment [3], using a new method for model construction. This new method develops a model approximating the mathematics of the simulation via perturbations and iterations [19]. The models developed by the new method in each case yield a minimum simulation accuracy of 90 percent in circuit applications. In comparison, models developed by the conventional method, which uses measured data to complete physical constructs of SPICE 2G.6 [5], offer a lower accuracy for the same circuits. Hence, the new method is more effective than the old method and also much faster, since the model generation process is now automated and does not require time-consuming manual measurements and calculations spread out over a long period of time. With further development, a computer model can also be developed for the theoretical model presented in this thesis for the Gallium Arsenide Metal Semiconductor Field Effect Transistor (GaAs MESFET) device using the same methodology that has been used to develop the computer model for the Bipolar Junction Transistor (BUT) device. Hence this research, in addition to developing a library of a hundred and fifty odd successful models in the PRECISE and SPICE formats for the diode and BUT, can also be used to develop a new model for the GaAs MESFET, which would make both PRECISE and SPICE easier and more user friendly as circuit simulators. / Master of Science