Device modeling

Models of electronic devices have to express the physical structure of the device, represent its significant properties, and lead to useful equivalent circuits for circuit analysis. The criteria by which a model can be said to achieve each of these three functions are explored and it is shown, with particular reference to transistors, that the demands of each function are so different that no previously proposed model is wholly satisfactory. The wisdom of trying to achieve a general theory of modelling is consequently questioned, and it is suggested that a deliberate separation might be preferable.