Silicon-germanium alloys have proven remarkably reliable in radioisotope thermoelectric generators and are the current materials of choice for the SP-100 space reactor program. However, even after three decades of study and extensive practical experience (including the spectacular Voyager missions) the best available models of the thermoelectric properties of these materials are little more than interpolation schemes based on experimental data. The present study extends a previous model for the thermal conductivity and, for the first time, applies conventional transport theory to develop a model for the electrical properties of n-type silicon germanium. Moreover, it is shown that treating the electrical and thermal models self-consistently predicts the observed properties of silicon-germanium with good accuracy using only three, physically interpretable parameters. The nature of the model and agreement with experiment are discussed. (3 References).