Isotype heterojunctions make it possible to produce new microwave devices. At present, technological problems are still important and often, there is not a full understanding of their behaviour. In the paper, the fundamental parameters ofnGe -nGaAs heterojunctions are determined from the frequency variations of their impedancesZ(ω, V). To account for the large impedance variations, a heterojunction model is proposed. By using the values of the capacitance and the parallel conductance for different frequencies, the fundamental parameters of several isotype heterojunctions are then determined. The calculated characteristics agree well with the experiment.