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This paper presents a systematic and rigorous analytical parameter-extraction method for a heterojunction bipolar transistor (HBT) small-signal equivalent-circuit model. The proposed method relies exclusively on S-parameter measurements. Exact closed-form equations are used for the direct extraction of circuit elements. The method is characterized by its simplicity and ease of implementation. It is applied to predict the small-signal characteristics of transistors from different foundries. Excellent agreement between modeled and measured S-parameters is observed up to 20 GHz.