Large-Signal Static Compact Circuit Model of SiGe Heterojunction Bipolar Phototransistors: Effect of the Distributed Nature of Currents

This paper presents a physically based opto-electrical compact model of a SiGe/Si heterojunction phototransistor (HPT). This static large-signal model is intended to model the static electrical and optical static operation of the HPT. The measured HPT was developed by extending the structure of an existing SiGe heterojunction bipolar transistor (HBT) from the Telefunken GmbH process technology. Unusual behaviors are observed compared to the standard HBT behaviors when the optical access on the top of the HPT is created. The proposed compact model then includes a phenomenon also called “2-D electrical extension effect” that explains the 2-D (vertical and lateral) charge carriers flow within the SiGe HPT. The validity and accuracy of the proposed model in the static mode of operation are confirmed through I
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-V
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and Gummel characteristics, both under dark and light conditions. The dc responsivity of the HPT is also extracted from the model and agrees quite well to measurement.