Modeling of non-linear large-signal dynamic effects in SiGe heterojunction bipolar transistors

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The availability of ultra-high-speed silicon-germanium (SiGe) heterojunction bipolar transistor (HBT) process technology has spurred rapidly increasing interest in utilizing the millimeter-wave and THz frequency spectrum for electronic applications. Integrated circuit design and optimization at such high frequencies requires accurate compact (i.e. lumped) models for HBTs with cut-off frequencies of several hundred GHz. Unfortunately, the large-signal (LS) behavior of existing compact models cannot be verified beyond 67 GHz due to the lack of adequate measurement equipment. Numerical device simulation (so-called TCAD) has indicated significant compact model errors during LS switching operation at high frequencies, especially when entering the high-current region where the transconductance peaks. These errors are caused mainly by the distributed character (i.e. non-quasi-static (NQS) effects) of the mobile charge distribution within the transistor in both vertical and lateral dimension, which is not adequately taken into account in present compact models. In the first phase of this project, a physics-based understanding of the NQS effects in vertical and lateral direction has been obtained from TCAD and transistor theory. Based on a closed-form solution of the one-dimensional (1D) transport and continuity equation, a reference for evaluating the impact of simplifications for analytical charge modeling has been established. Furthermore, investigations of the LS operation of two-dimensional (2D) HBT structures show promising results for capturing lateral NQS effects with a lumped mode. Finally, LS measurements revealed so far unexplained third-harmonic distortion behavior. The proposed second project phase addresses the observed problems as follows: (i) Derivation of a physics-based formulation for vertical NQS effects from 1D transistor theory with special emphasis on a more accurate modeling of the high-current region. (ii) Extension of 1D to 2D transistor theory for enabling a fully consistent definition of a compact internal base impedance with the transfer current and the derivation of a physics-based current dependent formulation of the lateral NQS effect related charge partitioning factor in a lumped transistor model. (iii) Model application to advanced SiGe HBT structures and verification of the LS model with both mixed-mode device simulation and measurements using not only load-pull but also a novel passive output filter based approach for obtaining the amplitudes of harmonics up to 325 GHz. The proposed work, subdivided into clearly defined work packages, also includes the design and fabrication of test structures and their experimental on-wafer characterization up to 325 GHz.