Low Temperature Modeling of I-V Characteristics and RF Small Signal Parameters of SiGe HBTs
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SiGe HBT has been attached great attention recently to be used for space exploration due to its high-quality performance compared with conventional Si bipolar transistor over an extremely wide temperature range. The currently used compact models fail to correctly function at very low temperature. This work investigates low temperature modeling of I-V characteristics and RF small signal parameters of SiGe HBTs. Compact model Mextram is used as the starting point. A brief introduction of Mextram model is made. Both main current and base current modeling and their temperature scaling in Mextram model are reviewed. New temperature scalable model of main current and base current is proposed and demonstrated with experimental data from 393 to 43 K. The temperature dependent ideality factor is proved necessary to model the low temperature current-voltage characteristics deviation from Shockley theory prediction. The relation delta_VBE = (kT/q)ln(JC1/JC2), which is widely used in bandgap references (BGR) circuits, is shown no longer valid at low temperature. The effect of tunneling on low temperature forward operation current is examined. Trap-assisted tunneling (TAT) current dominates the forward non-ideal base current. The way to distinguish tunneling current and main base current from forward gummel base current measurement is shown. A tunneling current model is developed to fit the lower bias region of forward base current from 110 to 43 K. Small signal model is used to extract device small signal parameters. A two-step hot-after-cold optimization procedure is successfully used to fit Y-parameters from 1 to 35 GHz. The temperature dependence of important equivalent circuit parameters and implication on cryogenic RF circuit are examined.