This Is AuburnElectronic Theses and Dissertations

Improvements of Current and Charge Models in Mextram 505.2-505.3




Li, Yiao

Type of Degree

PhD Dissertation


Electrical and Computer Engineering

Restriction Status


Restriction Type


Date Available



SiGe heterojunction bipolar transistors (HBTs) are rapidly growing as commercial IC technologies in the modern industry [1]. The relevant circuit design is closely related to the accuracy of the compact model. This work provides an overview of Mextram model, new features of Mextram 505.2-505.3, and its capabilities to meet the challenges of SiGe HBT modeling and circuit design. Compared to Mextram 504, 5 parameters are removed and 64 new parameters are added in Mextram 505, while it also improves on many aspects, such as bug fix, code format changed according to the Verilog-A standard and requirements from CMC [2], limiting and smoothing function added because of convergence problems, and improvements of the compact model to meet the modeling of specific technologies. The main changes of implementation are described in detail, Mextram 505.3-based parameter extraction, including DC, AC, and temperature scaling, is also listed in detail. Although Mextram model can satisfy most HBTs’ modeling, for some technologies, the current Mextram is still insufficient, hence based on the understanding of the physical characteristics of these technologies, we further improved the model. These feature improvements include: • Improved modeling of high injection substrate current in high voltage bipolar transistor. We examine parasitic PNP transistor current modeling in a high voltage bipolar transistor operating at high injection. We show that current parasitic PNP models are far from sufficient and propose a new model based on the epi-layer model of Mextram. The model is implemented in an experimental version of Mextram, and demonstrated to well fit measured substrate current at high 𝑉𝐵𝐸. • Improved compact modeling of recovery from saturation in SiGe HBTs. We present here the measurement and compact modeling of SiGe HBT recovery transients when the operation is switched abruptly from saturation to forward using Mextram 505.1. Accuracy of recovery modeling is found to be limited by the extrinsic base-collector (BC) junction diffusion charge model behavior in saturation. Parasitic resistances in the measurement setup are also important as they affect the intrinsic base-emitter (BE) junction bias during recovery, which in turn affect the collector-emitter (CE) transport current and hence the BC diffusion charge removal current. Based on detailed inspection of simulation details and comparison with measurement, and improved extrinsic BC diffusion charge model with better bias dependence and high injection accuracy is developed. The new model enables accurate modeling of recovery from all levels of saturation and its main features has been implemented in Mextram 505.2. • Improved modeling of neutral base recombination current in SiGe HBTs. The current neutral base recombination (NBR) current model is insufficient when modeling forward base current in high injection region and quasi-saturation region in newer SiGe HBT technologies. Measured base current reaches high injection point even earlier than the measured collector current. In addition, the quasi-saturation component share the same saturation current with NBR ideal base current, and is limited by the Early effect-related items. Through analyzing geometry scaling, the side-wall ideal base current is found comparable with the bulk current. Hence we propose an improved NBR ideal base current model, including high injection model and separated quasi-saturation components, and side-wall ideal base current model, including high injection model, as well as their corresponding temperature models. The new models are shown to fit the measurement data well. The new NBR model has been implemented in Mextram 505.3.