Simulation of Si-Ge Heterojunction Bipolar Transistors

Research project P 14483 Simulation of Silicon-Germanium Heterojunction Bipolar Transistors Siegfied SELBERHERR 09.10.2000 Nowadays Heterojunction Bipolar Transistors (HBTs) attract much industrial interest. This project aims at the development of new models for effective carrier masses, density of states, carrier mobility, and bandgap energies for strained and relaxed Silicon-Germanium (S,iGe), and their application to the simulation of SiGe HBTs. Modeling of the thermal material properties is needed for electro-thermal simulation. Attention will be paid to the hydrodynamic transport especially at the heterointerfaces. The developed models will be implemented in the two- dimensional device simulator MINIMOS-NT. The emphasis will be put on the simulation application of SiGe HBTs to industrial requirements. Physics-based DC-simulation, mixed-mode device/circuit simulation, small signal RF-parameter simulation, and device reliability investigations of high practical value will be performed. Our methodology is based on our previous experience with HBTs and other semiconductor devices based on III-V chemical group materials.

Nowadays Heterojunction Bipolar Transistors (HBTs) attract much industrial interest. In particular, Silicon- Germanium (SiGe) HBTs exhibit promising properties stretching the intrinsic advantages of the established CMOS technology into high-frequency electronics. SiGe HBTs increasingly challenge the III-V (GaAs, InP, etc.) devices in the highest frequency ranges. The devices employ strained SiGe layers to achieve better performance. SiGe devices have seen a major development to industrial maturity in the recent five years, further extending the applicability of the Silicon technology towards higher frequencies. Major contributions come from Hitachi (Japan), IBM (USA), Infineon (Germany). The Austrian company Austriamicrosystems AG has also attained a good position with these devices. In this project new models for the material properties of SiGe have been developed. This includes models for effective carrier masses, density of states, carrier mobility, and bandgap energies for strained and relaxed SiGe, and their application to the simulation of SiGe HBTs. The thermal material properties have been modeled to allow for electro-thermal simulation. Attention has been paid to the high-field/high-energy transport especially at the heterointerfaces. The developed models for SiGe were implemented in the two-dimensional device simulator MINIMOS-NT. The emphasis has been put on the simulation application of SiGe HBTs to industrial requirements. Physics-based DC- and AC- simulations were carried out for SiGe HBT structures provided by Austriamicrosystems AG. Model verification through a comparison between simulated and measured forward and reverse Gummel plots of several SiGe HBTs with different Ge content at several ambient temperatures was performed. Simulation of the output characteristics including self-heating effects and impact ionization is cross-checked against measured data. The successful match between measured and simulated S-parameters gives the possibility to go for large-signal modeling. Optimization of the doping for specific requirements (high speed or high breakdown voltage) was performed. In frame of the project a complete set of models which is needed to investigate industrial technologies by means of Technology Computer-Aided Design (TCAD), was developed.