The development of a hierarchical TCAD infrastructure for the simulation of extremely scaled SiGe HBTs has been completed. In particular, a new energy band model has been incorporated into device simulation software which correctly describes all transport mechanisms occurring in THz HBTs. This tool will be used to gain physical insight into the behaviour of scaled SiGe HBTs, develop guidelines to doping and Ge profile optimization, and explore device operation limitations.
New device architectures presently developed in the DOTFIVE project have been explored by calibrated TCAD (process and device) simulation. Special attention was paid in improving the maximum oscillation frequency. fmax values of 500GHz have been obtained in both architectures for the most aggressive scaled emitter window width. These results indicate the practical feasibility of 500 GHz HBTs.
By means of the calibrated TCAD infrastructure new device concepts based in strain engineering were investigated. A new SiGe HBT architecture utilizing a nitride layer in the collector region has been proposed. This nitride layer creates a mechanical strain which improves electron mobility. Process/device simulation results show only an 8% improvement in fT, and 5% improvement in fmax for the new device architecture.
Due to high current density operation and reduced cooling capabilities, thermal effects are exacerbated in scaled devices. For this reason, the thermal behavior of scaled HBTs has been investigated by three-dimensional thermal simulations.