Design issues for SiGe heterojunction bipolar transistors

Recent progress in the growth of strained SiGe epitaxial layers, demonstrating the feasibility of silicon-based heterojunction transistors, is examined. The specific design issues for SiGe HBTs are discussed. These are illustrated by reviewing the experimentally obtained electrical characteristics of recent SiGe-based devices. Analytical and numerical calculations are used to explain the device physics and to assess the potential circuit advantage of SiGe for present and future technologies. Experimental results for a double-polysilicon structure with a non-self-aligned emitter opening are reported. For this type of device the additional degree of freedom available for profile optimization can be employed to enhance f _T, shift the peak f_T to lower current density, or lower the base sheet resistance. Very good DC operation has been observed at room and low temperature, suggesting improved speed performance over homojunction devices