Design issues for SiGe heterojunction bipolar transistors

Abstract

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 fT, shift the peak fT 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.