Base Transport and Vertical Profile Engineering in SILICON/SILICON(1-X) Germanium(x)/silicon Heterojunction Bipolar Transistors
Abstract
Recent advances in low-temperature epitaxial growth of strained silicon-germanium alloys on silicon substrates allow bandgap engineering in silicon-based devices, with profound consequences for device design. In this thesis the improved control by Rapid Thermal Chemical Vapor Deposition of the vertical profile of a Si/Si_{1-x}Ge _{x}/Si heterojunction bipolar transistor (HBT) is used to study the effect of the shape of the conduction band in the base on device performance. Near-ideal base currents in Si/Si_ {1-x}Ge_{x}/Si HBT's, limited by hole injection into the emitter, are achieved using a non-ultra-high vacuum (UHV) technique for the first time, proving that high-lifetime Si_{1-x}Ge _{x} material can be fabricated using processes compatible with standard silicon technology. Graded-base Si/Si_{1-x}Ge_{x} /Si HBT's are fabricated in a non-UHV epitaxial technology for the first time, and their electrical characteristics are modeled analytically. The formation of parasitic potential barriers for electrons in the base of HBT's resulting from base dopant outdiffusion or non-abrupt interfaces is studied, together with the concurrent degradation of the electrical performance of the devices. This deleterious effect is especially severe in devices with narrow, heavily doped bases fabricated in an integrated circuit (IC) process because of the thermal budget employed. To alleviate this problem, intrinsic Si_{1-x}Ge_{x}^acer layers can be inserted on both sides of the base to greatly improve device performance. The tradeoff between the common-emitter current gain beta and the Early voltage V_{A} (output resistance) in heterojunction bipolar transistors is investigated for the first time. This tradeoff is important for analog application of HBT's, and it is shown that thin, narrow -gap layers in the base close to the base-collector junction reduce the Early effect dramatically leading to a high Early voltage. It is further demonstrated that even small amounts of dopant outdiffusion from the Si_ {1-x}Ge_{x} base into the silicon collector degrade the Early voltage drastically. Finally, a novel Double-Base HBT is developed which increases the functionality of a HBT. Temperature -dependent measurements prove that the DC characteristics of the DB-HBT can be modeled using a version of charge -control theory. Switching is demonstrated in a single -transistor NAND gate at temperatures up to 150 K.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- January 1992
- Bibcode:
- 1992PhDT.......109P
- Keywords:
-
- SILICON-GERMANIUM;
- SILICON;
- Engineering: Electronics and Electrical; Physics: Condensed Matter