The SiGe heterojunction bipolar transistor (HBT) is basically a Si-based bipolar junction (BJT) transistor with a small amount of Ge added to the base region. The SiGe base region turned out to be a significant performance improver, making SiGe HBTs now accepted as a standard bipolar transistor for high-speed applications. When first demonstrated in 1987 by researchers at IBM based on molecular beam epitaxy (MBE)-grown SiGe/Si epitaxial layers , SiGe HBTs were in fact intended for high-speed digital applications. However, partly due to the huge power dissipation when highly integrated and also due to the concurrent emergence of complementary metal-oxide-semiconductor (CMOS) circuits with low-power operation, SiGe HBTs had to redefine their application path. It was fortunate for the devices to find a new fertile application area, namely, the radio frequency (RF) circuits and systems. The first report on the RF characteristics of SiGe HBT were brought out in 1989 by a group of researchers at Stanford and Hewlett-Packard, who exhibited an fT of 28 GHz . Since then, a series of significant improvements in the RF performance followed, notably the one achieving the first fT exceeding 100 GHz in 1993 . Another great milestone from a practical point of view was the release of the world’s first commercial SiGe HBT technology (in the form of a BiCMOS technology) by IBM in 1996, which exhibited fT and fmax of 47 GHz and 65 GHz, respectively . Continuing scaling efforts, combined with structural innovation such as raised extrinsic base, led to SiGe HBTs operating beyond 200 GHz based on a 0.13 μm lithography node , which was soon followed by a device with an fT of 375 GHz . With the abundant circuit and system applications of SiGe HBTs that benefited from the technology development, it became obvious for technology developers that fmax is more desired than fT from the circuit designers’ side. Consequently, the technology innovation made a small change in its roadmap in more favor of high fmax of the device. With such efforts, SiGe HBTs with fmax higher than 400 GHz were reported by STM , and soon after, a 500 GHz fmax SiGe HBT was released by IHP , owing to the organized efforts to achieve a half-terahertz operation as supported by the Dot Five project . The recent technological advancements in SiGe HBTs significantly contributed to the emergence of the Si-based THz era.
ASJC Scopus subject areas
- Physics and Astronomy(all)