SiGe HBT technology and circuits for THz applications

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 [1], 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 f_T of 28 GHz [2]. Since then, a series of significant improvements in the RF performance followed, notably the one achieving the first f_T exceeding 100 GHz in 1993 [3]. 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 f_T and f_max of 47 GHz and 65 GHz, respectively [4]. 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 [5], which was soon followed by a device with an f_T of 375 GHz [6]. With the abundant circuit and system applications of SiGe HBTs that benefited from the technology development, it became obvious for technology developers that f_max is more desired than f_T from the circuit designers’ side. Consequently, the technology innovation made a small change in its roadmap in more favor of high f_max of the device. With such efforts, SiGe HBTs with fmax higher than 400 GHz were reported by STM [7], and soon after, a 500 GHz f_max SiGe HBT was released by IHP [8], owing to the organized efforts to achieve a half-terahertz operation as supported by the Dot Five project [9]. The recent technological advancements in SiGe HBTs significantly contributed to the emergence of the Si-based THz era.