Pushing the Boundaries of Silicon-Germanium: Cryogenics, Space, and Ocean Worlds
TSRB’s John D. Cressler, Regents Professor and Schlumberger Chair Professor in the School of Electrical and Computer Engineering at Georgia Tech, has spent over three decades exploring the frontiers of silicon-germanium (SiGe) technology. His research has advanced the fundamental understanding of semiconductor devices and opened up new possibilities for applications in some of the most extreme environments imaginable: cryogenic temperatures, outer space, and the icy moons of Jupiter. Cressler’s work is a testament to how engineering can solve some of humanity’s most challenging problems.
Dr. Cressler and his students have been working with SiGe HBTs for decades and have found them to have unique advantages in extreme environments.
Cryogenic Applications: Unlocking the Quantum Realm
One of the most fascinating aspects of Cressler’s research is the exploration of SiGe heterojunction bipolar transistors (HBTs) at cryogenic temperatures. As detailed in his invited paper, “The SiGe HBT at Cryogenic Temperatures,” SiGe transistors exhibit remarkable performance improvements when cooled to temperatures as low as the millikelvin (mK) range, the domain of quantum computing. SiGe HBTs show enhanced current gain, transconductance, and noise performance at these ultra-low temperatures, making them ideal for quantum computing applications where signal integrity and low noise are critical. Cressler’s team has demonstrated that SiGe HBTs can operate robustly in these conditions, paving the way for their use in quantum control and readout circuits. The ability to operate at cryogenic temperatures without significant performance degradation makes SiGe technology a strong candidate for next-generation quantum computing systems. Companies like IBM and GlobalFoundries, with whom Cressler has collaborated, are already exploring these applications, indicating a clear path toward commercialization.
Space Exploration: Surviving the Final Frontier
Jupiter’s moon Europa is one of the most promising places in the solar system to look for extraterrestrial life. Exploring Europa could be possible in the coming years thanks to new applications for silicon-germanium transistor technology research at Georgia Tech. (Image: NASA)
Space is an unforgiving environment characterized by extreme radiation, temperature fluctuations, and vacuum conditions. Cressler’s research has demonstrated that SiGe technology is uniquely well-suited to thrive in these conditions, as highlighted in his plenary presentation at Si RF 2024 in Puerto Rico. SiGe HBTs are inherently resistant to high levels of ionizing radiation, a critical requirement for electronics in space. Cressler’s work has demonstrated that SiGe devices can withstand multi-Mrad doses of radiation, making them ideal for satellites and deep-space probes. Additionally, SiGe transistors can operate seamlessly across a wide temperature range, from the frigid shadows of lunar craters (-248°C) to the scorching heat of direct sunlight (+120°C). This capability is crucial for missions to the Earth’s Moon, Mars, and beyond, where temperature fluctuations are extreme and rapid. The rise of low-cost, small satellite constellations (CubeSats) has created a demand for robust, high-performance electronics that can be produced at scale. Cressler’s vision of integrating SiGe HBTs with CMOS and photonics into a single "superchip" could revolutionize space systems, enabling advanced communication, imaging, and data processing capabilities in a compact, cost-effective package.
Ocean Worlds: Europa and Beyond
Cressler’s research also extends to the icy moons of Jupiter, particularly Europa, which is believed to harbor a vast ocean beneath its frozen surface. His paper, Environmentally Invariant SiGe Electronics for On-Surface Exploration of Ocean Worlds, which was published for the 2023 IEEE Aerospace, outlines how SiGe electronics can enable the exploration of these "ocean worlds." Europa’s surface presents a unique challenge, with temperatures as low as -180°C and radiation levels up to 5 Mrad. Traditional electronics require protective shielding, which adds weight and complexity. Cressler’s team has demonstrated that SiGe HBTs can operate robustly in these conditions without shielding, offering significant advantages in size, weight, and power (SWaP). By enabling electronics to function directly on the surface of Europa, SiGe technology could support distributed sensor networks, data computation, and communication systems, enhancing the scientific return of future missions. This capability is not limited to Europa; it can also be applied to other icy moons, such as Enceladus and Ganymede.
Looking Ahead
While Cressler’s research applications span vastly different environments, they are united by a common thread: the ability of SiGe technology to operate reliably in extreme conditions. Whether it’s the ultra-cold temperatures of quantum computing, the harsh radiation of space, or the icy surfaces of distant moons, SiGe HBTs offer a versatile and robust solution. As the demand for reliable electronics in extreme environments continues to grow, the potential for SiGe technology becomes even more apparent. Cressler’s work in silicon-germanium technology is a shining example of how fundamental research can lead to transformative applications, paving the way for new frontiers in science and exploration.