Breaking the Terahertz Barrier | ECE | Virginia Tech


Breaking the Terahertz Barrier

Infographic showing cross layer design problems

Koh's high frequency research is a vision of the future. His efforts could mean downloading full movies in mere seconds, and potentially to see through walls.

As a young boy, Kwang-Jin Koh was captivated by science fiction novels and still remembers the impact of a story of a half man-half frog. Since the world was becoming more polluted, he pondered that perhaps with changes to our biology or technology, humankind could breathe underwater and have the whole sea as a place to live. These boyhood ruminations triggered a passion for biology and engineering.

Koh was raised in a small rural town on Jeju Island the southernmost island of South Korea. It was such an isolated town that there was only one rotary phone in the entire village back in the early 1980s. One day it was announced via a loud speaker at the village hall that his father had received a phone call. For the first time in his life, Koh picked up a phone. "Through the cumbersome black phone came my grandma's lovely voice," he recalls. "It was such an electrifying moment for my 7-year-old self that I could not speak a single word. Ever since, I have been fascinated by communication gadgets," he says.

That passion and intellectual interest led him, as a Ph.D. student, to design a phased-array integrated circuit (IC) system that has become an industry standard in defense and commercial applications. Today, as a new assistant professor of ECE, Koh is developing integrated high frequency electronics technology that feels like science fiction with potential applications of seeing through walls or downloading whole movies in seconds. He has also found an unanticipated joy in teaching and sharing his passion for the field.

"The educational infrastructure was very weak in my home town," Koh recalls, "my parents had completed only elementary school, and a good education was just not available. Although I was the top student throughout elementary and middle schools, I was not prepared well." When he went to high school in a big city, he found a wide gap between his background and that of the other students who had had access to private schooling and better schools in general.

"I had to help my parents with farm work after school and on weekends, and study at night to catch up with the other students, but I kept making progress." He was at the top of the electronic engineering class for his whole four-year period at Chung-Ang University where he earned a B.S. in 1999. He then went to the well-known Korea Advanced Institute of Science & Technology (KAIST) for his Master's degree under a Korean government scholarship.

Following graduation in 2001, he worked for Korea's Electronics and Telecommunications Research Institute (ETRI), where he designed silicon CMOS RF chips for cell phones. After about four years there, he went to the University of California San Diego for his Ph.D.

Kwang-Jin Koh

Kwang-Jin Koh shows a chip in a test carrier from the Wireless Microsystems Laboratory. Researchers in the laboratory work on chips that measure just a couple millimeters.

It was at UCSD where Koh designed the first successful phased array on a silicon chip. Koh's research involved creating an integrated phased array radar defense system funded by DARPA. "When there is just a single antenna, you get a very weak signal sensitive to jamming from counterparts. When you use an array, you can focus the direction and get a stronger signal," he explains. "The main issue, however, was making the system on a tiny inexpensive silicon chip."

His chip was only 2.2 by 2.3 mm and spawned several additional chips, including a 16-element phased array chip that was 3.2 by 2.6 mm then the most complex silicon phased array chip in the world. The chip designs were reported to the Pentagon as one of the major achievements in 2007 and transferred to the U.S. Navy, Raytheon, Lockheed Martin, Boeing, Teledyne Scientific Corp., ViaSat and Cobham for the development of advanced defense radar systems.

After getting his Ph.D. in 2008, Koh worked for Intel for two years, then moved to Broadcom. He joined Virginia Tech in November 2011 and is concentrating on three different issues of integrated systems: low power, high frequency, and terahertz electronic systems.

The low power research is urgent for today's commercial technology, he says. "Low power circuit design," he explains, "extends battery life," Koh's high frequency work is "more futuristic," he says. With extremely high frequency (millimeter-wave) electronics, full-length feature movies could be downloaded in less than a minute. Commercial radios, WiFi and cellphones currently operate below 5 GHz, and a higher frequency means higher data transfer rates. Millimeter-wave electronics allow hidden objects to be sensed and identified non-obtrusively as well, and could be used in applications in image sensing.

One project, funded by DARPA, is to develop phased arrays that operate at 94 GHz. A major challenge is the cost of defense radars: in order to make inexpensive radar systems, these phased arrays must be fabricated in silicon. Koh explains that "the silicon process itself has some limits to operate in high-frequency systems. There's a loss problem. If the frequency gets higher, the loss can be larger." Also, the signal can be weak and noisy. "To resolve these issues, we must develop new design techniques or approaches," Koh says.

Koh also is pursuing terahertz electronics systems. "It's an extremely challenging area for electronic design. Every device has some speed limitation, and the speed of the solid-state transistor itself will be lower than of terahertz." The basic issue for terahertz electronics is the devices themselves, he says. "For now, silicon transistors cannot reach 1000 GHz and we may need a combined technology of silicon and compound semiconductor for active transistors. Advanced nano-technology such as nanowires should also be investigated as an interconnection material to achieve terahertz speed."

One of Koh's goals is to be an excellent teacher. "I was worried about how to develop effective ways to transfer this technical knowledge to students. I believe teaching is the greatest act of optimism and find it's very rewarding. When students ask me technical questions and then they understand, I feel something."