Signal-amplification process could transform communications, imaging, computing

Jan. 21, 2015

Researchers at the University of California, San Diego, have discovered a new signal-amplification process that could serve new generations of electrical and photonic devices. The researchers describe their work in the journal Applied Physics Letters, from AIP Publishing.

“For many years, the semiconductor industry has relied on photodetectors for optoelectrical conversion, followed by low-noise electronic amplifiers to convert optical signals into electronic signals with amplification to enable information detection and processing,” explained Yu-Hwa Lo, a professor of electrical and computer engineering at the University of California, San Diego, at Newswise.

Lo noted that avalanche photodetectors that use impact ionization have been the devices of choice for optoelectrical conversion, but they require high operating voltages and exhibit increasing noise with amplification.

“Thanks to insights of the complex interactions among electrons in localized and extended states and phonons (a unit of vibrational energy that arises from oscillating atoms within a crystal), we’ve discovered a far more efficient mechanism—the cycling excitation process (CEP)—to amplify the signal,” Lo said.

Concepts involved in the cycling excitation process (Courtesy of Yuchun Zhou/UCSD)

The device employs a heavily compensated p/n junction. “The only unique feature is that both sides of the p/n junction contain a substantial amount of counter doping—a large number of donors exist in the p-region, with acceptors in the n-region,” said Lo.

Counter impurities in the compensated p/n junction are responsible for the team’s highly efficient signal (photocurrent) amplification process. Electrons or holes crossing the depletion region gain kinetic energy and, in turn, excite new electron-hole pairs using the compensating impurities (donors in the p-side and acceptors in the n-side) as intermediate states.

“An energetic electron, for example, can excite an electron from an occupied acceptor to the conduction band, while a phonon is absorbed subsequently to fill the acceptor with an electron from the valence band—producing a hole in the valence band to complete the generation of an electron-hole pair,” said Yuchun Zhou, first author of the paper and a doctoral student in Lo’s group. “This type of process occurs on both sides of the p/n junction and forms cycles of electron-hole excitation to produce high gain.”

The key discovery and innovation for the amplification process is to use the compensating impurities as the intermediate steps for electron-hole pair generation. “Impurity states are localized, so the conservation of momentum that limits the efficiency for conventional impact ionization can be greatly relaxed and leads to higher signal amplification efficiency and reduced operation voltage,” added Lo.

When asked what is the most striking implication of the team’s discovery, Lo said, “Perhaps that an entirely new physical mechanism can be found in the most common device structure—a p/n junction—that has been used since the semiconductor industry’s heyday. It appears that a small modification, such as heavy doping compensation, from a common structure can be used to take advantage of the unusual physical process that results from concerted interactions between electrons in extended and localized (impurity) states and phonons.”

With further improvements, according to the team, the discovered signal amplification mechanism can be used in a variety of devices and semiconductors—presenting a new paradigm for the semiconductor industry.

“With an efficient gain mechanism at an operation voltage compatible with CMOS integrated circuits, it’s possible to produce communication and imaging devices with superior sensitivity at a low cost,” Lo said. “By using other methods along with optical excitation to produce the seed carriers that initiate the cycling excitation process, we can conceive new types of transistors and circuits and extend the scope of applications beyond optical detection.”

The article, “Discovery of a Photoresponse Amplification Mechanism in Compensated PN Junctions,” is authored by Yuchun Zhou, Yu-Hsin Liu, Samia N. Rahman, David Hall, L.J. Sham and Yu-Hwa Lo. It is published in the journal Applied Physics Letters on January 20, 2015 (DOI: 0.1063/1.4904470). After that date, it can be accessed at http://scitation.aip.org/content/aip/journal/apl/106/3/10.1063/1.4904470

The authors of this paper are affiliated with the University of California, San Diego.


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