June 10, 1998 NEW HAVEN, Conn.--Scientists at Yale University have developed the world's most sensitive electrometer, a transistor so sensitive it can count individual electrons as they pass through a circuit. The detector could be useful not only in developing and testing miniaturized electronic devices but also as a highly sensitive light detector in powerful new microscopes and telescopes.
Made from aluminum, the device is about 1,000 times faster than the best electrometer on record and 1 million times faster than other single electron transistors, according to a report by Yale applied physicist Daniel E. Prober in the May 22 issue of the journal Science. Working with him on the device were Yale postdoctoral associate Robert J. Schoelkopf; former graduate student Peter Wahlgren, now in Göteberg, Sweden; and graduate students Alexay A. Kozhevnikov and Per Delsing.
"Single electron transistors have been around for about a dozen years, but our laboratory has developed a new type called a Radio Frequency Single Electron Transistor (RF-SET) that can measure charges as small as 15-millionths of an electron. It detects an extremely large bandwidth," said Prober, an expert in high-temperature superconductivity as well as electron conduction in metal films, wires and semiconductors.
The goal of many scientists for the last 10 years has been to develop more precise frequency measurements and to devise current voltage standards, said Schoelkopf, who began working on the RF-SET design while a graduate student at California Institute of Technology. Without that, researchers cannot study and perfect extremely miniaturized electronic devices and computer chips at the level where quantum mechanical effects become important.
Currently, the RF-SET works only at temperatures near absolute zero Kelvin, or about -459 degrees Fahrenheit, thus requiring a large refrigerator. The Yale scientists are exploring ways to make the detector work more effectively at higher temperatures.
On the plus side is the device's high operational speed. Conventional single electron transistor electrometers have been limited by slow speeds, typically below frequencies of 1 kilohertz (1,000 cycles per second), Schoelkopf said. The RF-SET can operate even at frequencies exceeding 100 megahertz (100 million cycles per second), where the noise due to background charge motion is completely negligible. In their report, the Yale researchers describe how improved versions of this device could even approach the quantum limit, yielding the best electron detectors possible.
Because the device effectively monitors a wide range of photons -- including X-rays, ultraviolet radiation, light, infrared radiation, and microwaves -- the RF-SET design is "the best by many criteria, very exciting," Prober said. Among the many potential applications are far-infrared detectors, being considered by the National Aeronautic and Space Agency (NASA) for use in astronomy, and high-resolution electron microscopes that can amplify light for the study of molecular structure in medicine.
For more detailed studies in astronomy, the new detectors could replace CCD (charged coupled device) detectors to provide information about the energy and color of photons coming from very faint stars and galaxies. "It could serve as a CCD detector and spectrometer combined into one, capable of defining chemical composition, density and temperature of a distant star," Prober said.
By combining the RF-SET electrometer with a new type of photometer, researchers hope to boost the capability of a CCD detector to produce images of stars and galaxies at the resolution of 1,000 pixels or more. If successful, the device also could amplify images received by the next generation of the Deep Field camera on the Hubble Space Telescope.
Another potential use for the device would be in the computer field, where scientists are developing radically new technologies for smaller, faster and cheaper computers containing billions of transistors on a single chip. The lack of a highly sensitive electrometer has been a significant impediment in testing and perfecting these miniaturized technologies, Prober said.
When working with quantum components, scientists must deal with special laws of physics that can be ignored when working with larger components. For example, electrons behave more like waves than particles at quantum scales and can do unexpected things like tunnel through barriers. Scientists still have much to learn about quantum mechanics in extremely small electronic devices, Schoelkopf said, and "the importance of a highly sensitive electrometer for basic research cannot be overstated."
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