By realizing that a bad tunnel diode could form the basis for a good sensor, researchers at Ohio State University in Columbus, the US Naval Research Laboratory in Washington and the University of Notre Dame in Indiana may have found a new technology to detect concealed weapons without x-rays. A variation of the same approach could allow pilots to see through bad weather.The sensor works on ambient radiation at 94 GHz — corresponding to a wavelength of 3.2 mm, or 3200 µm. At this wavelength, the atmosphere is transparent, and scattering from rain, fog, smoke, clothing and other materials is limited, making long-range sensing possible.Paul R. Berger, a professor of electrical and computer engineering and physics at Ohio State and the leader of the group, noted that the contrast between asphalt and a field or between a metallic weapon and flesh is very high in this spectral region.Because the researchers had been working on making good silicon-based tunnel diodes, they knew how to make bad tunnel diodes with a particular characteristic: a low forward tunneling current and a large reverse tunneling current. This yields a large nonlinearity or reaction at zero bias, which is key to making a highly responsive sensor.To achieve this, the researchers used molecular beam epitaxy to grow stacked layers of P-type silicon, silicon germanium, silicon and N-type silicon atop a P-type silicon substrate. The backward diode prototypes constructed in this way had a curvature coefficient — a measure of sensor sensitivity — of 31. This is somewhat less than that of other ambient millimeter-wave sensors under development, but twice that of the best other silicon- or germanium-based systems.Berger noted that this silicon-based approach does not require exotic materials and could be easily integrated with other semiconductor electronics — two advantages that other ambient detectors under development lack.The researchers plan to continue to optimize the device design to boost its performance. Given the right cost and performance, the devices could find their way into public places and buildings as well as into aircraft cockpits.