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October 2014 • The PCB Design Magazine 49 CliCk To View Video Interview Matt Kelly of IBM explains his company's strategy for attract- ing more young technologists. Kelly believes that getting en- gineering students involved in their local SMTa chapters can go a long way toward making them feel more confident and less nervous among the older, more experienced engineers. IBM Seeking the Next Generation of Technologists by Real Time with... SMTAI by Real Time with... NEPCON South China Shining light on a semiconductor will make it more conductive. But now researchers have discovered that in a special semiconductor, light can have the opposite effect, making the mate- rial less conductive instead. The phenomenon was discovered in an ex- otic two-dimensional semiconductor — a sin- gle layer of molybdenum disulfide (MoS 2 ) just three atoms thick. The finding is reported in a paper inPhysical review Letters by MIT postdoc Joshua Lui; nuh Gedik, the Lawrence C. and Sarah W. Biedenharn Career Devel- opment associate Professor of Physics; and six others at MIT, Harvard university, and in Taiwan. When a semiconductor is illuminated by light, its conductivity tends to in- crease. This is because light absorption generates pairs of loose electrons and holes—places in a material with "missing" electrons—that facilitate the flow of electrical current through the material. The MIT team, however, observed the oppo- site behavior in a two-dimensional semiconduc- tor. In single-layer MoS 2 , trions have the same net charge as an electron, but a mass roughly three times that of an electron. Instead of increasing the population of free charges, the illumination actually converts the original free electrons into heavier trions with the same charge density. This is the reason for the reduction of conductivity of single-layer MoS 2 under illumination. So far, the team has only studied the effect in MoS 2 , which belongs to a family of new two-dimensional semi- conductors. "There are other types of two-dimensional materials with [similarly] strong trionic effect." Lui says. "They are likely to ex- hibit the same photoconduc- tion phenomenon." New Mechanism of Photoconduction for Next-gen Devices

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