Transparent transistors: a new class of semiconductors makes for fast, flexible, and invisible electronics

Two groups, in Japan and the United States, have reported making see-through circuits out of a new class of semiconductors. Besides holding out the possibility of building displays into the windows of cars and trains, the materials´ low cost and low-temperature fabrication may suit them to future applications that don´t need transparency, notably roll-up electronic displays. Standard silicon-based techniques can´t compete in this area, because even if they could be made flexible, their processing temperatures, generally around 250 °C, are so high they would melt any plastic substrate holding the silicon in place. To get around the problem, several academic and corporate laboratories are developing pentacene and other organic semiconductors - so called because they consist of carbon, hydrogen, and oxygen. But although organic transistors can be processed at low temperatures and even printed like ink, they don´t let electrons and other charge carriers move around very quickly; therefore, they perform poorly. Besides, organic materials tend to be thermally and chemically unstable. The semiconductor is n-type, meaning that electrons carry charges through it. The speed at which the electrons move in the device - called the field-effect charge carrier mobility - is the key, because it limits how fast a transistor can switch. In a-IGZO devices, the mobility is 6-10 square centimeters per volt-second, about five times that in similar organic thin-film transistors and more than seven times that of the hydrogenerated amorphous silicon in flat-panel displays today.