Most sensitive photoelectric semiconductor.

Most sensitive photoelectric semiconductor

Scientists at the University of Southern California in Los Angeles report they have developed a new semiconductor that is 100 times more sensitive to light than any other. Called hetero NIPI, this crystalline superlattice of gallium arsenide and gallium-aluminum arsenide is named for its structure -- an ordered sandwich of layers designated N, I, P and I. Between the N layer, containing negatively charged electrons, and the P layer of "holes" -- to which electrons naturally gravite -- is an insulating I layer of uncharged material. Below the P layer is another I layer.

Normally, electrons move around freely inside a crystal in a continuum of energy states. But hetero NIPI's electrons are trapped within its I layers -- in what are called quantum wells. Here they "can only move in two dimensions, instead of three," explains Elsa Garmire, a developer of the new material. That limits the energy states in which the electrons can reside, she says, and makes them more susceptibel to interactions with light -- in this case the near-infrared (830 nanometers) wavelength of a gallium-arsenide laser.

Hetero NIPI has been designed for use as a photosensitive switch -- and on/off circuit-shunting device -- for the coming generation of optical computers. The circuits of these computers will be configured "in a fundamentaly new way," Garmire says -- "allowing for dramatically increased parallelism," or processing of massive amounts of data simultaneously. But in developing hetero NIPI, she notes, "we are trading off speed for parallelism": Though the material is 100 times more sensitive, "it's also 100 times slower." Garmire anticipates that energy-efficient optical switches, based on materials like hetero NIPI, will prove especially valuable in high-volume data processing, such as the analyzing or enhancing of satellite images.