Mission planners for future flights to the moon, Mars, and beyond rightly worry about how to keep astronauts safe from the hazards of high-energy protons, neutrons, and ions streaming through space. These particles can cause cancer and brain damage, but they can be just as damaging to a spacecraft's electronics.
NASA is now testing how well a new type of transistor, shown to be radiation-resistant on Earth, will hold up in space. The transistors went up on the space shuttle Endeavor , and astronauts placed them in a test setup on the outside of the International Space Station on 22 March. After a year, researchers will check how radiation affects the transistors' operating voltages and currents.
A typical transistor is made up of a gate separated from the channel between the source and the drain by an insulating dielectric, usually silicon dioxide. When the gate sees a voltage above a certain threshold, it creates a conductive path that lets charge flow between the source and the drain.
Radiation's effects on the silicon dioxide dielectric are what may cause a transistor to fail. Radiation ionizes the oxide, creating electron-hole pairs. The electrons flow out but the holes get stuck, building up a positive charge. This leads to two problems: it causes current to leak across the channel, draining batteries, and it decreases the transistor's threshold voltage, putting it out of sync with other transistors and disrupting the delicate timing of a circuit. ”Every piece of circuitry is designed to operate in synchrony in a predictable manner,” says Tobin Marks, materials science and engineering professor at Northwestern University, in Evanston, Ill., who led the work on the new transistor that NASA is testing. ”You want [everything] to be rock stable.”
Instead of silicon dioxide, the new transistors contain an organic dielectric, which assembles itself from three chemical solutions into a 15â''nanometer-thick layer. The organic dielectric is more radiation-proof than silicon dioxide because it conducts holes well, says David Janes, an electrical and computer engineering professor at Purdue University, in West Lafayette, Ind. The cosmic radiation still generates electron-hole pairs, but the holes ”bleed out” rather than building up, he says.
Marks and Janes have already shown that the transistors can withstand 2.85 kilograys (285 kilorads) of proton radiation. The space station test, scheduled to last 12 to 18 months, will show how the transistors hold up when exposed to ultraviolet radiation, as well as zero gravity and the corrosive effects of atomic oxygen. A one-way mission to Mars, including eight months of travel and three to six months on the planet, exposes electronics to about 10 kG of radiation--the limit of today's space electronics.
The custom processing required to make traditional radiation-hardened circuits ups their cost and keeps them several generations behind commercial computer chips in speed and power consumption. The electronics also require radiation shielding and backup circuits and devices, which add bulk. ”We're always trying to save weight,” says Geetha Dholakia, a research scientist at the NASA Ames Research Center, at Moffett Field in Mountain View, Calif. ”If [the electronics are] inherently radiation-resistant, then one doesn't have to employ these other precautions.” The new transistor, which contains a tiny zinc oxide nanowire as the semiconductor material, is lighter and could be cheaper to fabricate, claim Marks and Janes.
NASA might have other dielectric options for space electronics. To speed their transistors, Intel and other chip makers are turning to a new class of dielectrics, including oxides of hafnium and zirconium. But so far, there is little data about their radiation tolerance, Dholakia says.