Using a fast-pulsing laser, physicists have recorded the first moments of electrical resistance, the friction that generates heat as electricity travels through circuits.
It’s quite the feat: electrons in a computer’s semiconductor slow from near-light speed to a snail’s pace in roughly 300 femtoseconds, or about 10,000 times faster than it takes light to travel one foot.
“We had to use very fast laser pulses to measure such short time periods. You could never do it with typical electronics,” said Klaus Reimann of the Max Born Institute in Berlin, a physicist who co-authored a study of the effect Dec. 16 in Physical Review Letters.
Semiconductors are materials that borrow properties of both electrical conductors, such as copper, and electrical resistors, like ceramic. They’re found in everything from transistors and LEDs to solar panels and microprocessors. Depending on the material, semiconductors perform a circus of physical feats, including producing light. When a voltage is applied to gallium arsenide, for example, the material spits out infrared photons (which makes them great hidden light sources for security cameras).
Semiconductors are also crucial components of computer processors. When a voltage is applied, they store and shuttle bits of information. As this happens, the friction of electrons in the material — electrical resistance — heats them up.
Physicists knew electrical resistance didn’t kick in the moment a voltage was applied. Electrons experience some freedom before slowing to a crawl and scattering. What wasn’t certain was how quickly they make that transition (illustrated in the animation above, with electrons in blue, “electron holes” left by departing electrons in red, and voltage signified by the green arrow).
“Any scattering processes will take some time, but we didn’t know how much,” Reimann said.
To find out, Reimann and six colleagues set up a terahertz laser, able to emit 1 trillion pulses of light per second, and split its beam in two. One half shined on a strip of gallium arsenide and helped its electrons create a current. The other measured the electrons’ movement.
Because a standard computer was too slow to compile the data in one shot, the researchers ran the experiment hundreds of times, taking a reading at a slightly different moment with each iteration. Data point by data point, a picture of resistance emerged.
In gallium arsenide, it took 300 femtoseconds for electrons to begin slowing and scattering. Reimann said the speed of electrical resistance’s onset scaled with the number of electron holes, where an electron had popped out and moved. The more holes, the faster electrons slowed to a crawl.
Someday, when computers reach processing speeds 1,000 times faster than what’s possible now, the effect may be crucial.
“We have no idea if or when that will happen, but you might be able to use it to make computers that are faster and use less electricity,” he said.
Citation: “Strong correlation of electronic and lattice excitations in GaAs/AlGaAs semiconductor quantum wells revealed by two-dimensional terahertz spectroscopy.” By W. Kuehn, K. Reimann, M. Woerner, T. Elsaesser, R. Hey, and U. Schade. Physical Review Letters, Vol. 107, No. 067401 Dec. 16, 2011. DOI: 10.1103/PhysRevLett.107.067401