Berlin, Germany--Researchers at the Max Born Institute have observed the femtosecond-scale onset of electrical resistance in semiconductors by following electron motions in real time.1 They did this by measuring the transmission of intense femtosecond pulses of terahertz radiation through the semiconductor with and without the presence of a separate 885-nm-wavelength photoexcitation pulse that introduces an electron-hole plasma.
Typical electrons in a semiconductor's electrical current move at only a speed of 1 m per hour. But for the first few femtoseconds after a current is turned on, they move freely before slowing down. To see what's happening on this scale, the scientists used 100-fs bursts of terahertz light to accelerate optically generated free electrons in a piece of gallium arsenide. The accelerated electrons generated another electric field, which, when measured with femtosecond time resolution, indicated exactly what they are doing. The researchers saw that the electrons travelled unperturbed in the direction of the electric field when the battery was first turned on. About 300 femtoseconds later, their velocity slowed down due to collisions.
The experiments allowed the researchers to determine which type of collision is mainly responsible for the velocity loss. Interestingly, they found that the main collision partners were not atomic vibrations, but the positively charged holes. Optical excitation of the semiconductor generates both free electrons and holes which the terahertz bursts moved in opposite directions. Because the holes have such a large mass, they do not move very fast, but they do get in the way of the electrons. Such a direct understanding of electric friction will be useful in the future for designing more efficient and faster electronics, and perhaps for finding new tricks to reduce electrical resistance.
REFERENCE:
1. P. Bowlan et al., Physical Review Letters, Vol. 107, p. 256602 (2011).
