PTB News 2/2000 (209 kB) PTB News 2/2000, English edition, Issue August 2000PTB has developed semiconductor devices in which single electrons are transported with the help of surface acoustic waves. The novel device was used to produce a current of up to 0.75 nA determined by the charge of the electron and the applied frequency. The device has possible applications as a quantum current standard.
The quest for tracing back the ampere, the unit of electric current and one of the seven base units of the international system of units (SI), to quantum standards is a hitherto unsolved problem. Worldwide research now focuses on the investigation of single-electron devices. In these devices, an electron with electric charge e transported with the frequency f realises a current I = e · f.
Novel techniques may allow to increase the realised currents by orders of magnitude as compared to single-electron tunnelling devices (SET) where the current is limited to a few picoamperes. In the GaAs devices developed at PTB, an applied high frequency produces a surface acoustic wave by means of the piezo-electric effect. The associated propagating modulation of the electrostatic
potential near the surface can act as a microscopic «scoop» which transports single electrons with the speed of sound through a narrow constriction. Recently, PTB successfully manufactured a device in which a DC current of 0.75 nA was generated applying a frequency of about 4.7 GHz. By using devices in parallel, the achieved current level can easily be increased by an order of magnitude. This now allows to set the goal for a quantum current standard with metrologically relevant small uncertainties even after transfer to the technologically more suitable microampere range.
F. J. Ahlers,
fax: (+49 531) 592-24 05,
e-mail: 
franz-josef.ahlers(at)ptb.de