Noise in Al single electron transistors of stacked design

We have fabricated and examined several Al single electron transistors whose small islands were positioned on top of a counterelectrode and hence did not come into contact with a dielectric substrate. The equivalent charge noise figure of all transistors turned out to be surprisingly low, (2.5-7)x10(-5)e/root Hz at f = 10 Hz. Although the lowest detected noise originates mostly from fluctuations of background charge, the noise contribution of the tunnel junction conductances was, on occasion, found to be dominant. (C) 1998 American Institute of Physics.