  We present the fabrication and measurement of a radio frequency single
electron transistor (rf-SET), that displays a very high charge sensitivity of
1.9 microlectrons/sqrt(Hz) at 4.2 K. At 40 mK, the charge sensitivity is 0.9
and 1.0 microlectrons/sqrt(Hz) in the superconducting and normal state
respectively. The sensitivity was measured as a function of radio frequency
amplitude at three different temperatures: 40 mK, 1.8 K and 4.2 K.
