Measurements of the temperature and bias dependence of Single Electron
Transistors (SETs) in a dilution refrigerator show that charge noise increases
linearly with refrigerator temperature above a voltage-dependent threshold
temperature, and that its low temperature saturation is due to SET
self-heating. We show further that the two-level fluctuators responsible for
charge noise are in strong thermal contact with the electrons in the SET, which
can be at a much higher temperature than the substrate. We suggest that the
noise is caused by electrons tunneling between the SET metal and nearby
potential wells
