I present the results of experimental investigations into single electron transistors made on doped silicon substrates, with the ultimate goal of individual dopant manipulation at millikelvin temperatures. The sensitivity of single electron transistors to local charge motion should enable observations of single donor ionization. Here I formulate a model for the electrostatic control of a donor electron near an oxide interface and describe a device geometry that should enable its measurement. I give data from several Al-AlO_x-Al single electron transistors below 100 mK that provide evidence for field-induced dopant ionization, as well as for the motion of individual charges whose origins are not yet understood. I also describe a cryogenic scanning force microscope that I built to measure large arrays of single electron transistors.