Single-electron transistor strongly coupled to an electrostatically defined quantum dot

A system is described in which an Al-based superconducting single-electron transistor ~S–SET! is fabricated directly above an electrostatically defined quantum dot formed in a two-dimensional electron gas. This arrangement allows the coupling capacitance between the S–SET central island and the dot to be comparable to the tunneling capacitances of the dot. As a result, the measured sensitivity of the S–SET referred to charge on the dot is 1.2310 e/AHz, about an order of magnitude better than previously reported results. The increased sensitivity makes this system ideally suited for broadband measurements such as study of individual tunneling events. © 2000 American Institute of Physics. @S0003-6951~00!04243-1#