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 1.2x10(-4) e/root Hz, 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. (C) 2000 American Institute of Physics. [S0003-6951(00)04243-1].