Alloyed, Thin-Base Diode Capacitors for Parametric Amplification

Abstract

The design of double alloyed, thin-base diode capacitors for parametric amplification is described which indicates that capacitors with maximum Q's well in excess of 200 at 1 kmc and capacitive swings in excess of 15 are obtainable. The high Q's resulting from the type of fabrication and the small base thickness used; the large capacitive swing resulting from the abrupt p+n junction created. Three figures of merit are defined and evaluated: (1) an over-all device figure, FD, proportional to the product of the maximum Q and capacitance ratio evaluated in terms of material parameters, degree of base doping, and geometry factors; (2) a Q factor, FQ, proportional to just the capacitor Q; and (3) a material factor, FM, by which different materials used for fabrication can be compared. Fixing the choice of material (with germanium shown to be superior to silicon for this purpose) and achieving the best geometry, FD and FQ are examined as a function of doping. It is shown that FQ is nearly constant over the entire practical range of doping while FD, because of the capacitive swing, increases substantially with lighter doping. Results are presented for units with all soldered connections (no pressure contacts) fabricated from double alloyed pellets with a p+nn+ structure. To date, diode capacitors with maximum Q's in excess of 200 at 1 kmc and with capacitive swings in excess of 10 have been made. It is anticipated that this fabrication process will yield Q's in excess of 500 at 1 kmc with closer alloying control.