  We report on the effects of a global top gate on low-frequency noise in
Schottky gate-defined quantum point contacts (QPCs) and quantum dots (QDs) in a
modulation-doped Si/SiGe heterostructure. For a relatively large top gate
voltage, the QPC current shows frequent switching with 1/f2 Lorentzian type
charge noise. As the top gate voltage is decreased, the QPC pinch-off voltage
becomes less negative, and the 1/f2 noise becomes rapidly suppressed in a
homogeneous background 1/f noise. We apply this top-gating technique to double
QDs to stabilize the charge state for the electron number down to zero.
