We investigate the gating properties of Si/SiGe two-dimensional electron gas systems with various gate materials and fabricate a lateral Si/SiGe quantum dot by gating through an Al2O3 film. In comparison to the conventional surface Schottky gates, gating through a thin Al2O3 layer provides a strong suppression of leakage current. The fabricated quantum dot shows a periodic current oscillation or Coulomb oscillation with negligible gate leakage, indicating that the gating employed may be good for implementing Si/SiGe-based spin qubit devices with quantum dots.