At least one layer including a scavenging material and a dielectric material is deposited over a gate stack, and is subsequently anisotropically etched to form a oxygen-scavenging-material-including gate spacer. The oxygen-scavenging-material-including gate spacer can be a scavenging-nanoparticle-including gate spacer or a scavenging-island-including gate spacer. The scavenging material is distributed within the oxygen-scavenging-material-including gate spacer in a manner that prevents an electrical short between a gate electrode and a semiconductor material underlying a gate dielectric. The scavenging material actively scavenges oxygen that diffuses toward the gate dielectric from above, or from the outside of, a dielectric gate spacer that can be formed around the oxygen-scavenging-material-including gate spacer.