Additional electron mobility due to remote scatterers in n(+)-polysilicon 1.65-nm gate oxide (SiO2) n-channel metal-oxide-semiconductor field-effect transistors is experimentally extracted at three different temperatures (i.e., 233, 263, and 298 K). The core of the extraction process consists of simulated temperature-dependent universal mobility curves and Matthiessen's rule in a mobility universality region. Resulting additional mobility for the first time experimentally exhibits a negative temperature coefficient, confirming interface plasmons in a polysilicon depletion region to be dominant remote Coulomb scatterers. We also present corroborative evidence as quoted in the literature, including: 1) calculated temperature-dependent remote Coulomb mobility due to ionized impurity atoms in a polysilicon depletion region; 2) experimentally assessed additional mobility at room temperature; and 3) simulated remote Coulomb mobility due to interface plasmons in a polysilicon depletion region as well as its temperature coefficient. Validity of Matthiessen's rule used in this paper is verified.