The single-tone power of the AlGaN/GaN heterostructure field-effect transistors (HFETs) with different gate widths was measured. A distinct improvement in device linearity was observed in the sample with a larger gate width. The analysis of the variation of the parasitic source access resistance showed that, as the gate bias is increased, the polarization Coulomb field scattering can offset the increased polar optical phonon scattering and improve the device linearity. This approach is shown to be effective in improving the device linearity of AlGaN/GaN HFETs.