We report enhanced performance of Al _0.3 Ga _0.7 N / GaN high electron mobility transistors (HEMTs) grown on sapphire substrates incorporating isoelectronic indium surfactant doping in the two-dimensional electron gas channel, the AlGaN spacer, and the AlGaN cap layer. We found that the isoelectronic In doping plays the role of surfactant and can effectively reduce the defect density. The In surfactant also lowers the interface roughness scattering and improves the surface morphology, leading to higher electron mobility at 300 and 77 K . More than 30% increase in the drain saturation current and peak transconductance were observed in the In-doped devices that also showed negligible current collapse and high cutoff frequencies. The on-wafer output power, linear gain and power-added efficiency of an unpassivated 1×100 μ m device measured at 2 GHz were 3.01 W / mm , 20.9 dB , and 35.7% , respectively.