The optical properties of graded InxGa1-xAs/GaAs high electron mobility transistors (g-HEMTs) with different channel-band slopes are characterized by making photoreflectance (PR) and photoluminescence (PL) measurements at 300 K. Analyses of experimental PL and PR spectra clarify the channel-well transitions and the built-in electric fields of g-HEMTs. The numbers of channel-well transitions for two channel band-slope g-HEMTs of the same indium composition differ because the effects of In segregation on the growth of graded InxGa1-xAs channel layers of opposite gradient differ. Intersubband energies, the Fermi-level energy, the sheet carrier density of two-dimensional-electron gas, and the built-in electric fields of three g-HEMTs with three different channel-band slopes are evaluated. The device characteristics are also discussed. (c) 2006 The Electrochemical Society.