High performance pseudomorphicInxGa1−xAs/GaAsmodulation doped field effect transistors (MODFETs) grown by metalorganic chemical vapor deposition have been characterized at dc using modulation spectroscopy. A transconductance as high as 175 and 253 mS/mm was obtained at 30&percent; and 40&percent; indium content, respectively. In order to identify the origin of the MODFETs’ features we have performed a self-consistent Schrodinger–Poisson calculation of the subband and intersubband energies. The photoreflectance spectra can be divided into two parts by the photon energy; one belongs to the GaAs bulk transition and the other belongs to theInxGa1−xAs/GaAsquantum well transition including two-dimensional electron gas (2DEG) signals. The built-in electric fields are20.6×104 V/cmand22.6×104 V/cm.From photoreflectance spectra peaks at 1.035 and 1.06 eV show that their energies agree with line shape fitting and 2DEG theory. ©1997 American Institute of Physics.