Characterization of pseudomorphic InGaAs/AlGaAs MODFET structures grown by molecular beam epitaxy

InyGa1-yAs/Al0.15Ga0.85As pseudomorphic modulation doped field effect transistor (MODFET) structures with varying InAs mole fractions and InGaAs quantum well thicknesses were grown and characterized by transmission electron microscopy (TEM), Hall measurement, and photoreflectance. Conduction subband to valence subband transition energies were calculated, and the theoretical values were in good agreement with observed transition energies. Two dimensional electron was (2DEG) concentrations were also determined from calculated subband energies and Fermi levels and were found to be consistent with Hall measurement results. We have observed a peak in device performance with InAs mole fractions in the 0.15-0.20 range (for a 1 µm gate device with a 150 Å, In0.20Ga0.80As quantum well, an extrinsic transconductance of 310 mS/mm at 300K was obtained). Further, the performance of layers with InAs mole fractions higher than 0.25 is often degraded. This is probably growth related, since our calculations indicate that, provided a quantum well think enough, much higher 2DEG concentrations are possible than in lower InAs mole fraction devices. This, coupled with the higher saturation velocities that accompany higher InAs mole fractions, should result in even better device performance if growth problems can be surmounted.