We have measured surface photovoltage (SPV), photoreflectance (PR), and photoluminescence (PL) spectra of two InAlAs/InxGa1-xAs/InAlAs metamorphic high-electron mobility transistor (MHEMT) structures. One possesses a V-shaped InxGa1-xAs (x= 0.3-0.5-0.3) tensile-strained channel in In0.5Al0.5As/InxGa1-xAs/In0.5Al0.5As heterostructures, and the other is an In0.42Al0.58As/In0.53Ga0.47As/In0.42Al0.58As MHEMT structures with InxGa1-xAs (x= 0.53) compressively-strained channel grown on GaAs by molecular beam epitaxy. The comparison of SPV, PR, and PL spectra facilitates the identification of channel-well transitions in the MHEMT structures with different InxGa1-xAs channels. Inter-subband transitions, Fermi-level energies, and built-in electric field of the two MHEMT structures with dissimilar InxGa1-xAs channel are evaluated and discussed from the experimental analyses of SPV, PR and PL measurements. The results showed that the design of tensile-strainedMHEMT structure enhances sheet-carrier density and avoids surface-roughness scattering by increasing V-shape electric field between the two channel interfaces. The electron mobility of the tensile-strained MHEMT structure is hence being promoted. (C) 2012 Elsevier B.V. All rights reserved.