Capacitance deep-level transient spectroscopy (DLTS) was used to study surface states on aluminum compounds. Two hole-like traps were observed in pseudomorphic high-electron-mobility transistor with a multifinger gate. No hole-like signals were observed in the DLTS spectra of the fat field-effect transistor (FATFET) having negligible ratio of the ungated surface to the total area between the source and the drain. The activation energies of both surface states were measured to be 0.50±0.03 and 0.81±0.01 eV.