Ferroelectric BaMgF 4 films have been grown on AlGaAs/GaAs(100) high-electron-mobility transistor (HEMT) structures. BaMgF 4 films in which the direction of spontaneous polarization has a finite angle to the substrate surface can be obtained on the HEMT structure at growth temperatures of 500-600 o C. However, it is shown that the growth temperature is limited to about 550 o C, because the AlGaAs/GaAs interface becomes disordered by the diffusion of Mg atoms into the HEMT structure when the growth temperature is 600 o C.