Sidegating characteristics of AlGaAs/GaAs heterostructure field‐effect transistors, fabricated on molecular‐beam epitaxially grown layers, were investigated with emphasis on the material properties. A systematic analyses of the epitaxial layers concluded with the identification of the substrate–superlattice‐buffer‐layer interface as the predominant cause of the sidegating effect. Remnant carbon contamination on the GaAs surface was found to produce ap‐type, conducting interfacial region. Controlled oxidation of the carbon on the wafers was accomplished using ultraviolet radiation. This oxide was desorbedinsitubefore epitaxial growth. Secondary‐ion‐mass spectroscopy was employed to estimate the carbon concentration at the substrate–epitaxial‐layer interface for standard cleaned and ultraviolet‐ozone‐treated wafers. The carbon concentration of the interfacial region decreased by two orders of magnitude for the wafers exposed to the ultraviolet radiation. Hall‐effect measurements of standard cleaned and ultraviolet‐ozone‐treated heterostructure wafers, prepared with various buffer layer thicknesses, demonstrated the dominant influence of the interfacialp‐type region on the electronic properties of the material. A comparison of sidegating characteristics for devices fabricated on the two types of wafers is presented and discussed. A dramatic improvement in sidegating was observed for the wafers subjected to the ultraviolet‐ozone cleaning procedure.