Device characteristics of AlGaAs/GaAs heterostructure field effect transistors fabricated by molecular‐beam epitaxial growth are related to the condition of the substrate‐epitaxial layer interface. The presence of carbon on the GaAs wafer surface prior to growth has been found to produce ap‐type, conducting interfacial region. We demonstrate that the concentration of carbon on the wafer surface can be significantly reduced by exposing the wafers to ultraviolet radiation. Depletion‐ and enhancement‐mode device transfer characteristics and transconductance curves have been obtained on heterostructure wafers that were subjected to an ultraviolet‐ozone surface preparation. A comparison of these results with the device properties of wafers receiving a standard cleaning procedure is presented. A model describing the interaction between the interfacialp‐type region and the two‐dimensional electron gas channel is also included.