A method for controlling a concentration of donors in an Al-alloyed gallium oxide crystal structure includes implanting a Group IV element as a donor impurity into the crystal structure with an ion implantation process and annealing the implanted crystal structure to activate the Group IV element to form an electrically conductive region. The method may further include depositing one or more electrically conductive materials on at least a portion of the implanted crystal structure to form an ohmic contact. Examples of semiconductor devices are also disclosed and include a layer of an Al-alloyed gallium oxide crystal structure, at least one region including the crystal structure implanted with a Group IV element as a donor impurity with an ion implantation process and annealed to activate the Group IV element, an ohmic contact including one or more electrically conductive materials deposited on the at least one region.