Heterojunction energy band analysis is employed to elucidate the energy band structure of injector (i.e. a metal or heavily-doped semiconductor)/insulator (i.e. an insulator or semi-insulating wide-bandgap semiconductor) heterojunctions. Although both blocking and injecting contacts are considered, the primary focus is to clarify the energy band nature of injecting contacts. An important aspect of this work is the invocation of spatial truncation of the insulator region as a means of obtaining a physically realistic energy band picture of the interface in real devices, thereby avoiding confusion associated with insulator bulk boundary conditions in mathematical device physics models. The topic of insulator injecting contacts is perceived to be one of current interest, since such contacts appear to play a critical role in the operation of transparent thin-film transistors and perhaps other transparent electronics devices.