A novel detector and harmonic multiplier – a barrier-injection heterostructure transistor (BIHT) – is proposed and evaluated. The structure of a BIHT is similar to that of a heterostructure field-effect transistor. The device utilizes thermionic injection of electrons to the BIHT channel in which the standing plasma waves are excited by an incoming signal. The combination of the strong nonlinearity of the dependence of the injected current on the channel potential with the high amplitude of the oscillation of the latter at resonant frequencies results in a significant enhancement of the responsivity and harmonic multiplication efficiency. The BIHT detectors and multipliers can be effective in the terahertz range of frequencies, surpassing the performance of Schottky-diode-based devices.