Voltage-tunable plasmon resonances in a InGaAs/InP high electron mobility transistor (HEMT) are reported. The gate contact consisted of a 0.5 micron period metal grating formed by electron-beam lithography. Narrow-band resonant absorption of THz radiation was observed in transmission in the range 10 - 50 cm-1. The resonance frequency red-shifts with increasing negative gate bias as expected. Photo-response to a tunable far-IR laser is reported. The device may have application in high-frame-rate THz array detectors for spectral imaging with real-time chemical analysis.© (2009) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.