We present a theory of tunable infrared response in field‐effect transistors. The linear &agr;(&ohgr;) and second‐order &bgr;(&ohgr;) polarizability are calculated numerically as functions of frequency of an applied electric field. Transitions between electron subbands in the inversion layer cause resonances in the frequency response, which are tunable with a gate voltage. ©1996 American Institute of Physics.