The strong bias dependence of the far‐infrared laser‐induced negative photoeffect is here attributed to an intrinsic bias dependent internal photoemission effect. Experimental results fit remarkably well with the carrier temperature and the transition potential predicted by our model. We demonstrate that any asymmetric heavily doped junction (P+/N,N+/P) which is properly forward biased is suitable to detect far infrared radiation. The sensitivity is reasonable (≊1 mV per kW at 300 K) for detection of pulsed radiation and the rise time is intrinsically fast (≤1 ns). ©1996 American Institute of Physics.