The concept of homojunction internal photoemission far‐infrared (FIR) detectors has been successfully demonstrated using forward biased Sip‐i‐ndiodes at 4.2 K. The basic structure consists of a heavily doped IR absorber layer and an intrinsic (or lightly doped) layer. An interfacial workfunction between these regions defines the long‐wavelength cutoff (&lgr;c) of the detector. Three types of detectors are distinguished according to the emitter layer doping concentration level. Our model shows that high performance Si FIR detectors (≳40 &mgr;m) can be realized using the type‐II structures with a tailorable &lgr;c, in which the absorber/emitter layer is doped to a level somewhat above the metal‐insulator transition value. Analytic expressions are used to obtain the workfunction versus doping concentration, and to describe the carrier photoemission processes. The photoexcitation due to free‐carrier absorption, emission to the interfacial barrier, hot‐carrier transport, and barrier collection due to the image force effect, are considered in calculating the spectral response and quantum efficiency as functions of device parameters for Sin+‐istructures, leading to a detailed photoresponse analysis of type‐II detectors. These results are useful for the design and optimization of type‐II detectors. ©1995 American Institute of Physics.