A three-level, generation-recombination model has been developed to explain the unusual response law and ultraphotosensitivity of the Avalanche Breakdown far infrared detector1,2). The detector consists of Sb-doped n-germanium operated at a bias level above the point of impact ionization breakdown. Under these conditions, the signal response current, is, has been shown to depend on far infrared signal power, P, as is = K(P)1/2, where K is a proportionality constant, over more than six orders of magnitude in P. Furthermore, an apparent photomultiplication factor of the order 106 - 107 has been observed. Signals as low as 10-14 watts in a 10-7 sec pulse have been seen, corresponding to the arrival at the detector of only 4-5 photons per pulse. Earlier attempts to provide a model of this detector3), including a two- level scheme by Penin4), have not proven satisfactory.