The influence of incoherent tunneling in double-barrier resonant-tunneling (DBRT) diodes, originating in carrier scattering, is investigated using a previously published model in which the device is divided into three regions (accumulation region, barrier region and depletion region), each of which is analyzed by a separate model. In the barrier region a self-consistent quantum mechanical model is used. Scattering is assumed to contribute a damped amplitude to the coherent wave function and the non-coherent carriers are assumed to tunnel out through one of the two barriers in proportion to the transmittivities of the latter. The damping factor can be related to the momentum relaxation time used in transport calculations. A good match of calculated to experimental I-V characteristics, measured on GaAs/AlGaAs DBRTs, can be obtained using a scattering time constant substantially shorter than that of bulk GaAs, indicating that there are extra scattering mechanisms to take account of.