In this paper, we propose trap-assisted tunneling (TAT) to account for the discrepancy between theoretical predictions and measured valley currents (also called excess currents) in double barrier resonant tunneling diodes (DB RTDs) with observed generation–recombination noise spectra. This proposed mechanism in DB RTDs is required, since predictions of excess currents from the models involving no gap states, such as quantum coherent tunneling, are too low. The model is also supported by the observed strong correlation between excess current and low frequency noise spectra. The trap states are assumed to be at or near the interface between the emitter and the first barrier in DB structures. The electrons trapped in the trap states emit out to tunnel through the double barriers. The empty trap states are then refilled by the electrons from the conduction band of the emitter. The conservation of the energy and momentum are incorporated through the emission–absorption of phonons. Based on these mechanisms, we propose a semiempirical formula for calculating valley current with three parameters, which are related to some physical parameters. A simple semiphysical model is set up to justify the three parameters and the detailed derivation is given. This two-step TAT currents are calculated for our examples of AlAs–GaAs–AlAs DB RTDs and are found to be in agreement with the measured excess currents. The variation of excess currents with temperature is also discussed. Improvement of the peak-to-valley current ratio, among other device design and fabrication considerations, depends on the reduction of these trap states.