Electron thermal transport in semiconductor thermionic devices is investigated numerically. The efficiency of thermionic devices is dramatically reduced by the conduction heat current, DeltaT/Rth . One approach to reduce the thermal conductivity is to use layered structures where interface scattering can increase the thermal resistivity. It is found that the temperature gradient across the devices can be increased by a factor of 2 in the presence of phonon scattering. The device is more efficient at elevated temperature.