Design of Heterostructures for High Efficiency Thermionic Emission

We use two heterostructure designs to improve the energy conversion efficiency of solidstate thermionic devices. The first method is to use a non-planar heterostructure with roughness in order of electron mean free path. This has some combined benefits of increased effective interface area, and reduced total internal reflection for the electron trajectories arriving at the interface. Monte Carlo simulations of various geometries show that the electrical conductivity and thermoelectric figure of merit can be improved for non-planar barrier compared to the planar counterpart. The second method is to use planar high barrier heterostructures with different effective masses for charge carriers in emitter and barrier regions. When an electron passes from a lower effective mass emitter and arrives at a barrier with higher effective mass, since both the lateral momentum and total energy are conserved, part of the lateral energy is coupled to the vertical direction and the electron gains momentum in the direction perpendicular to the interface to enter the barrier region. For high potential barriers, the improvement of thermionic current is about the same as the ratio of the effective masses of the two materials, which can be a factor of 5-10 for typical heterostructure material systems.