What is the main physical difference behind these two theories? 1) I understand that the thermionic emission theory is applied in metal-semiconductor contacts and heterostructures where the energy band off-sets are large. Whereas the diffusion theory is applied in a simple homojunction, of which the gradient of the quasi-fermi levels are continious. 2) On the other hand, I understand that the diffusion current and the thermionic emission current work in series, where the slowest process limits the current. There seems to be contradictions between statement 1 and 2. What have I misunderstood?