Specifics of the current flow in a tunnel diode Those are two diagrams depicting current flow along with equivalent band diagrams form two different textbooks. I believe one first one is resonant diode and second is tunnel diode only.
From the first one I would make out that the electron tunnels from the conduction band through the forbidden gap to the allowed energy state in the quantum well.
While from the second one it looks to me as the current consists of the electron tunneling to the valance band where it recombines with the hole.

So which mechanism is the correct one or is it both? Perhaps I am looking at two similar but totally unrelated diagrams and inferring wrong ideas? I am also not sure how Fermi level can sit above the conduction band.
 A: You show two different diodes with electron tunneling as a charge transfer mechanism. The first is a so-called resonant tunneling diode, where electrons tunnel through a double barrier in the conduction bands formed by two different semiconductors with different band gaps and a conduction band offset. Such semiconductor structures are called heterostructures. Because this tunneling occurs through discrete energy states in the quantum well between the barriers, the I-V characteristic exhibits two regions of negative differential resistance (NDR) related to the tunneling transport through the first two energy states in the quantum well. This resonant tunneling diode has highly doped n-type regions at both contacts.
The second diode is a tunnel diode, which is simply a p-n diode an a single semiconductor with very highly ("degenerately") doped n- and p-regions, where depletion zone and thus potential barrier due to the built-in voltage of the p-n diode is very thin. The Fermi levels in the highly doped n- and p-region are in the conduction and valence band, respectively. The electron tunneling is between the filled conduction band of the n-type side and the empty valence band of the p-type side through the very thin triangularly shaped barrier formed by the conduction band and band gap the p-n diode. The relative energetic alignment of the filled and empty states in the conduction and valence bands on the n- and p-side of the diode causes the NDR of the I-V characteristic.
Thus both diodes use tunneling for electron tranport  but the structures of the diodes and the mechanisms of the NDR in the I-V characteristics are completely different.
