Which diode (semiconductor material) has the lowest voltage in the direction of conductivity?
In fact, Schottky Diodes
have the lowest forward voltages. This means, that this is not a question of band gap "voltage" (this is a energy difference originally!) but of technology. Second are Germanium point contact diodes with gold wires.
"Diodes" made from Galena maybe are very low too, but due to the wiggely properties I would not dare to write about the "impressions" I had some times for some seconds on a scope.
Superconducting Niobium Chip Smashes Silicon Power Consumption Standards A superconducting logic chip with a clock speed of 6 GHz beats silicon energy efficiency by two orders of magnitude
Ref: Ultra-Low-Power Superconductor Logic