Forward voltage of AlN diode
The reference for the 210 nm diode is:


  Yoshitaka Taniyasu, Makoto Kasu, and Toshiki Makimoto, "An aluminium
  nitride light-emitting diode with a wavelength of 210 nanometres",
  Nature 441, 325 (2006) [doi:10.1038/nature04760]


In this paper, the authors state that they constructed diodes having both a MIS and a PIN structure. The forward voltages for 20 mA forward current were 37 and 45 V, respectively, although the operating current (to produce 0.02 µW of 210 nm emission) was 40 mA, not 20 mA, and they don't directly state the forward voltage for this condition.  The large forward voltages were (seemingly, without having checked it too rigorously) the result of having to drive such a large current through the device given the rather low hole mobility, which was around 30 cm^2 V^-1 s^-1 at room temperature.

The external quantum efficiency was stated to be of the order of 0.000001%.