I have wondered myself why is that value fixed to around 0.7 V (0.3 Ge). I have researched about this topic over and over again, but I always find the same answer. They say "Because the voltage for Silicon Diodes is 0.7". That is just like saying that the sky is blue because blue is the color of the sky.
I am familiar with the Shockley diode equation, but I don't see the connection with the threshold voltage (I'm saying this because people have given me a link to its Wikipedia page).
I have also read something about the concentration of impurities near the junction being related to the voltage barrier (I am hoping to get an answer related to that, and the manufacture process).
Another answer I have been given is that that is silicons nature (I kind of hate this answer, because what I get from it is that the voltage is an intensive property, instead of extensive – which would make materials more "workable").
So the question per se is: Why 0.7 and not 0.4, 0.11, 1.2 (for Silicon)?