Why 0.3 voltage in germanium diode?

Answer 1

the reason for 0.3V as barrier potential in Ge is:-

Electrons near the junction drift into the P region and recombine with holes. At the junction, the P-side has a layer of negative charges or negative ions (since p-type material is electrically nuetral addition of an electron makes it a negative ion). At the junction, the N-side has given up electrons thus creating holes i.e. it has positive charges or positive ions.This ion build up creates an area that is depleted of any conduction electrons or holes i.e. it has positive charges or positive ions cancelled out by negative charges or negative ions This ion build up creates an area that is depleted of any conduction electrons or holes. This represents a potential difference of 0v to 0.3v volts Ge diodes Semiconductor devices are controlled by controlling the depletion region of the device. The potential difference is called the barrier potential.

It's basically what makes the semiconductor work, so its a really good question.