Why is silicon diode 0.6 V and germanium diode 0.2 V?

Answer 1

It probably has to do with the arrangement of their electrons in their electron shells, which in turn effects things like their atomic radius and resistivity. Remember that these are "semi-conductor" metals.

The term "forward voltage" has to do with devices, such as diodes and transistors, not the elements themselves. Elemental Silicon and Germanium do not have "forward voltages" per se; it is only when these elements are "grown" into a wafer and fused to a circuit that we have a device. The terms forward voltage, reverse voltage, breakover voltage, thermal breakdown, and a lot of other measurements and factors then become part of the "specs" for that particular type of device.

Answer 2

Silicon has energy level (orbit) of 3 (14 e) while Germanium has 4 (32 e). That is the 4 valence electrons of Si are found in the 3rd orbit from the nucleus while 4th for Ge. The closer the electrons to the nucleus the higher energy is needed to knock it down from its orbit.

Answer 3

Germanium has a much greater tendency than does silicon to produce minority carriers through thermal excitation of its valence band electrons (because the atom is larger these electrons are further from the nucleus and thus looser bound to it).

It is the quantity of minority carriers that determines the amount of leakage when reverse biased and thus the reverse current of the junction. Thus germanium devices have more reverse current and silicon devices less reverse current, at the same junction temperature.

Answer 4

Those are the approximate forward bias voltage drops across P-N junctions made with each material.

Answer 5

Silicon diode can bear more temperature than germenium

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