Why A PN junction does not conduct appreciably until the applied forward voltage overcomes the junction's built-in barrier potential. That barrier depends on the semiconductor material: germanium junctions start conducting at roughly 0.3 volts, while silicon junctions need about 0.7 volts. The lower drop is why germanium diodes were favored for small-signal detection in low-level receiver circuits.
Watch out The 0.7 volt figure is the classic silicon diode threshold, the number the pool asks about in the companion question, so it is easy to swap the two materials.
G comes before S in the alphabet and takes less voltage: Germanium 0.3 V, Silicon 0.7 V.