Hi mate. Germanium Diodes principally have a PN Junction barrier charge that requires a 0.2V Voltage drop.
For Silicon Diode PN Junctions, this value is increased to around 0.6V for the voltage across the PN Junction to be sufficient, that the energy provided will cause a flow of electrons across that junction.
This occurs for all Germanium and Silicon PN junctions respectively.
Therefore, a transistor comprising a PN or one comprising an NP junction of Germanium, will have a break down barrier voltage required of 0.2 volts before electrons will flow across the junction, and therefore a transistor made of Silicon shall require a breakdown voltage of around 0.6V to achieve the same result.
Whilst this is an ideal voltage of 0.2V or 0.6V most of these components use Silicon now anyway, it does not mean the figures are concise. Some components might be rated at 0.5 or 0.4V or otherwise. Germanium Diodes are used principally in RF applications, for example, to provide half wave rectification in an AM Receiver.
In fact, one germanium diode, is really all you need (save for an antenna) to create a crystal radio.
Germanium, diodes replaced the Cats Whisker which used a small amount of Galena, to form a Junction.
I have a Cats Whisker at home, and you can purchase them online, for kits.
Anyways hope that helps.