Why germanium controlled rectifiers are not used instead of silicon controlled rectifiers?

Answer 1

The first solid state four layer thyristors made were indeed made of germanium and therefor could be called "germanium controlled rectifiers" (although this name was never used in the literature, they were just called thyristors). But when it became practical in the early 1960s to make solid state devices using silicon, the higher operating junction temperature and the ability to self passivate junctions in silicon devices soon made germanium devices obsolete especially for high power applications (which include many of the applications of solid state four layer thyristors).

The term thyristor was coined originally for these devices as they were seen as a solid state version of the gas filled thyratron tube. The idea of calling them "controlled rectifiers" only came years later when silicon had replaced germanium for most purposes in solid state devices.