Silicon is used as a CPU because it has too many advantages and disadvantages that can be overcome. Although germanium also has advantages (such as opening voltage, carrier mobility), it has several disadvantages that are difficult to overcome.
The first is the price. Silicon can be made directly with sand, although the process is complex, but the raw material cost is close to 0. Germanium is very dispersed in the crust, and the price of finished germanium (not yet semiconductor grade) has exceeded silver, which is nearly 2,000 US dollars a kilogram.
The second big problem is that the oxide of germanium is unstable and difficult to use. Silica is a dense insulator, mechanical, electrical and chemical properties are very stable, insoluble in water; Germanium oxide is not so dense and is still soluble in water. This basically declares the CPU hopeless.
There is also the problem that germanium devices do not perform well at slightly higher temperatures, and germanium itself is heavier than silicon, and softer than silicon, and more easily broken; Let's wait. And now that the entire semiconductor industry is based on silicon, no one will develop germanium cpus.
At present, the future of germanium is largely in photonics, solar cells, light sensors, infrared leds, germanium lasers (which have been made by MIT, but not the laser pointer that you think of), and so on. Because silicon is completely impossible to use as a laser, and germanium can be grown relatively easily on silicon, the ideal is to integrate optical devices made of germanium with electronic devices made of silicon on a silicon chip