Why germanium semiconductor is more preferable than silicon?
Not necessarily more preferable, but Germanium exhibits a narrower band gap, making it more ideal for infrared applications, and also is superior to Silicon when used in rectifiers because it has a higher resistance to destruction by reverse voltage surge. It is far more expensive than Silicon, has a narrower operating temperature, and is poorly suited to LSI/VLSI circuitry (modern computers, etc.), however and has since fallen out of favor in most places except Russia (there are very few Silicon reserves in Russia, and massive amounts of Germanium, which is why many Russian semiconductors still use it, especially in high-power applications.