A germanium transistor is a variation on a standard transistor built on the element silicon, where, instead, a silicon-silicon-germanium alloy is commonly used to increase transmission speed of electrical signals. Individual electrical component speed adds up as an aggregate, and, therefore, a germanium transistor array can significantly increase the processing speed of a circuit. The germanium transistor predates standard silicon designs, and they were commonly used in the 1950s and 60s. Their throughput speed or lower cut off voltage is superior to silicon, but, today, they only have specialized applications.
Semiconductor germanium silicon transistors are also alloyed with indium, gallium, or aluminum, and have been used as replacements for another alternative to pure-silicon transistor arrays, those built on gallium-arsenide. In solar cell applications, germanium and gallium-arsenide are used together as they have similar crystal lattice patterns. Optics applications are a common place where a germanium transistor is employed now, in part because pure germanium metal is transparent to infrared radiation.
Germanium alloys offer enhanced transmission rates in high-speed circuitry over silicon, but they are not without their drawbacks. Most properties of a germanium transistor fall below those of a standard silicon transistor, including the maximum power distribution they offer, at around 6 watts versus over 50 watts for silicon, and lower levels of current gain and operating frequencies. The germanium transistor also has poor temperature stability as compared to silicon. As the temperature increases, they allow more current to pass, eventually resulting in their burning out, and circuits must be designed to prevent this possibility.
One of the biggest drawbacks of a germanium transistor is that it displays current leakage due to the tendency of germanium to develop screw dislocations. These are fine outgrowths of the crystalline structure, known as whiskers, that, over time, can short-out a circuit. Current leakage of over 10 micro-amps can be a method of determining that a transistor is built on a base of germanium instead of silicon.
Compared to silicon, germanium is a rare and expensive metal to mine. Whereas silicon is easy to obtain as quartz in raw form, the process of refining semiconductor grade silicon (SGS) is still a highly technical one. Nevertheless, it doesn't pose the health hazards that germanium does, where germanium and germanium oxide produced in the refining process have been shown to have neurotoxic effects on the body.
Though germanium is primarily used as transistors in solar cell and optical applications, the germanium diode is also employed as an electrical component due to its lower cut-off voltage of about 0.3 volts versus 0.7 volts for silicon diodes. This unique advantage of germanium semiconductor components makes them a target for incorporation into future high-speed components, such as the silicon-germanium carbon transistor. Such transistors offer the lowest noise transmission levels and are best-suited to radio frequency applications for oscillators, wireless signal transmission, and amplifiers. This reflects the fact that one of the original uses for germanium components decades ago was in radio design.