The most commonly used semiconductor material is Silicon. This is an element, it has 14 electrons, and its pure solid form melts at 1420 °C. Used for
thousands of years to make ordinary glass, Silicon is a very common element. Silicon turns up in lots of rocks and forms the sand on beaches.
The earliest commercial semiconductor devices mostly used Germanium. This element has 32 electrons per atom and melts at 985 °C. It has now
largely fallen into disuse because it is much rarer and more expensive than Silicon and has no real advantages for most purposes.
The second most common modern material is Gallium Arsenide, GaAs. This is a combination of Gallium, an element with 31 electrons per atom, and Arsenic,
with 33 electrons per atom. This is a crystalline compound, not an element. Hence we can get an extra degree of control over its properties by
varying the relative amount of Gallium and Arsenic.
GaAs has the advantage of making semiconductor devices which respond very quickly to electrical signals. This makes it better than Silicon for doing tasks
like amplifying the high frequency (1GHz to 10GHz) signals from TV satellites, etc. The main disadvantage of GaAs is that it is more difficult to make and the
chemicals involved are quite often poisonous!
GaAs can be used with signal frequencies up to about 100 GHz. At even higher frequencies more esoteric materials such as Indium Phosphide (InP) may be used. At present, however, the MMWave region (frequencies above about 50 GHz) is only used for special purposes, so most of the electronics in the world thends to be based on Silicon, with some GaAs, and only a few InP devices.
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