- Single Crystal
Silicon, Germanium are single crystal. Gallium Arsenide (GaAs), Cadmium Sulfide (CdS), Gallium Nitride (GaN) and Gallium Arsenide Phosphide (GaAsP) are compound semiconductors. Most popularly used semiconductors are Silicon (Si), Germanium (Ge) and Gallium Arsenide (GaAs). In 1939 diode was discovered. In 1947 transistor was discovered. Germanium was the first semiconductor materials commercially and widely used. Germanium is easy to find, easy to refine and largely available in nature. Germanium is very sensitive to change in temperature. So, reliability of Germanium diode is low. After Germanium, Silicon comes into market as semiconductor.
Silicon is less sensitive to change in temperature than Germanium. But refining process of silicon is much more complicated and expensive than that of germanium. In 1954 first silicon transistor came into the market. Silicon became most popular choice as a semiconductor after 1954. GaAs transistor was first introduced in the year of 1970. The speed of operation of a GaAs transistor is five times more than that of the silicon transistor. GaAs is more difficult to refine. GaAs is more expensive than Si. As the speed of operation is quite high in GaAs semiconductor devices, it is often used as the base material of VLSI (very large scale integrated) circuit. But still, Silicon is the most popular semiconductor. Gallium has 31 electrons. Electrons configuration of Ga is,
Therefore, 2 electrons in the 4S subshell and 1 electron in the 4p subshell. That means Gallium has three electrons in the outermost orbit, i.e. 4th orbit. Hence, Gallium has three valence electrons.
Arsenic has 33 electrons.
Electrons configuration of As is
Therefore, 2 electrons in 4S subshell and 3 electrons in the 4p subshell. That means Arsenic has 5 electrons in the outermost shell, i.e. 4th shell. Hence, Arsenic has five valence electrons.
The potential energy required to remove these valence electrons from their parent atoms, quite smaller than that of any other inner electrons, in the atomic structure.
Since GaAs is a compound, each gallium atom in the structure is surrounded by Arsenic atoms, and gallium atoms surround each Arsenic atom in the structure. Three valence electrons of gallium atoms and five valence electrons of Arsenic atoms share each other. In this way, each of the arsenic and gallium atoms gets 8 electrons in its outermost shell.
That means, there are covalent bonds between arsenic and gallium atoms, in a gallium arsenide compound. Although covalent bonds are stronger bonds, still it is possible to break the bonds, if sufficient energy is supplied externally. Due to the breaking of covalent bonds between arsenic and gallium, electrons are come out from the lattice structure of the GaAs compounds. As soon as, an electron is separated from the covalent bond, it leaves a vacancy behind it in the bond. The separated electrons from the bonds are free to move anywhere in the lattice. These free to move electrons are referred to as free electrons. The vacancies created in the bonds are referred to as holes. Both free electrons and holes are called free charge carriers. At room temperature, the number of free carriers is 1.7 × 106 approximately. The concentration of free charge carriers, at room temperature in a pure semiconductor material is denoted as ni. Here, n denotes the number of free charge carriers per unit volume of semiconductor lattice and the suffix ‘i’ used for the term intrinsic. Intrinsic semiconductor means, absolutely pure semiconductor that means ideally zero impurity contain.
The energy gap between valence band and conduction band in GaAs is 1.43 eV. Among, three most popular semiconductor materials are Silicon (Si), Germanium (Ga) and Gallium Arsenide (GaAs). GaAs has the largest energy gap between valence band and the conduction band.
From early 1990, the use of GaAs is growing up. For manufacturing very large scale integrated circuits, nowadays GaAs are used widely instead of silicon. This is because of its very high speed operating characteristics, low reverse saturation currents, excellent temperature sensitivities and high breakdown voltages. Also, became of these advantages, GaAs is widely used for different optoelectronics applications, like light emitting diode, solar cells and other photodetector devices.