Gallium Arsenide Semiconductor
between that of a conductor
There are two types of semiconductor-
- 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 market. After this year silicon became most popular choice as semiconductor. GaAs transistor was first introduced in the year of 1970. Speed of operation of a GaAs transistor is 5 times more than that of silicon transistor. GaAs is more difficult to refine. GaAs is more expensive then 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 most popular semiconductor. Gallium has 31 electrons. Electrons configuration of Ga is,
Therefore, 2 electrons in 4S sub shell and 1 electron in 4p sub shell. That means Gallium has 3 electrons in outermost shell i.e. 4th shell. Hence, Gallium has 3 valence electrons.
Arsenic has 33 electrons.
Electrons configuration of As is
Therefore, 2 electrons in 4S sub shell and 3 electrons in 4p sub shell. That means Arsenic has 5 electrons in outermost shell i.e. 4th shell. Hence, Arsenic has 5 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 each Arsenic atom in the structure is surrounded by gallium atoms. 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 arsenics and gallium atoms, in gallium arsenide compound. Although, covalent bonds are stronger bonds, but still it is possible to break the bonds, if sufficient energy is supplied externally. Due to 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 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 as free electrons. The vacancies created in the bonds, are referred 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 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 largest energy gap between valence band and 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 sensivities 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 photo detector devices.