The Semiconductors, such as Germanium, Silicon, Carbon, Selenium, etc. are the materials which are neither conductors nor insulators. Their conductivity lies in between or middle of the conductivity of conductors and insulators. Semiconductors have some useful properties and are being extensively used for the preparation of solid state devices like the diode, transistor, etc.
Although the valence band of these substances is almost filled and the conduction band is almost empty as in case of insulators. But the forbidden energy gap between the valence band and the conduction band is very small (nearly 1 electron volt) as shown in the figure below.
Therefore, an electric field, smaller than insulators and greater than conductors is required to push the valence electrons to the conduction band. Few of the electrons cross the conduction band even at room temperature as some of the heat energy are imparted to the valence electrons.
As the temperature increases, more and more number of valence electrons cross over to the conduction band and the conductivity of the material increases. Thus, these materials have negative temperature co-efficient of resistance.
Commonly Used Semiconductor Materials
There are a large number of Tetravalent materials available such as carbon in the diamond stat, silicon, germanium and gray tin. The minimum energy required for breaking the covalent bond in these materials is 7, 1.12, 0.75 and 0.1 electron volt respectively.
Carbon in diamond state behaves like as an insulator having forbidden energy gap of 7 eV. Gray having a forbidden energy gap of 0.1 eV behaves like a conductor. Therefore, Germanium and Silicon has an energy gap of 0.75 and 1.12 eV respectively, are considered most suitable for semiconductor materials.
The two semiconductor materials are discussed below.
Germanium was discovered in 1886. It is an earth element recovered from the ash of certain coaks or from the flue dust of zinc smelters. The recovered germanium is in the form of germanium dioxide powder. It is then converted into pure germanium.
The atomic structure of germanium is shown below.
Its atomic number is 32. It has 32 protons in the nucleus and 32 electrons distributed in the four orbits around the nucleus. The number of electrons in the first, second, third and fourth orbit are 2, 8, 18 and 4 respectively. It is clear that the germanium has four valence electrons. The various germanium atoms are held together through covalent bonds as shown in the figure below.
The forbidden energy gap (i.e. Gap between the valence band and conduction band) in this material is very small. Hence, a very small energy is sufficient to lift the electrons from the valence band to the conduction band.
Silicon is the element available in most of the common rocks. Actually, sand is silicon dioxide. It is treated chemically and reduced to pure silicon, which can be used for the preparation of electronic devices.
The figure below shows the atomic structure of silicon.
Its atomic number is 14. Therefore, it has 14 protons in the nucleus and 14 electrons distributed in the three orbits around the nucleus. The number of electrons in the first, second and third orbit are 2, 8 and 4 respectively. The various silicon atoms are held together through covalent bonds as shown in the figure below.
The forbidden energy gap in this material is quite small. It also needs a small energy to lift the electrons from the valence band to the conduction band.
Therefore, even at room temperature, a minute quantity of valence electrons is lifted to the conduction band and constitute current conduction if a high electric field is applied. However, at room temperature, the number of electrons lifted to the conduction band in the case of silicon is quite less than germanium.
This is the reason why silicon semiconductor devices are preferred over germanium devices.