![]() These free electrons, which are in conduction bands, cause the conductivity of semiconductor. Not only in silicon or germanium semiconductor, in all other semiconductors there are numbers of free electrons at room temperature, because of the same reason. Even at room temperature, there are numbers of free electrons available in a silicon or germanium semiconductor crystal. As the forbidden energy gap is quite moderate, an electron in valence band requires a small energy to cross the forbidden energy gap to become free. The forbidden energy gap of silicon is 1.1 eV and same for germanium is 0.7 eV. We call the energy gap between conduction and valence as forbidden energy gap. ![]() Hence, for any external energy supplied to the crystal, the electrons of valence band can acquire an ability to migrate to the conduction band and increase the conductivity. But the typicality about the semiconductor is that the band gap between conduction band and valence band is moderately small. Hence, in ideal condition, the entire conduction band is empty. Due to atom to atom covalent bonds, the entire valence band is filled with valence electrons in ideal condition. In a semiconductor crystal, the valence bands are filled with valence electrons. For same reasons as in the silicon crystal, germanium atoms in germanium crystal arrange themselves in an orderly manner. In the similar way of silicon atoms in the crystal, germanium atoms in germanium crystal make four covalent bonds with four adjacent germanium atoms. The fourth or outermost orbit of the germanium consists of 4 electrons. The first, second and third orbit of the germanium atom consists of 2, 8, 18 electrons respectively. Germanium has 32 electrons in its isolated atom. The atom to atom covalent bonds arrange the silicon atoms in the crystal in an orderly manner. In this way, each atom of a silicon crystal gets 8 electrons in its outermost orbit. As there are four electrons in the outer most orbit of a silicon atom, the silicon is a tetravalent element.Įach silicon atom in silicon crystal creates covalent bonds with four neighbouring silicon atoms. The second orbit consists of 8 electrons and the third orbit consists of 4 electrons. Silicon has total 14 electrons in its isolated atom. Silicon requires 1.1 eV for breaking any covalent bond in its crystal and germanium requires This is the main reason for using these two semiconductors most commonly. The silicon and germanium require lower energy to break their covalent bonds in the crystal. Two most commonly used semiconductors are silicon and germanium. There are many semiconductors but few of them are used for electronic circuits. This forms the crystal structure of a semiconductor. In a semiconductor, the atoms are orderly arranged due to atom to atom covalent bonds. In this way, each of them in the crystal will have eight electrons in its outermost orbit.īy forming these covalent bonds, each of the valence electrons in the crystal becomes associated with atoms, hence there will not be any free electron in the crystal in ideal condition. The third orbit consists of 18 electrons and rests 4 electrons are in fourth or outer most orbit.īut in a germanium crystal, each atom shares 4 valence electrons from four neighbouring atoms to fill its outermost orbit with eight electrons. The second orbit consists of 8 electrons. But in an isolated single germanium atom, there are 32 electrons. ![]() In germanium crystal, each atom has eight electrons in its last orbit. The figure below shows the covalent bonds formed in a germanium crystal. That means, one covalent bond is created with each of four neighbouring semiconductor atom. Copper is good conductor and glass is insulater.Įach semiconductor atom creates four covalent bonds with four neighbouring atoms in the crystal. ![]() Where the resistivity of copper is about 1.7 × 10 – 8 Ω – m in room temperature and resistivity of glass is about 9 × 10 11 Ω – m. The resistivity of a semiconductor ranges from 10 – 4 Ω – m to 0. Although a semiconductor has many physical properties, semiconductor name is given to this material because of its moderate electrical conductivity. Because of these four valence electrons, the semiconductor elements do have some special electrical characteristics and properties, which make them useful in using extensively in electronic circuit elements like diodes, transistors, SCRs, etc. The atoms of semiconductor elements have exactly four valence electrons. As the conductivity of those materials lies between good conductors and insulators, and these materials are called semiconductors. The examples of such materials are germanium, silicon, carbon etc. They have a moderate range of electrical conductivity. There are some materials, which have neither good conductivity nor bad conductivity of electricity.
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