1. Unit-2 Dr.A.L.Jerald Antony Raj, M.Sc.,M.Ed.,M.Phil(Che).,M.Phil(Edn).,Ph.D.,NET.,D.Acu Associate Professor, Pope John Paul II College of Education

2. Classification of solids based on electrical conductivity or resistivity On the basis of the relative values of electrical conductivity (σ) or resistivity ( ρ = 1/σ ), the solids are broadly classified into conductors, semiconductors, and insulators. Metals possess high conductivity or very low resistivity and insulators have low conductivity or high resistivity whereas semi-conductors have resistivity or conductivity intermediate to metals and insulators.

3. The electrical properties of solids can be indicated by their resistivity. Conductors such as gold, silver and copper have low resistance and conduct electricity easily. Insulators such as rubber, glass and ceramics have high resistance and are difficult for electricity to pass through. Semiconductors have intermediate properties between these two. SEMICONDUCTORS Semiconductors are the crystalline solids act as intermediates in electrical conductivity between the conductors and insulators. Semiconductors are neither a good conductor nor a good insulator but conduct more electricity when heat, light or voltage is added.

4. Applications of Semiconductors Semiconductors are employed in the manufacture of various kinds of electronic devices, including diodes, transistors, and integrated circuits. Such devices have found wide application due to their compactness, reliability, power efficiency and low cost. As discrete components, they have found use in power devices, optical sensors and light emitters including solid-state lasers. They have a wide range of current and voltage-handling capabilities and hence used in various microelectronic circuits.

5. TYPES OF SEMICONDUCTORS Semiconductors comprising a single element are called elemental semiconductors. The examples for elemental semiconductors are silicon (Si), germanium (Ge), tin (Sn), selenium (Se) and tellurium (Te). Semiconductors made up of two or more compounds are called compound semiconductors. Gallium arsenide (GaAs), mercury indiumtelluride (HgIn 2 Te 4 ) and gallium arsenide (Al x Ga 1 –x As) are the examples for compound semiconductors.

6. Semiconductors which exist in their purest state with no impurities are called as intrinsic semiconductors and the semiconductors that are added with impurities purposefully to change their electrical properties are called as extrinsic semiconductors. Based on the type of impurities added, extrinsic semiconductors are classified into n-type and p-type semiconductors. Impurities which act as donors (for example, phosphorus or arsenic) are added to the purest element of semiconductors to form n-type semiconductors and acceptors (for example, boron) are added to form p-type semiconductors.

7. ENERGY BAND An atom is consisting of a nucleus and the electrons revolving around the nucleus in orbitals. The electrons cannot orbit the nucleus at any distance in the atomic space surrounding the nucleus, but only certain very specific orbits are allowed and only exist in specific discrete levels. These energies are called energy levels. In a solid material, a large number of atoms gather and interact with other. Here, the energy levels become so closely spaced and they form bands. This is known as energy band. Metals, semiconductors and insulators are distinguished from each other by their band structures.

9. In the above diagram, the valence band refers to the collection of energy levels associated with the valence electrons which can jump out of the valence bond and move into the conduction band when excited. On the other hand, the valence band is simply the outermost electron orbital of an atom of any specific material that electrons occupy. The conduction band refers to the collection of energy levels of the orbitals that are high in energy and are generally empty. In reference to conductivity in semiconductors, it is the band that accepts the electrons from the valence band.

10. In solids, Fermi energy is defined as the highest occupied energy level of a material at absolute zero temperature. During the conduction process, only electrons that have an energy that is close to that of the fermi energy can be involved in the process. This concept of Fermi energy is useful for describing and comparing the behaviour of different semiconductors. For example, an n-type semiconductor will have a Fermi energy close to the conduction band, whereas a p-type semiconductor will have a Fermi energy close to the valence band.

12. The energy difference between the valence band and conduction band is known as the ‘band gap’. In semiconductors and insulators, the valance band and conduction band are separated by a forbidden energy gap (Eg) of sufficient width and the Fermi energy (E f ) is between the valence and conduction band. To get into the conduction band, the electrons have to gain enough energy to jump the band gap. Once this is done, it can conduct electricity. Forbidden energy band is defined as the amount of energy that is needed to release an electron from its valence band to its conduction band.

13. The density of electrons in conduction band at room temperature is not as high as in metals, thus cannot conduct current as good as metal. The electrical conductivity of semiconductor is not as high as metal but also not as poor as electrical insulator. That is why, this type of material is called semiconductor - means half conductor. The band gap for insulators is large so very few electrons can jump the gap. Therefore, current does not flow easily in insulators.

14. Effect of Temperature In semiconductors, the band gap is smaller at room temperature and enough thermal energy will allow the electrons to jump the gap fairly and make the transitions in conduction band will give the semiconductor a limited conductivity. At low temperature, no electron possesses sufficient energy to occupy the conduction band and thus no movement of charge is possible. At absolute zero temperature, semiconductors are perfect insulators. The difference between insulators and semiconductors is the size of the band gap energy.

15. In insulator, the forbidden gap is very large and as a result the energy required by the electron to cross over to the conduction band is practically large enough. That means the electrical conductivity of insulator is very poor. Conductivity of n-type and p-type Semiconductors In semiconductors, the band gap is smaller at room temperature and enough thermal energy will allow the electrons to jump the gap fairly and make the transitions in conduction band will give the semiconductor a limited conductivity. On the other hand addition of impurities with pure elements of semiconductors will promote the number of free electrons which in turn jump the band gap from valence band to conduction band and conduct electricity.

16. The addition of impurities converts them into n-type and p-type semiconductors. Impurities which act as donors (for example, phosphorus or arsenic) are added to the pure element of semiconductors to form n-type semiconductors. The acceptors (for example, boron) are added to form p-type semiconductors. Impurity atoms with 5 valence electrons produce n- type semiconductors by contributing extra electrons Impurity atoms with 3 valence electrons produce p- type semiconductors by producing a "hole" or electron deficiency.

17. For example, semiconductor crystal used for Integrated Circuit (IC) is made up of single crystal silicon of high purity, but when actually making a circuit, impurities are added to control the electrical properties. Depending on the added impurities, they become as n-type and p-type semiconductors.

18. The addition of pentavalent impurities such as antimony, arsenic or phosphorus contributes free electrons and increases the conductivity of the intrinsic semiconductor. These added impurities are called donors and this type is known as ‘n-type’ semiconductors. In this type, the energy level of the donor is located closer to the conduction band, i.e., the energy gap is small. Then, electrons at this energy level are easily excited to the conduction band and contribute to the conductivity.

19. The addition of trivalent impurities such as boron, aluminium or gallium to an intrinsic semiconductor creates deficiencies of valence electrons or "holes". These added impurities are called acceptors and this type is known as ‘p-type’ semiconductors. The energy level of the acceptor is closer to the valence band. Since there are no electrons here, electrons in the valence band are excited here. As a result, holes are formed in the valence band, which contributes to the conductivity.

20. Sl.No Intrinsic SemiconductorsExtrinsic Semiconductors 1. It is a pure semi-conducting material and no impurity atoms are added to it. It is prepared by doping a small quantity of impurity atoms to the pure semi- conducting material. 2. Examples: crystalline forms of pure silicon and germanium. Examples: silicon “Si” and germanium, “Ge” crystals with impurity atoms of As, Sb, P etc. or In B, Al etc. 3. The number of free electrons in the conduction band and the number of holes in valence band is exactly equal and very small indeed. The number of free electrons and holes is never equal. There is an excess of electrons in n-type semi-conductors and excess of holes in p-type semi- conductors. 4. Fermi energy level lies at the center of forbidden gap. In n-type, Fermi energy level lies at the bottom of conduction band where as in p- type, Fermi energy level at the top of valence band. 5. Its electrical conductivity is low.Its electrical conductivity is high. 6. Its electrical conductivity is a function of temperature alone. Its electrical conductivity depends upon the temperature as well as on the quantity of impurity atoms doped the structure.

21. Evaluation 1. How do we classify the solids based on their electrical conductivity or resistivity? 2. Compare the conductivity and resistivity of metals, insulators and semi-conductors. 3. Give few examples for metals, insulators and semi- conductors. 4. What happens to resistivity if the temperature is increased? 5. Give the resistivity range of conductors, insulators and semi-conductors. 6. What are the symbols used to represent the electrical conductivity and resistivity? How are related?

22. 7. What are called as Semi-conductors? 8. How do we classify semi-conductors based on the number of elements present in them? 9. Differentiate between energy levels and energy band. 10. Define the term ‘Fermi Energy’. 11. Which electrons would involve in the conduction of electricity with reference to Fermi energy? 12. Differentiate between valence band and conduction band. 13. How does Fermi energy help to compare semiconductors? 14. Why do metals have high electrical conductivity?

23. 15. What is meant by ‘Band gap’? 16. How do electrons overcome the band gap to conduct electricity? 17. What is meant by ‘forbidden energy band’? 18. When do semiconductors act as insulators and conductors? 19. Why do insulators not carry electricity? 20. What are intrinsic and extrinsic semiconductors? 21. How are ‘n-type’ and ‘p-type’ semiconductors formed?