Conductor semiconductor insulator

it is useful for physics and chemistry students

1. CONDUCTOR, SEMICONDUCTOR, INSULATOR. Presentation by S.RAVIKUMAR DEPARTMENT OF CHEMISTRY SCSVMV UNIVERSITY S.RAVIKUMAR M.Sc 1

2. ENERGY BANDS IN SOLIDS In solid materials, electron energy levels form bands of allowed energies, separated by forbidden bands . Valence band = Outermost (highest) band filled with electrons (“filled” = all states occupied). Conduction band = Next highest band to valence band (empty or partly filled). Gap = Energy difference between valence and conduction bands, = width of the forbidden band. S.RAVIKUMAR M.Sc 2

3. Points to Understanding Electrons in a completely filled band cannot move, since all states occupied (Pauli principle). Only way to move would be to “jump” into next higher band - needs energy. Electrons in partly filled band can move, since there are free states to move to. S.RAVIKUMAR M.Sc 3

4. Classification of solids into three types, according to their band structure Insulators semiconductor conductor S.RAVIKUMAR M.Sc 4

5. conductor (metal) Material capable of carrying electric current, i.e. material which has “mobile charge carriers” . (e.g. electrons, ions,..). A metal which is very good carrier of electricity is called conductor. In a conductor (metal) - The valence and conduction bands overlap, so practically the energy gap is zero. Thus, electrons need very little energy to stay in the conduction band, and conduct electricity. valence band only partially filled, or (if it is filled), the next allowed empty band overlaps with it. S.RAVIKUMAR M.Sc 5

6. conductor (metal) If the electron to become free to conduct means, it must be promoted into an empty available energy state. S.RAVIKUMAR M.Sc 6

7. conductor (metal) For metals, these empty states are adjacent to the filled states. Generally, energy supplied by an electric field is enough to stimulate electrons into an empty state. e.g. metals, liquids with ions (water, molten ionic compounds), plasma, copper and aluminium are good examples of a conductor. S.RAVIKUMAR M.Sc 7

8. insulators Materials with no or very few free charge carriers Gap = forbidden region between highest filled band (valence band) and lowest empty or partly filled band (conduction band) is very wide, about 3 to 6 eV ; In an insulator the valence band is filled with electrons, so electrons can not move within the valence band. In order to produce conduction of electricity, the electrons from the valence band must go into the conduction band. Thus, energy of more than the energy gap must be supplied to the electrons in the valence band, in order to transfer them into the conduction band. Because the energy gap in insulator is large, it prevents this change in energy by the electrons. Thus, insulators are poor conductors . e.g. quartz, most covalent and ionic solids, plastics, glass, wood, mica, paper S.RAVIKUMAR M.Sc 8

9. insulators Practically it is impossible for an electron to jump from the valence band to the conduction band. Hence such materials cannot conduct and called insulators. Such materials may conduct only at very high temperatures or if they are subjected to high voltage. Such conduction is rare and is called breakdown of an insulator. S.RAVIKUMAR M.Sc 9

10. Semiconductor A semiconductor material is one whose electrical properties lie in between those of insulators and good conductors. In terms of energy bands, semiconductors can be defined as those materials which have almost an empty conduction band and almost filled valence band with a very narrow energy gap ( 0.1eV to1 eV ). S.RAVIKUMAR M.Sc 10

11. Semiconductor The energy gap is very small , and very little energy is needed to transfer electrons from the valence band into the conduction band . Even the thermal energy at room temperature is enough. By raising the temperature, more and more electrons will be transferred to the conduction band. This process results in an increase in conductivity with increase in temperature. Examples are: germanium and silicon. S.RAVIKUMAR M.Sc 11

12. Semiconductor IN PERIODIC TABLE S.RAVIKUMAR M.Sc 12

13. SEMICONDUCTOR MATERIALS S.RAVIKUMAR M.Sc 13

14. Types of Semiconductors S.RAVIKUMAR M.Sc 14

15. Intrinsic Semiconductors An intrinsic semiconductor is one which is made of the semiconductor material in its extremely pure form. Examples of such semiconductors are: pure germanium and silicon which have forbidden energy gaps of 0.72 eV and 1.1 eV respectively. The energy gap is so small that even at ordinary room temperature; there are many electrons which possess sufficient energy to jump across the small energy gap between the valence and the conduction bands. Alternatively, an intrinsic semiconductor may be defined as one in which the number of conduction electrons is equal to the number of holes. S.RAVIKUMAR M.Sc 15

16. Intrinsic Semiconductors Electrons moving to conduction band leave “hole” (covalent bond with missing electron) behind; under influence of applied electric field, Neighboring electrons can jump into the hole, thus creating a new hole, etc. Holes can move under the influence of an applied electric field, just like electrons; both contribute to conduction. S.RAVIKUMAR M.Sc 16

17. Intrinsic Semiconductors S.RAVIKUMAR M.Sc 17

18. Extrinsic Semiconductors ( “doped semiconductor”) semiconductor with small admixture of trivalent or pentavalent atoms. Those intrinsic semiconductors to which some suitable impurity or doping agent or doping has been added in extremely small amounts (about 1 part in 108) are called “Extrinsic or Impurity semiconductors”. S.RAVIKUMAR M.Sc 18

19. TYPES OF EXTRINSIC SEMICONDUCTOR Depending on the type of doping material used, extrinsic semiconductors can be sub-divided into two classes: N-type semiconductors (donor) P-type semiconductors ( acceptor ) S.RAVIKUMAR M.Sc 19

20. N-type Extrinsic Semiconductor This type of semiconductor is obtained when a Pentavalent material like antimony ( Sb ) is added to pure silicon crystal. dopant with 5 valence electrons. 4 electrons used for covalent bonds with surrounding Si atoms. each antimony atom forms covalent bonds with the surrounding four silicon atoms with the help of four of its five electrons. The fifth electron is loosely bound to the antimony atom. it is mobile charge carrier. This electron needed only small amount of energy to lift it into conduction band (0.05 eV in Si). which improves the conduction ability to some extent. S.RAVIKUMAR M.Sc 20

21. N-type Extrinsic Semiconductor Hence, it can be easily excited from the valence band to the conduction band by the application of electric field or increase in thermal energy. The resultant material is known as an n-type semiconductor. It is seen from the above description that in N-type semiconductors, electrons are the majority carriers while holes constitute the minority carriers. has conduction electrons, no holes. e.g.of dopant with 5 valence electrons P, As, Sb S.RAVIKUMAR M.Sc 21

22. N-type Extrinsic Semiconductor S.RAVIKUMAR M.Sc 22

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24. P-type Extrinsic Semiconductor This type of semiconductor is obtained when traces of a trivalent like boron (B) are added to a pure germanium crystal. dopant with 3 valence electrons (e.g. B, Al, Ga , In) only 3 of the 4 covalent bonds filled and vacancy in the fourth covalent bond is hole. In this case, the three valence electrons of boron atom form covalent bonds with four surrounding germanium atoms but one bond is left incomplete and gives rise to a hole. hole is left free as a mobile charge carrier, which improves the conduction ability to some extent. S.RAVIKUMAR M.Sc 24

25. P-type Extrinsic Semiconductor Thus, boron which is called an acceptor impurity causes as many positive holes in a germanium crystal as there are boron atoms there by producing a P-type (P for positive) extrinsic semiconductor. In this type of semiconductor, conduction is by the movement of holes in the valence band. The resultant material is known as a p-type semiconductor. Examples for trivalent dopant B, Al, Ga , In S.RAVIKUMAR M.Sc 25

26. P-type Extrinsic Semiconductor S.RAVIKUMAR M.Sc 26

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28. P and N-type Extrinsic Semiconductor S.RAVIKUMAR M.Sc 28

29. P and N-type Extrinsic Semiconductor S.RAVIKUMAR M.Sc 29

30. p-n JUNCTION It is two terminal devices consisting of a P-N junction formed either in Ge or Si crystal. It is circuit symbol is shown in fig. (1-a). The P and N type regions are referred to as anode and cathode respectively. In fig. (1-b) arrowhead indicates the conventional direction of current flow when forward biased. It is the same direction in which hole flow takes place. S.RAVIKUMAR M.Sc 30

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32. REFERENCES http://mediatoget.blogspot.in/2011/06/classification-of -materials based-on-energy-band -theory.html . HorstWahl,QuarkNet presentation, June 2001 Semiconductors, diodes, transistors. http://engineering-electrical-equipment.org/electrical-distribution/introduction-to-semiconductors..html . S.RAVIKUMAR M.Sc 32

33. THANK YOU S.RAVIKUMAR M.Sc 33