SEMI CONDUCTOR MATERIAL

1. SEMI CONDUCTOR MATERIAL PRESENTATION BY: Er. Satnam Singh Lecturer(Electrical engg.) GPC Mohali (Khunimajra)

2. Contents • Introduction of different electrical materials • Atomic theory • Energy band • Classification of material on the basis of energy band structure • Crystal structure of semiconductor • Semiconductor material Germanium Silicon • Intrinsic semiconductor • Extrinsic semiconductor N- type semiconductor P-type semiconductor

3. Classification of Electrical and Electronics Engineering Material The electrical and electronics material can be classified into following types: • Conducting materials • Semi conducting material • Insulating material • Magnetic material

4. Conducting material • The conducting material are those material in which the electric current can easily flow. In other word, we can say that materialls which offer very low resistance to electric current is called conducting material. • The resistivity of these materials is very low, its values lies between 10-8 -10-6 Ω-m. • The most well known conducting materials are copper, aluminum, brass bronze etc.

5. Copper Atom Conductor Atomic Structure • The atomic structure of good conductors usually includes only one electron in their outer shell. • It is called a valence electron. • It is easily striped from the atom, producing current flow.

6. Insulating material • The insulating material are those material which do not allow the passage of electric current or we can say that material which offer very high resistance to electric current is called insulating materials. • The resistivity of these materials is very high, its values lies between 1012 -1018 Ω-m. • The most well known insulating material are rubber, wood, glass ceramic, mica etc • Most insulators are compounds of several elements. • The atoms are tightly bound to one another so electrons are difficult to strip away for current flow.

7. Semi-conducting material • Thesemi-conducting material are those material whose resistivity is less than insulator but more than conductor. In other word we can say that conductivity of these material is less than conductor but more than insulator. • The resistivity of these materials is medium, its values lies between 100-102Ω-m. • The most well known insulating material are silicon, germanium, gallium etc.

8. Semiconductor Valence Orbit • The main characteristic of a semiconductor element is that it has four electrons in its outer or valence orbit.

9. Magneticmaterial • The magnetic material are those materials which can be magnetized and which are attracted towards the magnet. • Such material create a magnetic field on the surrounding space • Iron nickel and cobalt are most well known used magnetic materials.

10. Semi - conductor • Semiconductor material are those which possess the conductivity higher than insulators but lesser than conductor and are used for manufacturing most of the active components.

11. Atomic Theory • The atomic theory state that all the matter weather solid, liquid or gases consist of minute particles called molecules which can be further divided into atoms • The atom is defined as tiny discrete particles which is incapable of division. • Those substance whose molecule consist of similar atoms are called elements and the material whose molecules consist of dissimilar atoms are called compounds

12. Bohr’s Atomic Theory • Bohr’s theory tell us that the atom of an element consist of two main parts • Nucleus • Extra nucleus • The central part of an atom is known as nucleus. It contain proton and neutrons. The nucleus hold the entire mass of an atom. • The proton are the positive charge particles, while neutron are chargeless particles. Therefore the net charge on nucleus remain positive. • The outermost part of atom arround the nucleus is known as extra-nucleus. It contain electron only. As the number of electron is equal to no. of protons, so an atom is therefore electrically neutral. • The no. of electrons in any orbit is given by 2n2,where n is the number orbit from nucleus. • E.g. 1st orbit has 2(1)2 = 2 electron as n = 1 • 2nd orbit has 2(2)2 = 8 electron as n = 2 • 3rd orbit has 2(3)2 = 18 electrons as n = 3 • 4th orbit has 8 electron

13. Bohr’s Atomic Theory • Bohr’s atomic theory can be summarized as follow: • The atomic no. of an atom is the no. of proton present in the nucleus • The algebraic sum of proton and neutron of an atom is called the atomic weight or mass of the atom. • The number of proton inside the nucleus is equal to the number of electron outside of it. • The electron on various energy level are arranged in definite shells named as K,L,M,N…………… • The outermost orbit of an atom can contain maximum 8 electron

14. Classification of material on the basis of atomic structure • Conducting material : On the basis of atomic structure conductor is defined as a substance, whose atom has its outermost orbit incomplete. Such a substance act as good conductors of an electricity e.g. aluminum or copper

15. Classification of material on the basis of atomic structure • Copper : It is denoted by Cu having atomic no. = 29 and atomic weight = 64 • no. of electrons = no. of protons • no. of electron = atomic no. • Hence no. of electron in Cu =29 • no. of neutrons = atomic weight - atomic no. • Therefore no. of neutron = 64 – 29 = 35 • no. of electron in K shell = 2(1)2 = 2 (n=1) • L shell = 2(2)2 = 8 (n=2) • M shell = 2(3)2 = 18 (n=3) • N shell = 1 • So there is one electron valence on it, it is called free electron (or valence electron) • Valence electron of an atom are defined as the electron which are loosely attached to the nucleus at an atom and can be easily detached

16. Classification of material on the basis of atomic structure • Aluminium (Al) : It is denoted by Al having atomic no. = 13 and atomic weight 27 • no. of electrons = no. of protons • no. of electron = atomic no. • Hence no. of electron in Al =13 • no. of neutrons = atomic weight - atomic no. • Therefore no. of neutron = 27 – 13 = 14 • no. of electron in K shell = 2(1)2 = 2 (n=1) • L shell = 2(2)2 = 8 (n=2) • M shell = 3 (n=3) • So there is 3 electron valence on it, it is called free electron (or valence electron) • Valence electron of an atom are defined as the electron which are loosely attached to the nucleus at an atom and can be easily detached

17. Classification of material on the basis of atomic structure • Semiconducting material : In semiconducting materials, if a potential difference is applied across the end of the material, a partial flow of electron takes place i.e. some of electron can be detached from the atom and some of them cannot be detached, hence the conduction is partial and therefore the properties of semiconductor are in between those of conductor and insulator e.g. silicon and germanium.

18. Classification of material on the basis of atomic structure • Silicon(Si) : it is denoted by Si having atomic no. = 14 And atomic weight = 28 • no. of electrons = no. of protons • no. of electron = atomic no. • Hence no. of electron in Si =14 • no. of neutrons = atomic weight - atomic no. • Therefore no. of neutron = 28-14 = 14 • no. of electron in K shell = 2(1)2 = 2 (n=1) • L shell = 2(2)2 = 8 (n=2) • M shell = 4 • So there is 4 electron valence in its outermost orbit

19. Classification of material on the basis of atomic structure • Germanium (Ge) : it is denoted by Ge having atomic no. = 32 And atomic weight = 73 • no. of electrons = no. of protons • no. of electron = atomic no. • Hence no. of electron in Si =32 • no. of neutrons = atomic weight - atomic no. • Therefore no. of neutron = 73-32 = 41 • no. of electron in K shell = 2(1)2 = 2 (n=1) • L shell = 2(2)2 = 8 (n=2) • M shell = 2(3)2 = 18 (n=3) • N shell = 4 • So there is 4 electron valence in its outermost orbit

20. Classification of material on the basis of atomic structure • Insulator : • In insulating material the electron are firmly held to their atom and hence if a potential difference is applied then a little or no electron flow occurs • There fore no electron can be detached from outermost orbit so it is not easy to pass the electric current through them. e.g. neon • Neon (Ne) atomic no. 10 • no. of electron = atomic no. =10 • no. of electron in K shell =2(1)2 = 2 n=1 • L shell =2(2)2 = 8 n=2 • It outer most orbit does not contain any free electron as its outer most orbit is completely filled.

21. Energy band theory • In an atom, the electron revolving in different orbits possesses certain energy level. • The amount of energy on each orbit is fixed. but when we talks about solids, where the atoms are closely packed the electron on various orbits are influenced by energy of electrons of neighboring orbits. In that case, energies of various bands are considered in a single band. • So the range of energy possessed by various electrons of the same orbit of different atoms in a solid is known as energy band.

22. Energy band theory • Following are the important types of energy bands: • Valence band : The range of energy possessed by valence electron (electron present in outer most orbit) is called valence band. This band may be completely or partially filled by valence electrons. • Conduction band : The range of energy possesed by free electron (electrons responsible for conduction) is called conduction band. The conduction in solid is possible only when some electron are present in conduction band. • Forbidden energy gap: The energy gap existing between a conduction band and valence band is known as forbidden energy gap. There is no electron in this region. The electrons can only jump to conduction band when they receive extra energy from external source.

23. Classification of material on the basis of energy band • Insulators: Insulator are those material in which we cannot pass the electric current easily. In insulator, conduction band remain empty, while valence band has many electrons. • In this case forbidden energy gap between valence band and conduction band is quite large approx 20eV) so it is not easy to conduct electric current through them.

24. Classification of material on the basis of energy band • Semi conductor : Semiconductor material are those material whose resistivity lies between conductor and insulator. In semiconductor valence band is full of electron while conduction band is empty. But in this case, energy gap is very small (1 eV) with the application of small potential difference electron can jump from valence band to conduction band.

25. Classification of material on the basis of energy band • Conductor : Conductor are those material which offer least resistance to electric current. Here conduction band is overlapped with valence band. So large no. of electrons can jump from valence band to conduction band with the application of small potential difference

26. Semiconductor material • Semiconductor material are those which possess the conductivity higher than insulators but lesser than conductor and are used for manufacturing most of the active components.

27. Semiconductor materials • Silicon and germanium each have 4 electrons in their outer orbital. To fill the outer most shell each atom acquire four more electron by sharing one electron each from the four adjacent atoms and hence form the crystal. • In a crystal the atom are bonded together in cohesive manner. • In the fig, the core represent the nucleus and all other electron as valence electrons. The valence electron take part in forming covalent bonds with four neighbouring atoms. • At absolute zero all four covalent bond are intact and no electron is free to conduct. A chip, an LED and a transistor are all made from semiconductor material.

28. Germanium • Germanium (Ge) is an earth element pure germanium is obtained from coal ash in the form of germanium dioxide, which is then reduced to pure germanium.

29. Germanium Atomic structure : • The atomic no. Ge is 32. therefore it has 32 protons in its nucleus along with 32 neutrons and 32 electron which revolves around the necleus in fixed paths known as orbits • These 32 electrons can be distributed in various orbits or shells according to the rule 2n2 where n is orbit no. except that there are four electrons in the outermost orbit. • This outermost orbit is also known as valence orbit. The germanium atom has two electron in its first orbit four electron in second orbit eighteen electron in its third orbit, and only four electron in its valence shell.

30. Germanium • Crystal structure : A substance in which the atoms or molecules are arranged in an orderly pattern is known as a crystal • All semiconductor have crystalline structure. Each atom is surrounded by a neighbouring atoms in a repetitive manner. Therefore in a germanium crystal a large number of atoms are bonded together • This bonding is due to the fact that each atom has tendency to complete its outermost or valence orbit by acquiring eight electron in it. • However in most of the substance the last orbit is incomplete i.e. last orbit does not have 8 electrons. • This makes the atom active to bargain with the other atoms to acquire 8 electron in the last orbit. To do so the atom may lose gain or share valence electron with other atoms.

31. Germanium • In semiconductor bonds are formed by sharing the valence electrons of an atom with the neighboring atoms. This type of bonding in which which bonds are formed by sharing the valence electrons are called covalent bonds. • The various germanium atom are held together through covalent bond to form a germanium crystal as shown in fig • The covalent bond in between the various Ge atoms are formed by sharing their valence electron with the neighboring atoms. • Since germanium is tetravalent element each atom forms four covalent bonds with the four neighboring atoms.

32. Silicon Atomic structure : • The atomic no. Si is 14. therefore it has 14 protons in its nucleus along with 14 neutrons and 14 electron which revolves around the nucleus in fixed paths known as orbits • The no. of electrons in first, second and third orbit are 2,8 and 4 respectively as shown in fig. • The silicon has 4 electrons in its valence shell so it is also known as tetravalent.

33. Silicon • Crystal structure : Like germanium silicon atoms are also arranged in an orderly manner to form a silicon crystal. • The crystal structure of Si is shown in fig it is quite similar to that of germanium crystal.

34. Concept of electron and hole • At absolute zero, the valence band in the intrinsic semiconductors is totally filled and the conduction band is empty. • Since there are four covalent bond each bond is not so strong. When the temperature is increased the electron get sufficient energy to make a valence electron to move away from the influence of its nucleus.when this happen the electron becomes free to conduct and hence appears in the conduction band. • When an electron moves away to the conduction band, a vacancy is created in the valence band. This vacancy is called hole. Whenever a free electron is generated a hole is created simultaneously • Fig shows the generation of electron hole pair in semiconductor. The amount of energy required to break a covalent bond in germanium is 0.72eV and that in silicon is 1.12eV. • When such energy is supplied, one of the covalent bond is broken and the electron appears in conduction band. This result in generation of holes. • The hole does remain stable and is quickly filled by the electron from the adjacent covalent bond. The hole is generated there. • This way electron and hole moves in opposite direction

35. Intrinsic semiconductor • An extremely pure semiconductor is called intrinsic semiconductor

36. INTRINSIC SEMICONDUCTOR At zero Kelvin all of the four valence electrons of each atom in the silicon crystal form part of the covalent bond with the four neighboring atoms. The valence band is completely full and the conduction band completely empty. The semiconductor behaves as a perfect insulator because there are no conducting electrons present.

37. INTRINSIC SEMICONDUCTOR At temperatures above zero Kelvin some of the valence electrons are able to break free from their bonds to become free conduction electrons. The vacancy that is left behind is referred to as a hole. This hole is treated as a positive carrier of charge. Conduction due solely to thermally generated electron-hole pairs is referred to as intrinsic conduction.

38. Extrinsic semiconductor • An doped semiconductor is called extrinsic semiconductor. • The process by which the an impurity is added to a semiconductor is known as doping. • A semiconductor to which an impurity at controlled rate is added to make it conductive is known as extrinsic semiconductor.

39. Doping Silicon to Create n-Type and p-Type Silicon • The process of doping, creates p-type and n-type silicon. • Materials introduce an atom of another element into the silicon crystal to alter the electrical properties. • The "dopant” has either 3 or 5 valence electrons; silicon has 4. • Substituting a phosphorus atom with 5 valence electrons for a silicon atom in a silicon crystal leaves an extra, unbonded electron that is relatively free to move around the crystal. • Substituting a boron atom with 3 valence electrons for a silicon atom in a silicon crystal leaves a hole (a bond missing an electron) that is relatively free to move around the crystal.

40. When a dopant atom with a valence of less than four is substituted into the silicon structure, a hole is created in the structure and an acceptor energy level is created just above the valence band. Little energy is required to excite the holes into motion.

41. EXTRINSIC CONDUCTION A pure or intrinsic conductor has thermally generated holes and electrons. However these are relatively few in number. An enormous increase in the number of charge carriers can by achieved by introducing impurities into the semiconductor in a controlled manner. The result is the formation of an extrinsic semiconductor. This process is referred to as doping. There are basically two types of impurities: donor impurities and acceptor impurities. Donor impurities are made up of atoms (arsenic for example) which have five valence electrons. Acceptor impurities are made up of atoms (gallium for example) which have three valence electrons.

42. N-TYPE EXTRINSIC SEMICONDUCTOR Arsenic has 5 valence electrons, however, only 4 of them form part of covalent bonds. The 5th electron is then free to take part in conduction. The electrons are said to be the majority carriers and the holes are said to be the minority carriers.

43. P-TYPE EXTRINSIC SEMICONDUCTOR Gallium has 3 valence electrons, however, there are 4 covalent bonds to fill. The 4th bond therefore remains vacant producing a hole. The holes are said to be the majority carriers and the electrons are said to be the minority carriers.

44. Revision • Atomic Theory • Energy band • Classification of material on the basis of energy band structure • Semiconductor material • Crystal structure of semiconductor Germanium Silicon • Intrinsic semiconductor • Extrinsic semiconductor N- type semiconductor P-type semiconductor