1. Dr. N.M. Safri/SEU3003_SemiconductorMaterial1 SEU 3003 ELEKTRONIK (ELECTRONICS) Chapter 1 SEMICONDUCTORS MATERIALS Dr. Norlaili Mat Safri

2. Dr. N.M. Safri/SEU3003_SemiconductorMaterial2 In this chapter, we will learn: 1.Atomic structure 2.Energy band 3.Materials classification 4.Covalent bondsCovalent bonds 5.Conduction in semiconductorsConduction in semiconductors 6.Free electron and hole as carrier of currentFree electron and hole as carrier of current 7.Doping of semiconductor materialsDoping of semiconductor materials 8.The p-n junctionThe p-n junction 9.Biasing the p-n junction

3. Dr. N.M. Safri/SEU3003_SemiconductorMaterial3 Atomic Structure All matter is made of atoms. All atoms consist of electrons, proton, and neurons. An atom is the smallest particle of an element that retains the characteristics of that element. Each of the known 109 elements has atoms that are different from the atoms of all other elements. This gives each element a unique atomic structure. According to the classical Bohr model, atoms have a planetary type of structure that consists of a central nucleus surrounded by orbiting electrons.

4. Dr. N.M. Safri/SEU3003_SemiconductorMaterial4 Activity According to the classical Bohr model, atoms have a planetary type of structure that consists of a central nucleus surrounded by orbiting electrons. Draw the atomic structure according to Bohr model

5. Dr. N.M. Safri/SEU3003_SemiconductorMaterial5 Atomic Structure (The Bohr Model) ElectronProton - - - + + + + + + - - - - + Neutron The nucleus consists of positively charged particles called protons and uncharged particles called neutrons. The basic particles of negative charge are called electrons.

6. Dr. N.M. Safri/SEU3003_SemiconductorMaterial6 Atomic Number Each type of atom has a certain number of electrons and protons that distinguishes it from the atoms of all other elemens. All elements are arranged in the periodic table of the elements in order according to their atomic number. The atomic number = the number of protons in the nucleus, which is the same number of electrons in an electrically balanced (neutral) atom. Eg. Atomic number of hydrogen = 1; helium = 2. + Electron Nucleus Hydrogen atom + Electron Nucleus Helium atom - - - + Electron

7. Dr. N.M. Safri/SEU3003_SemiconductorMaterial7 Energy band Electrons orbit the nucleus of an atom at certain distances from the nucleus. Electrons near the nucleus have less energy than those in more distant orbits. It is known that only discrete values of electron energies exist within atomic structures. Therefore, electrons must orbit only at discrete distances from the nucleus. Each discrete distance (orbit) from the nucleus corresponds to a certain energy level.

8. Dr. N.M. Safri/SEU3003_SemiconductorMaterial8 Energy band In an atom, the orbits are grouped into energy bands known as shells. A given atom has a fixed number of shells. Each shell has a fixed maximum number of electrons at permissible energy levels (orbits). The differences in energy levels within a shell are much smaller than the difference in energy between shells. The shells are designated 1,2,3,… with 1 being closest to the nucleus. Some references designate shells by the letters K,L,M,….

9. Dr. N.M. Safri/SEU3003_SemiconductorMaterial9 Energy band - - - - - - + + + + + + Nucleus Shell 1 (K) Shell 2 (L) Energy increases as the distance from the nucleus increases Energy Right figure shows the 1 st shell with one energy level and the 2 nd shell with two energy levels. Additional shells may exist in other types of atoms, depending on the element.

10. Dr. N.M. Safri/SEU3003_SemiconductorMaterial10 Energy band (Valence Electrons) Electrons that are in orbits farther from the nucleus have higher energy and are less tightly bound to the atom than those close to the nucleus. This is because the force of attraction between the positively charged nucleus and the negatively charged electron decreases with increasing distance from the nucleus. Electrons with the highest energy exist in the outermost shell of an atom and are relatively loosely bound to the atom. This outermost shell is known as the valence shell and electrons in this shell are called valence electrons. These valence electrons contribute to chemical reactions and bonding within the structure of a material and determine its electrical properties.

11. Dr. N.M. Safri/SEU3003_SemiconductorMaterial11 Energy band (The number of electrons in each shell) The maximum number of electrons (N e ) that can exist in each shell of an atom is a fact of nature and can be calculated by the formula, N e = 2n 2 where n is the number of the shell. The innermost shell is number 1, the next shell is number 2, and so on. The maximum number of electrons that can exist in the innermost shell (shell 1) is N e = 2n 2 = 2(1) 2 = 2

12. Dr. N.M. Safri/SEU3003_SemiconductorMaterial12 Activity Calculate the maximum number of electrons that can exist in the 2 nd shell, 3 rd shell and 4 th shell

13. Dr. N.M. Safri/SEU3003_SemiconductorMaterial13 Energy band (The number of electrons in each shell) All shells in a given atom must be completely filled with electrons except the outer (valence shell). Eg. Silicon has 14 electrons. Therefore, it has ___ electrons in the 1 st shell, ___ electrons in the 2 nd shell and ___ electrons in the 3 rd shell. Silicon also has ___ valence electrons.

14. Dr. N.M. Safri/SEU3003_SemiconductorMaterial14 MATERIALS CLASSIFICATION In terms of their electrical properties, materials can be classified into three groups: conductors, semiconductors, and insulators. The ability of a material to conduct current is based on its atomic structure. The orbit paths of the electrons surrounding the nucleus are called shells. Each shell has a defined number of electrons it will hold; determined by the formula, 2n 2. The outer shell is called the valence shell. The less complete a shell is filled to capacity the more conductive the material is.

15. Dr. N.M. Safri/SEU3003_SemiconductorMaterial15 Activity The less complete a shell is filled to capacity the more conductive the material is. Which means that it is the valence shell that determines the ability of material to conduct current. Draw the atomic structure of Silicon (14), Copper (29), and Argon (18) and determine the most and less conductive of the three materials.

16. Dr. N.M. Safri/SEU3003_SemiconductorMaterial16 MATERIALS CLASSIFICATION - ---- - +14 - - - - - - - - - - - - - ---- - +29 - - - - - - - - - - - - - - - - - - - - ---- - +18 - - - - - - - - - - - - SiliconCooperArgon Silicon, Cooper, and Argon have ____, ____, and ____ valence electron(s), respectively.

17. Dr. N.M. Safri/SEU3003_SemiconductorMaterial17 MATERIALS CLASSIFICATION ConductorsSemiconductorsInsulators Neon (10) Argon (18) Krypton (36) Xenon (54) Carbon (6) Silicon (14) Germanium (32) Cooper (29) Silver (47) Gold (79) Aluminium (13) Conductors, semiconductors, and insulators have ____, ____, and ____ valence electron(s), respectively.

18. Dr. N.M. Safri/SEU3003_SemiconductorMaterial18 MATERIALS CLASSIFICATION Energy band for each type of material Valence band Conduction band Valence band Conduction band Valence band Energy (eV) 000 Overlap Energy gap Energy (eV) ConductorsSemiconductorsInsulators

19. Dr. N.M. Safri/SEU3003_SemiconductorMaterial19 Activity Silicon and Germanium are semiconductive materials. However, Silicon is the most widely used in material in diodes, transistors, integrated circuits and other semiconductive devices. Use the valence shell and energy level to explain the reason behind the usage of Silicon rather than Germanium as semiconductive material in electronic devices.

20. Dr. N.M. Safri/SEU3003_SemiconductorMaterial20 COVALENT BONDS When atoms combine to form a solid, crystalline material, they arrange themselves in a symmetrical pattern. The atoms within the crystal structure are held together by covalent bonds, which are created by the interaction of the valence electrons of the atoms. Covalent bonding is a bonding of two or more atoms by the interaction (sharing) of their valence electrons.

21. Dr. N.M. Safri/SEU3003_SemiconductorMaterial21 COVALENT BONDS Shown in this figure is how each silicon atom positions itself with four adjacent silicon atoms to form a silicon crystal. A silicon (Si) atom with its four valence electrons shares an electron with each of its four neighbors. This effectively creates eight shared valence electrons for each atom and produces a state of chemical stability. Also, this sharing of valence electrons produces the covalent bonds that hold the atoms together; each valence electron is attracted by the two adjacent atoms which share it.

22. Dr. N.M. Safri/SEU3003_SemiconductorMaterial22 Activity Based from statements stated in the previous slide, illustrates the covalent bonds in silicon.

23. Dr. N.M. Safri/SEU3003_SemiconductorMaterial23 COVALENT BONDS

24. Dr. N.M. Safri/SEU3003_SemiconductorMaterial24 COVALENT BONDS Certain atoms will combine in this way to form a crystal structure. Silicon atoms combine in this way in their intrinsic or pure state. An intrinsic crystal is one that has no impurities (100% pure material). Covalent bonding for germanium is similar because it also has four valence electrons.

25. Dr. N.M. Safri/SEU3003_SemiconductorMaterial25 CONDUCTION IN SEMICONDUCTORS The way a material conducts electrical current is important in understanding how electronic devices operate. You can’t really understand the operation of a device such as diode or transistor without knowing something about the basic current mechanisms. As you have learned, the electrons of an atom can exist only within prescribed energy bands. Each shell around the nucleus corresponds to a certain energy band and is separated from adjacent shells by energy gaps, in which no electrons can exist.

26. Dr. N.M. Safri/SEU3003_SemiconductorMaterial26 CONDUCTION IN SEMICONDUCTORS Below figure shows the energy band diagram for an unexcited (no external energy such as heat) atom in a pure silicon crystal. This condition occurs only at a temperature of absolute 0 Kelvin. Nucleus 0 Energy gap Energy (eV) Fig: Energy band diagram for an unexcited atom in a pure (intrinsic) silicon crystal. There are no electrons in the conduction band. + + + + + + 1 st band (shell 1) 2 nd band (shell 2) Valence band Conduction band Energy gap Energy gap = 1.1 eV - - - - - - - - - - - - - -

27. Dr. N.M. Safri/SEU3003_SemiconductorMaterial27 CONDUCTION IN SEMICONDUCTORS An intrinsic (pure) silicon crystal at room temperature free electrons An intrinsic (pure) silicon crystal at room temperature has sufficient heat (thermal) energy for some valence electrons to jump the gap from the valence band into the conduction band, becoming free electrons. Free electronsconduction electrons Free electrons are also called conduction electrons. (Conduction Electrons and Holes) When an electron jumps to the conduction band, a vacancy is left in the valence band within the crystal. a hole This vacancy is called a hole. electron-hole pair For every electron raised to the conduction band by external energy, there is one hole left in the valence band, creating what is called an electron-hole pair. Recombination occurs when a conduction band electron loses energy and falls back into a hole in the valence band.

28. Dr. N.M. Safri/SEU3003_SemiconductorMaterial28 Activity Based from statements stated in the previous slide, illustrates the free electron, hole and electron-hole pair in an energy diagram of a silicon crystal when the silicon crystal receives heat energy.

29. Dr. N.M. Safri/SEU3003_SemiconductorMaterial29 CONDUCTION IN SEMICONDUCTORS Effect of Temperature (Conduction Electrons and Holes) Nucleus 0 Energy gap Energy (eV) + + + + + + 1 st band (shell 1) 2 nd band (shell 2) Valence band Conduction band Energy gap - - - - - - - - - - - - - - + + + + + + - - - - - - - - - - - - - - Free electron Heat energy Hole Electron-hole pair

30. Dr. N.M. Safri/SEU3003_SemiconductorMaterial30 CONDUCTION IN SEMICONDUCTORS At 0K, no bonds are broken. Si is an insulator. Si is an insulator. As temperature increases, a bond can break, releasing a valence electron and leaving a broken bond (hole). Current can flow. Effect of Temperature (bonding diagram) (Conduction Electrons and Holes) Si - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Free electron Heat energy Hole

31. Dr. N.M. Safri/SEU3003_SemiconductorMaterial31 CONDUCTION IN SEMICONDUCTORS (Conduction Electrons and Holes) To summarize, a piece of intrinsic silicon at room temperature has, at any instant, a number of conduction band (free) electrons that are unattached to any atom and are essentially drifting randomly throughout the material. There is also an equal number of holes in the valence band created when these electrons jump into the conduction band.

32. Dr. N.M. Safri/SEU3003_SemiconductorMaterial32 CONDUCTION ELECTRONS AND HOLES Fig: Electron-hole pairs in a silicon crystal. Free electrons are being generated continuously while some recombine with holes. Si - - - - - - - - - Generation of an electron-hole pair Heat energy Si - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Recombination of an electron with a hole - Outline

33. Dr. N.M. Safri/SEU3003_SemiconductorMaterial33 ELECTRON AND HOLE CURRENT When a voltage is applied across a piece of intrinsic silicon, the thermally generated free electrons in the conduction band, which are free to move randomly in the crystal structure, are now easily attracted toward the positive end. This movement of free electrons is one type of current in a semiconductive material and is called electron current. Activity Based from above statements, illustrates the movement of free electrons in figure shown in the next slide.

34. Dr. N.M. Safri/SEU3003_SemiconductorMaterial34 Si - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Fig: Electron current in intrinsic silicon is produced by the movement of thermally generated free electrons. ＋ － V Activity

35. Dr. N.M. Safri/SEU3003_SemiconductorMaterial35 Si - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Fig: Electron current in intrinsic silicon is produced by the movement of thermally generated free electrons. ＋ － V Activity - - - - - - - -

36. Dr. N.M. Safri/SEU3003_SemiconductorMaterial36 ELECTRON AND HOLE CURRENT Another type of current occurs in the valence band, where the holes created by the free electrons exist. Electrons remaining in the valence band are still attached to their atoms and are not free to move randomly in the crystal structure as are the free electrons. However, a valence electron can move into a nearby hole with little change in its energy level, thus leaving another hole where it came from. Effectively, the hole has moved from one place to another place in the crystal structure. This is called hole current.

37. Dr. N.M. Safri/SEU3003_SemiconductorMaterial37 Fig: Hole current in intrinsic silicon crystal. Si - - - - - - - - - Heat energy Si - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Outline ELECTRON AND HOLE CURRENT 1

38. Dr. N.M. Safri/SEU3003_SemiconductorMaterial38 Fig: Hole current in intrinsic silicon crystal. Si - - - - - - - - - Heat energy Si - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Outline ELECTRON AND HOLE CURRENT - 2

39. Dr. N.M. Safri/SEU3003_SemiconductorMaterial39 Fig: Hole current in intrinsic silicon crystal. Si - - - - - - - - - Heat energy Si - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Outline ELECTRON AND HOLE CURRENT - 3

40. Dr. N.M. Safri/SEU3003_SemiconductorMaterial40 Fig: Hole current in intrinsic silicon crystal. Si - - - - - - - - - Heat energy Si - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ELECTRON AND HOLE CURRENT - - 4

41. Dr. N.M. Safri/SEU3003_SemiconductorMaterial41 Fig: Hole current in intrinsic silicon crystal. ELECTRON AND HOLE CURRENT Si - - - - - - - - - - - - - --- - - - - 1. A free electron leaves hole in a valence shell.

42. Dr. N.M. Safri/SEU3003_SemiconductorMaterial42 Fig: Hole current in intrinsic silicon crystal. ELECTRON AND HOLE CURRENT Si - - - - - - - - - - - - - -- - - - - 2. A valence electron moves into 1 st hole and leaves a 2 nd hole 1. A free electron leaves hole in a valence shell. -

43. Dr. N.M. Safri/SEU3003_SemiconductorMaterial43 Fig: Hole current in intrinsic silicon crystal. ELECTRON AND HOLE CURRENT Si - - - - - - - - - - - - -- - - - - 2. A valence electron moves into 1 st hole and leaves a 2 nd hole 1. A free electron leaves hole in a valence shell. - 3. A valence electron moves into 2 nd hole and leaves a 3 rd hole -

44. Dr. N.M. Safri/SEU3003_SemiconductorMaterial44 Fig: Hole current in intrinsic silicon crystal. ELECTRON AND HOLE CURRENT Si - - - - - - - - - - - - - - - - - 2. A valence electron moves into 1 st hole and leaves a 2 nd hole 1. A free electron leaves hole in a valence shell. - 3. A valence electron moves into 2 nd hole and leaves a 3 rd hole - - 4. A valence electron moves into 3 nd hole and leaves a 4 rd hole

45. Dr. N.M. Safri/SEU3003_SemiconductorMaterial45 Fig: Hole current in intrinsic silicon crystal. ELECTRON AND HOLE CURRENT Si - - - - - - - - - - - - - - - - 2. A valence electron moves into 1 st hole and leaves a 2 nd hole 1. A free electron leaves hole in a valence shell. - 3. A valence electron moves into 2 nd hole and leaves a 3 rd hole - - 4. A valence electron moves into 3 rd hole and leaves a 4 th hole - 5. A valence electron moves into 4 th hole and leaves a 5 th hole

46. Dr. N.M. Safri/SEU3003_SemiconductorMaterial46 Fig: Hole current in intrinsic silicon crystal. ELECTRON AND HOLE CURRENT Si - - - - - - - - - - - - - - - 2. A valence electron moves into 1 st hole and leaves a 2 nd hole 1. A free electron leaves hole in a valence shell. - 3. A valence electron moves into 2 nd hole and leaves a 3 rd hole - - 4. A valence electron moves into 3 rd hole and leaves a 4 th hole - 5. A valence electron moves into 4 th hole and leaves a 5 th hole - 6. A valence electron moves into 5 th hole and leaves a 6 th hole

47. Dr. N.M. Safri/SEU3003_SemiconductorMaterial47 ELECTRON AND HOLE CURRENT Si - - - - - - - - - - - - - - 2. A valence electron moves into 1 st hole and leaves a 2 nd hole 1. A free electron leaves hole in a valence shell. - 3. A valence electron moves into 2 nd hole and leaves a 3 rd hole - - 4. A valence electron moves into 3 rd hole and leaves a 4 th hole - 5. A valence electron moves into 4 th hole and leaves a 5 th hole - 6. A valence electron moves into 5 th hole and leaves a 6 th hole - 7. A valence electron moves into 6 th hole and leaves a 7 th hole

48. Dr. N.M. Safri/SEU3003_SemiconductorMaterial48 ELECTRON AND HOLE CURRENT Si - - - - - - - - - - - - - 2. A valence electron moves into 1 st hole and leaves a 2 nd hole 1. A free electron leaves hole in a valence shell. - 3. A valence electron moves into 2 nd hole and leaves a 3 rd hole - - 4. A valence electron moves into 3 rd hole and leaves a 4 th hole - 5. A valence electron moves into 4 th hole and leaves a 5 th hole - 6. A valence electron moves into 5 th hole and leaves a 6 th hole - 7. A valence electron moves into 6 th hole and leaves a 7 th hole - 8. A valence electron moves into 7 th hole and leaves a 8 th hole

49. Dr. N.M. Safri/SEU3003_SemiconductorMaterial49 Activity Lets review (Covalent bond): 1.How are covalent bonds formed? 2. What is meant by the term intrinsic? 3. What is a crystal? 4. Effectively, how many valence electrons are there in each atom within a silicon crystal? Covalent bonds are formed by the sharing of valence electrons with neighboring atoms. An intrinsic material is one that is in a pure state. A crystal is a solid material formed by atoms bonding together in a fixed patterns. There are eight shared valence electrons in each atom of a silicon crystal.

50. Dr. N.M. Safri/SEU3003_SemiconductorMaterial50 Activity Lets review (Conduction in semiconductors): 1.Are free electrons in the valence band or in the conduction band? 2. Which electrons are responsible for current in a material? 3. What is a hole? 4. At what energy level does hole current occur? Free electrons are in the conduction band. Free (conduction) electrons are responsible for current in a material. A hole is the absence of an electron in the valence band. Hole current occurs at the valence level. Outline

51. Dr. N.M. Safri/SEU3003_SemiconductorMaterial51 DOPING Semiconductive materials do not conduct current well and are of limited value in their intrinsic state. This is because of the limited number of free electrons in the conduction band and holes in the valence band. Intrinsic silicon (or germanium) must be modified by increasing the number of free electrons or holes to increase its conductivity and make it useful in electronic devices. This is done by adding impurities to the intrinsic material. This process is called doping. Two types of extrinsic (impure) semiconductive materials are n-type and p-type.

52. Dr. N.M. Safri/SEU3003_SemiconductorMaterial52 DOPING (N-type Semiconductor) pentavalent To increase the number of conduction band electrons in intrinsic silicon, pentavalent impurity atoms are added. These are atoms with five valence electrons such as arsenic (As), phosphorus (P), bismuth (Bi) and antimony (Sb). Each pentavalent atom forms covalent bonds with four adjacent silicon atoms. Four of the pentavalent atom’s valence electrons are used to form the covalent bonds with silicon atoms, leaving one extra electron. This extra electron becomes a conduction electron because it is not attached to any atom. donor atom Because the pentavalent atom gives up an electron, it is often called a donor atom.

53. Dr. N.M. Safri/SEU3003_SemiconductorMaterial53 Activity Based from statements stated in the previous slide, illustrates the pentavalent impurity atom (antimony, in this case) with its four adjacent silicon atoms in a silicon crystal structure using bonding diagram.

54. Dr. N.M. Safri/SEU3003_SemiconductorMaterial54 DOPING (N-type Semiconductor) The number of conduction electrons can be carefully controlled by the number of pentavalent impurity atoms added to the silicon. A conduction electron created by this doping process does not leave a hole in the valence band because it is in excess of the number required to fill the valence band. Sb Si - - - - - - - - - - - - - - - - - - - - - Free electron Fig: Pentavalent impurity in a silicon crystal structure. An antimony (Sb) impurity atom is shown in the center. The extra electron from the Sb atom becomes a free electron.

55. Dr. N.M. Safri/SEU3003_SemiconductorMaterial55 DOPING (N-type Semiconductor) Majority and Minority Carriers n-type nn Since most of the current carriers are electrons, silicon or germanium doped with pentavalent atoms is an n-type semiconductor (the n stands for negative charge of an electron). majority carriers The electrons are called the majority carriers in n-type material. In n-type semiconductor, there are also a few holes that are created when electron-hole pairs are thermally generated. These holes are not produced by the addition of the pentavalent impurity atoms. minority carriers Holes in an n-type material are called minority carriers.

56. Dr. N.M. Safri/SEU3003_SemiconductorMaterial56 DOPING (P-type Semiconductor) trivalent To increase the number of holes in intrinsic silicon, trivalent impurity atoms are added. These are atoms with three valence electrons such as boron (B), indium (In) and gallium (Ga). Each trivalent atom forms covalent bonds with four adjacent silicon atoms; And, since four electron electrons are required, a hole results when each trivalent atom is added. acceptor atom Because the trivalent atom can take an electron, it is often referred to as an acceptor atom.

57. Dr. N.M. Safri/SEU3003_SemiconductorMaterial57 Activity Based from statements stated in the previous slide, illustrates the trivalent impurity atom (boron, in this case) with its four adjacent silicon atoms in a silicon crystal structure using bonding diagram.

58. Dr. N.M. Safri/SEU3003_SemiconductorMaterial58 DOPING (P-type Semiconductor) The number of holes can be carefully controlled by the number of trivalent impurity atoms added to the silicon. A hole created by this doping process is not accompanied by a conduction (free) electron. B Si - - - - - - - - - - - - - - - - - - - Fig: Trivalent impurity in a silicon crystal structure. A boron (B) impurity atom is shown in the center. Hole from B atom

59. Dr. N.M. Safri/SEU3003_SemiconductorMaterial59 DOPING (P-type Semiconductor) Majority and Minority Carriers p-type Since most of the current carriers are holes, silicon or germanium doped with trivalent atoms is an p-type semiconductor. majority carriers The holes are called the majority carriers in p-type material. In p-type semiconductor, there are also a few free electrons that are created when electron-hole pairs are thermally generated. These free electrons are not produced by the addition of the trivalent impurity atoms. minority carriers Electrons in p-type material are the minority carriers. Holes can be thought of as positive charges because the absence of an electron leaves a net positive charge on the atom.

60. Dr. N.M. Safri/SEU3003_SemiconductorMaterial60 In summary: Free electrons > Holes Holes > Free electrons Si - - Sb - Si - Sb - - - - - Si - - Sb - - Si - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - B - - B - - - - - - - B - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - P-type semiconductor N-type semiconductor Outline

61. Dr. N.M. Safri/SEU3003_SemiconductorMaterial61 PN JUNCTION pn junction If a piece of intrinsic silicon is doped so that part is n-type and the other part is p-type, a pn junction forms at the boundary between the two regions. The p region has many holes (majority carriers) from the impurity atoms and only a few thermally generated free electrons (minority carriers). The n region has many free electrons (majority carriers) from the impurity atoms and only a few thermally generated holes (minority carriers).

62. Dr. N.M. Safri/SEU3003_SemiconductorMaterial62 PN JUNCTION A p-type material consists of silicon atoms and trivalent impurity atoms such as boron. However, since the number of protons and the number of electrons are equal throughout the material, there is no net charge in the material and so it is neutral. An n-type silicon material consists of silicon atoms and pentavalent impurity atoms such as antimony. As you have learned, an impurity atom releases an electron when it bonds with four silicon atoms. Since there is still an equal number of protons and electrons (including the free electrons) throughout the material, there is no net charge in the material and so it is neutral.

63. Dr. N.M. Safri/SEU3003_SemiconductorMaterial63 Activity Based from statements stated in the previous slides, illustrates the p region, the n region, the pn junction and the majority and minority carriers.

64. Dr. N.M. Safri/SEU3003_SemiconductorMaterial64 PN JUNCTION - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Fig: Structure at the instant of junction formation showing only the majority and minority carriers. - Hole Randomly drifting free electron N region P region pn junction

65. Dr. N.M. Safri/SEU3003_SemiconductorMaterial65 PN JUNCTION (Formation of the depletion region (kaw. susutan) As you have seen, the free electrons in the n region are randomly drifting in all directions. At the instant of the pn junction formation, the free electrons near the junction in the n region begin to diffuse across the junction into the p region where they combine with holes near the junction. Before the pn junction is formed, recall that there are as many electrons as protons in the n-type material, making the material neutral in terms of net charge. The same is true for the p-type material.

66. Dr. N.M. Safri/SEU3003_SemiconductorMaterial66 PN JUNCTION (Formation of the depletion region (kaw. susutan) When the pn junction is formed, the n region loses free electrons as they diffuse across the junction. This creates a layer of positive charges (pentavalent ions) near the junction. As the electrons move across the junction, the p region loses holes as the electrons and holes combine. This creates a layer of negative charges (trivalent ions) near the junction. depletion region These two layers of positive and negative charges form the depletion region.

67. Dr. N.M. Safri/SEU3003_SemiconductorMaterial67 Activity Based from statements stated in the previous slides, illustrates the p region, the n region, the majority and minority carriers and the depletion region.

68. Dr. N.M. Safri/SEU3003_SemiconductorMaterial68 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Fig (a): At the instant of junction formation, free electrons in the n region near pn junction begin to diffuse across the junction and fall into holes near the junction in the p region. - Hole Randomly drifting free electron N region P region pn junction PN JUNCTION (Formation of the depletion region (kaw. susutan) - - - - - - - - - - - - - - + - - - - - - - - - - - - - - - - - - - Fig (b): For every electron that diffuses across the junction and combines with a hole, a positive charge is left in the n region and a negative charge is created in the p region, forming a barrier potential. This action continues until the voltage of the barrier repels further diffusion. + + + + + + - - - - - - - - - - - depletion region }

69. Dr. N.M. Safri/SEU3003_SemiconductorMaterial69 PN JUNCTION (Barrier potential) Any time there is a positive charge and a negative charge near each other, there is a force acting on the charges as described by Coulomb’s law. In the depletion region there are many positive charges and many negative charges on opposite sides of the pn junction. The forces between the opposite charges form a field of forces called an electric field. This electric field is a barrier to the free electrons in the n region, and energy must be expended to move an electron through the electric field. That is, external energy must be applied to get the electrons to move across the barrier of the electric field in the depletion region.

70. Dr. N.M. Safri/SEU3003_SemiconductorMaterial70 PN JUNCTION (Barrier potential) The potential difference of the electric field across the depletion region is the amount of voltage required to move electrons through the electric field. This potential difference is called the barrier potential and is expressed in volts. - - - - - - - - - - - - - - + - - - - - - - - - - - - - - - - - - - Fig (b): For every electron that diffuses across the junction and combines with a hole, a positive charge is left in the n region and a negative charge is created in the p region, forming a barrier potential. This action continues until the voltage of the barrier repels further diffusion. + + + + + + - - - - - - - depletion region } Barrier potential - Hole Randomly drifting free electron

71. Dr. N.M. Safri/SEU3003_SemiconductorMaterial71 PN JUNCTION (Energy diagram illustrating the formation of the pn junction and depletion region) - Majority carriers 0 - - - - -- - - - - - - - - - - - - Minority carriers Valence band Conduction band Energy P region N region pn junction - 0 - - - - -- - - - - - - - - Valence band Conduction band Energy P region N region pn junction and depletion region Fig (a): At the instant of junction formation (b): At equilibrium

72. Dr. N.M. Safri/SEU3003_SemiconductorMaterial72 PN JUNCTION (Barrier potential) A simplified 1-D sketch of a pn junction (a) has a doping profile (b). The 3-D representation (c) shows the cross sectional area of the junction.

73. Dr. N.M. Safri/SEU3003_SemiconductorMaterial73 ELECTRON AND HOLE (Concentration) n = electron concentration p = hole concentration n-type: n = N D, the donor concentrationp-type: p = N A, the acceptor concentration

74. Dr. N.M. Safri/SEU3003_SemiconductorMaterial74 ELECTRON AND HOLE (Intrinsic carrier concentration) B – coefficient related to specific semiconductor T – temperature in Kelvin E g – semiconductor bandgap energy k – Boltzmann’s constant

75. Dr. N.M. Safri/SEU3003_SemiconductorMaterial75 PN JUNCTION (Barrier potential@ Built-in potential) This movement of carriers creates a space charge or depletion region with an induced electric field near x = 0. A potential voltage, v bi, is developed across the junction.

76. Dr. N.M. Safri/SEU3003_SemiconductorMaterial76 V T – thermal voltage (= 0.026 V at room temperature (T = 300K)) PN JUNCTION (Barrier potential@ Built-in potential) Outline

77. Dr. N.M. Safri/SEU3003_SemiconductorMaterial77 BIASING THE PN JUNCTION As you have learned, no electrons move through the pn junction at equilibrium. bias Generally, the term bias refers to the use of a dc voltage to establish certain operating conditions for an electronic device. In relation to a diode, there are two bias conditions: forward and reverse. Either of these bias conditions is established by connecting a sufficient dc voltage of the proper polarity across the pn junction. Outline

78. Dr. N.M. Safri/SEU3003_SemiconductorMaterial78 FORWARD BIASED PN JUNCTION To bias a pn junction (a diode), you apply a dc voltage across it. Forward bias is the condition that allows current through the pn junction. 1 st requirement for forward bias: negative side of bias voltage is connected to n region of the diode and the positive side is connected to the p region. 2 nd requirement for forward bias: bias voltage > barrier potential

79. Dr. N.M. Safri/SEU3003_SemiconductorMaterial79 FORWARD BIASED PN JUNCTION - - - - - - - - - - - - - - + - - - - - - - - - - - - - - - - - - - + + + + + + - - - - - - - depletion region } Barrier potential + － R N regionP region V bias

80. Dr. N.M. Safri/SEU3003_SemiconductorMaterial80 FORWARD BIASED PN JUNCTION - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + - - - - - - - depletion region } Barrier potential + － R N regionP region V bias

81. Dr. N.M. Safri/SEU3003_SemiconductorMaterial81 FORWARD BIASED PN JUNCTION - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + - - - - - - - depletion region } Barrier potential + － R N regionP region V bias

82. Dr. N.M. Safri/SEU3003_SemiconductorMaterial82 FORWARD BIASED PN JUNCTION - - - + + - - - depletion region } Barrier potential + － R N regionP region V bias - - + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

83. Dr. N.M. Safri/SEU3003_SemiconductorMaterial83 FORWARD BIASED PN JUNCTION - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + - - - depletion region } Barrier potential + － R N regionP region V bias - - + - -

84. Dr. N.M. Safri/SEU3003_SemiconductorMaterial84 FORWARD BIASED PN JUNCTION - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + - - - depletion region } Barrier potential + － R N regionP region V bias - - + - -

85. Dr. N.M. Safri/SEU3003_SemiconductorMaterial85 FORWARD BIASED THE PN JUNCTION (Effect of forward bias on the depletion region) As more electrons flow into the depletion region, the number of positive ions is reduced. As more holes effectively flow into the depletion region on the other side of the pn junction, the number of negative ions is reduced. This reduction in positive and negative ions during forward bias causes the depletion region to narrow. Outline

86. Dr. N.M. Safri/SEU3003_SemiconductorMaterial86 FORWARD BIASED THE PN JUNCTION (Effect of barrier potential during forward bias) Recall that the electric field between the positive and negative ions in the depletion region on either side of the junction creates an “energy hill” that prevents free electrons from diffusing across the junction at equilibrium. barrier potential This is known as barrier potential. When forward bias is applied, the free electrons are provided with enough energy from the bias-voltage source to overcome the barrier potential and effectively “climb the energy hill” and cross the depletion region. Outline The energy that the electrons require in order to pass through the depletion region is equal to the barrier potential. In other words, the electrons give up an amount of energy equivalent to the barrier potential when they cross the depletion region.

87. Dr. N.M. Safri/SEU3003_SemiconductorMaterial87 FORWARD BIASED PN JUNCTION Applied voltage, v D, induces an electric field, E A, in the opposite direction as the original space-charge electric field, resulting in a smaller net electric field and smaller barrier between n and p regions.

88. Dr. N.M. Safri/SEU3003_SemiconductorMaterial88 REVERSE BIASED PN JUNCTION To bias a pn junction (a diode), you apply a dc voltage across it. Reverse bias is the condition that essentially prevents current through the diode. Requirement for reverse bias: positive side of bias voltage is connected to n region of the diode and the negative side is connected to the p region.

89. Dr. N.M. Safri/SEU3003_SemiconductorMaterial89 REVERSE BIASED PN JUNCTION - - - - - - - - - - - - - - + - - - - - - - - - - - - - - - - - - - + + + + + + - - - - - - - depletion region } Barrier potential + － R N regionP region V bias

90. Dr. N.M. Safri/SEU3003_SemiconductorMaterial90 REVERSE BIASED PN JUNCTION - - - - - - - - - - - - - - + - - - - - - - - - - - - - - - - - - - + + + + + + - - - - - - - depletion region } Barrier potential + － R N regionP region V bias + + + + + + +

91. Dr. N.M. Safri/SEU3003_SemiconductorMaterial91 REVERSE BIASED PN JUNCTION - - - - - - - - - - - - - - + - - - - - - - - - - - - - - - - - - - + + + + + + - - - - - - - depletion region } Barrier potential + － R N regionP region V bias + + + + + + + ---- -

92. Dr. N.M. Safri/SEU3003_SemiconductorMaterial92 REVERSE BIASED PN JUNCTION - - - - - - - - - - - - - - + - - - - - - - - - - - - - - - - + + + + + + - - - - - - - depletion region } Barrier potential + － R N regionP region V bias + + + + + + + -- - - - - - - - - - -

93. Dr. N.M. Safri/SEU3003_SemiconductorMaterial93 REVERSE BIASED PN JUNCTION (Reverse current) The extremely small current exists in reverse bias after the transition current dies out is caused by the minority carriers in the n and p regions that are produced by thermally generated electron-hole pairs. The small number of free minority electrons in the p region are “pushed” toward the pn junction by the negative bias voltage. When these electrons reach the wide depletion region, they “fall down the energy hill” and combine with the minority holes in the n region as valence electrons and flow towards the positive bias voltage, creating a small hole current.

94. Dr. N.M. Safri/SEU3003_SemiconductorMaterial94 REVERSE BIASED PN JUNCTION (Reverse breakdown) Normally, the reverse current is so small that it can be neglected. However, if the external reverse-bias voltage is increased to a value called the breakdown voltage, the reverse current will drastically increase. The high reverse-bias voltage imparts energy to the free minority electrons so that as they speed through the p region, they collide with atoms with enough energy to knock valence electrons out of orbit and into the conduction band. The newly created conduction electrons are also high in energy and repeat the process. avalanche If one electron knocks only two others out of their valence orbit during its travel through the p region, the number quickly multiply. (known as avalanche)

95. Dr. N.M. Safri/SEU3003_SemiconductorMaterial95 REVERSE BIASED PN JUNCTION Increase in space-charge width, W, as V R increases to V R +  V R. Creation of more fixed charges (-  Q and +  Q) leads to junction capacitance.