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SEU 3003 ELEKTRONIK (ELECTRONICS)

SEU 3003 ELEKTRONIK (ELECTRONICS). Chapter 1 SEMICONDUCTORS MATERIALS. Dr. Norlaili Mat Safri. In this chapter, we will learn:. Atomic structure Energy band Materials classification Covalent bonds Conduction in semiconductors Free electron and hole as carrier of current

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SEU 3003 ELEKTRONIK (ELECTRONICS)

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  1. SEU 3003 ELEKTRONIK (ELECTRONICS) Chapter 1 SEMICONDUCTORS MATERIALS Dr. Norlaili Mat Safri Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  2. In this chapter, we will learn: • Atomic structure • Energy band • Materials classification • Covalent bonds • Conduction in semiconductors • Free electron and hole as carrier of current • Doping of semiconductor materials • The p-n junction • Biasing the p-n junction Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  3. 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. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  4. 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 Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  5. + + + + + + - - - - - - - Atomic Structure (The Bohr Model) • The nucleus consists of positively charged particles called protons and uncharged particles called neutrons. • The basic particles of negative charge are called electrons. Electron Proton Neutron + Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  6. - - - 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. Nucleus Nucleus Electron + + + Electron Electron Dr. N.M. Safri/SEU3003_SemiconductorMaterial Hydrogen atom Helium atom

  7. 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 energiesexist 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. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  8. 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,…. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  9. + + + + + + - - - - - - Energy band Energy • Right figure shows the 1st shell with one energy level and the 2nd shell with two energy levels. • Additional shells may exist in other types of atoms, depending on the element. Nucleus Shell 1 (K) Shell 2 (L) Energy increases as the distance from the nucleus increases Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  10. 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. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  11. Energy band (The number of electrons in each shell) • The maximum number of electrons (Ne) that can exist in each shell of an atom is a fact of nature and can be calculated by the formula, • Ne = 2n2 • 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 • Ne = 2n2 = 2(1)2 = 2 Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  12. Activity • Calculate the maximum number of electrons that can exist in the 2nd shell , 3rd shell and 4th shell Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  13. 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 1st shell, ___ electrons in the 2nd shell and ___ electrons in the 3rd shell. Silicon also has ___ valence electrons. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  14. 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 atomicstructure. • 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, 2n2. • The outer shell is called the valence shell. • The less complete a shell is filled to capacity the more conductive the material is. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  15. 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. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  16. +14 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - MATERIALS CLASSIFICATION Silicon Cooper Argon +29 +18 Silicon, Cooper, and Argon have ____, ____, and ____ valence electron(s), respectively. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  17. MATERIALS CLASSIFICATION Conductors Semiconductors Insulators Neon (10) Argon (18) Krypton (36) Xenon (54) Cooper (29) Silver (47) Gold (79) Aluminium (13) Carbon (6) Silicon (14) Germanium (32) Conductors, semiconductors, and insulators have ____, ____, and ____ valence electron(s), respectively. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  18. MATERIALS CLASSIFICATION Energy band for each type of material Energy (eV) Energy (eV) Energy (eV) Conduction band Energy gap Conduction band Energy gap Conduction band Overlap Valence band Valence band Valence band 0 0 0 Conductors Semiconductors Insulators Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  19. 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. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  20. 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. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  21. 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. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  22. Activity • Based from statements stated in the previous slide, illustrates the covalent bonds in silicon. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  23. COVALENT BONDS Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  24. 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. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  25. 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. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  26. + + + + + + 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. Energy (eV) Conduction band Energy gap = 1.1 eV Valence band - - - - Energy gap Fig: Energy band diagram for an unexcited atom in a pure (intrinsic) silicon crystal. There are no electrons in the conduction band. 2nd band (shell 2) - - - - - - - - Energy gap 1st band (shell 1) - - Nucleus 0 Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  27. CONDUCTION IN SEMICONDUCTORS (Conduction Electrons and Holes) • 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 electrons are also called conduction electrons. • When an electron jumps to the conduction band, a vacancy is left in the valence band within the crystal. • This vacancy is called a hole. • 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. • Recombinationoccurs when a conduction band electron loses energy and falls back into a hole in the valence band. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  28. 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. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  29. + + + + + + + + + + + + CONDUCTION IN SEMICONDUCTORS (Conduction Electrons and Holes) Effect of Temperature Energy (eV) Electron-hole pair Conduction band Free electron - - Energy gap Hole Valence band - - - - - - Heat energy Energy gap 2nd band (shell 2) - - - - - - - - - - - - - - - - Energy gap 1st band (shell 1) - - - - Nucleus 0 Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  30. - Si - - - - - - - - Si Si Si - - - - - - - - Si - - - CONDUCTION IN SEMICONDUCTORS (Conduction Electrons and Holes) Effect of Temperature (bonding diagram) Free electron - - Si - - - - - Hole Heat energy - - Si Si Si - - - - - - - - Si - - - As temperature increases, a bond can break, releasing a valence electron and leaving a broken bond (hole). Current can flow. At 0K, no bonds are broken. Si is an insulator. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  31. 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. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  32. CONDUCTION ELECTRONS AND HOLES - - - - - - Recombination of an electron with a hole - - - - - - - Si Si Si Si Si Si - - - - - - - - - - - - - - - - - - - - - Si Si Si - Si Si Si Generation of an electron-hole pair - - - - - - - - - - - - - - - - - - - - - - - Si Si - Si Si Si Si - - - - - - - - - - - - - Heat energy Fig: Electron-hole pairs in a silicon crystal. Free electrons are being generated continuously while some recombine with holes. Dr. N.M. Safri/SEU3003_SemiconductorMaterial • Outline

  33. 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. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  34. Activity - - - - - - - - - - - - - Si Si Si Si Si Si - - - - - - - - - - - - - - - - - - - - - － ＋ Si Si Si - Si Si Si - - - - - - - - - - - - - - - - - - - - - - - Si Si - Si Si Si Si - - - - - - - - - - - - - V Fig: Electron current in intrinsic silicon is produced by the movement of thermally generated free electrons. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  35. Activity - - - - - - - - - - - Si Si Si Si Si Si - - - - - - - - - - - - - - - - - - - - - - - - － ＋ Si Si Si - Si Si Si - - - - - - - - - - - - - - - - - - - - - - - - Si Si - Si Si Si Si - - - - - - - - - - - V Fig: Electron current in intrinsic silicon is produced by the movement of thermally generated free electrons. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  36. 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. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  37. ELECTRON AND HOLE CURRENT - - - - - - - - - - - - - Si Si Si Si Si Si - - 1 - - - - - - - - - - - - - - - - - - - Si Si Si - Si Si Si - - - - - - - - - - - - - - - - - - - - - - - Si Si - Si Si Si Si - - - - - - - - - - - - - Heat energy Fig: Hole current in intrinsic silicon crystal. Dr. N.M. Safri/SEU3003_SemiconductorMaterial • Outline

  38. ELECTRON AND HOLE CURRENT - - - - - - - - - - - - - Si Si Si Si Si Si - - - - - - - - - - - - - - - - 2 - - - - - Si Si Si - Si Si Si - - - - - - - - - - - - - - - - - - - - - - - Si Si - Si Si Si Si - - - - - - - - - - - - - Heat energy Fig: Hole current in intrinsic silicon crystal. Dr. N.M. Safri/SEU3003_SemiconductorMaterial • Outline

  39. ELECTRON AND HOLE CURRENT - - - - - - - - - - - - - Si Si Si Si Si Si - - - - - - - - - - - - - - - - - 3 - - - - Si Si Si - Si Si Si - - - - - - - - - - - - - - - - - - - - - - - Si Si - Si Si Si Si - - - - - - - - - - - - - Heat energy Fig: Hole current in intrinsic silicon crystal. Dr. N.M. Safri/SEU3003_SemiconductorMaterial • Outline

  40. ELECTRON AND HOLE CURRENT - - - - - - - - - - - - - Si Si Si Si Si Si - - - - - - - - - - - - - - - - - - - - - - Si Si Si - Si Si Si - 4 - - - - - - - - - - - - - - - - - - - - - Si Si - Si Si Si Si - - - - - - - - - - - - - Heat energy Fig: Hole current in intrinsic silicon crystal. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  41. ELECTRON AND HOLE CURRENT 1. A free electron leaves hole in a valence shell. - - - - - - - - - Si Si - Si Si Si - - - - - - - - - - Fig: Hole current in intrinsic silicon crystal. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  42. ELECTRON AND HOLE CURRENT 1. A free electron leaves hole in a valence shell. 2. A valence electron moves into 1st hole and leaves a 2nd hole - - - - - - - - - Si Si - Si Si Si - - - - - - - - - - Fig: Hole current in intrinsic silicon crystal. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  43. ELECTRON AND HOLE CURRENT 3. A valence electron moves into 2nd hole and leaves a 3rd hole 1. A free electron leaves hole in a valence shell. 2. A valence electron moves into 1st hole and leaves a 2nd hole - - - - - - - - - Si Si - Si Si Si - - - - - - - - - - Fig: Hole current in intrinsic silicon crystal. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  44. ELECTRON AND HOLE CURRENT 3. A valence electron moves into 2nd hole and leaves a 3rd hole 1. A free electron leaves hole in a valence shell. 2. A valence electron moves into 1st hole and leaves a 2nd hole 4. A valence electron moves into 3nd hole and leaves a 4rd hole - - - - - - - - - Si Si - Si Si Si - - - - - - - - - - Fig: Hole current in intrinsic silicon crystal. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  45. ELECTRON AND HOLE CURRENT 3. A valence electron moves into 2nd hole and leaves a 3rd hole 5. A valence electron moves into 4th hole and leaves a 5th hole 1. A free electron leaves hole in a valence shell. 2. A valence electron moves into 1st hole and leaves a 2nd hole 4. A valence electron moves into 3rd hole and leaves a 4th hole - - - - - - - - - Si Si - Si Si Si - - - - - - - - - - Fig: Hole current in intrinsic silicon crystal. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  46. ELECTRON AND HOLE CURRENT 3. A valence electron moves into 2nd hole and leaves a 3rd hole 5. A valence electron moves into 4th hole and leaves a 5th hole 1. A free electron leaves hole in a valence shell. 2. A valence electron moves into 1st hole and leaves a 2nd hole 4. A valence electron moves into 3rd hole and leaves a 4th hole 6. A valence electron moves into 5th hole and leaves a 6th hole - - - - - - - - - Si Si - Si Si Si - - - - - - - - - - Fig: Hole current in intrinsic silicon crystal. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  47. ELECTRON AND HOLE CURRENT 3. A valence electron moves into 2nd hole and leaves a 3rd hole 5. A valence electron moves into 4th hole and leaves a 5th hole 1. A free electron leaves hole in a valence shell. 2. A valence electron moves into 1st hole and leaves a 2nd hole 4. A valence electron moves into 3rd hole and leaves a 4th hole 6. A valence electron moves into 5th hole and leaves a 6th hole - - - - - - - - - Si Si - Si Si Si - - - - - - - - - - 7. A valence electron moves into 6th hole and leaves a 7th hole Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  48. ELECTRON AND HOLE CURRENT 3. A valence electron moves into 2nd hole and leaves a 3rd hole 5. A valence electron moves into 4th hole and leaves a 5th hole 1. A free electron leaves hole in a valence shell. 2. A valence electron moves into 1st hole and leaves a 2nd hole 4. A valence electron moves into 3rd hole and leaves a 4th hole 6. A valence electron moves into 5th hole and leaves a 6th hole - - - - - - - - - Si Si - Si Si Si - - - - - - - - - - 8. A valence electron moves into 7th hole and leaves a 8th hole 7. A valence electron moves into 6th hole and leaves a 7th hole Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  49. Activity • Lets review (Covalent bond): • How are covalent bonds formed? Covalent bonds are formed by the sharing of valence electrons with neighboring atoms. 2. What is meant by the term intrinsic? An intrinsic material is one that is in a pure state. 3. What is a crystal? A crystal is a solid material formed by atoms bonding together in a fixed patterns. 4. Effectively, how many valence electrons are there in each atom within a silicon crystal? There are eight shared valence electrons in each atom of a silicon crystal. Dr. N.M. Safri/SEU3003_SemiconductorMaterial

  50. Activity • Lets review (Conduction in semiconductors): • Are free electrons in the valence band or in the conduction band? Free electrons are in the conduction band. 2. Which electrons are responsible for current in a material? Free (conduction) electrons are responsible for current in a material. 3. What is a hole? A hole is the absence of an electron in the valence band. 4. At what energy level does hole current occur? Hole current occurs at the valence level. Dr. N.M. Safri/SEU3003_SemiconductorMaterial • Outline

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