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  • Semiconductors

    1. 1. SOLIDS AND SEMICONDUCTOR DEVICES Electrical conductivity Energy bands in solids Band structure and conductivity Semiconductors Intrinsic semiconductors Doped semiconductors n-type materials p-type materials Diodes and transistors p-n junction depletion region forward biased p-n junction reverse biased p-n junction diode
    2. 2. ELECTRICAL CONDUCTIVITY  R = ρL/A, R = resistance, L = length, A = cross section area; resistivity at 20 o C in order of conductivity: superconductors, conductors, semiconductors, insulators  superconductors: certain materials have zero resistivity at very low temperature.  conductors: material capable of carrying electric current, i.e. material which has “mobile charge carriers” (e.g. electrons, ions,..) e.g. metals, liquids with ions (water, molten ionic compounds), plasma.  semiconductors: materials with conductivity between that of conductors and insulators; e.g. germanium Ge, silicon Si, GaAs, GaP, InP.  insulators: materials with no or very few free charge carriers; e.g. quartz, most covalent and ionic solids, plastics
    3. 3. 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 Note: 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
    4. 4. Classification of solids into three according to their band structure: types, Insulators: 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; semiconductors: gap is small - about 0.1 to 1 eV; conductors: valence band only partially filled, or (if it is filled), the next allowed empty band overlaps with it
    5. 5. Band structure and conductivity
    6. 6. What Is a Semiconductor? •Many materials, such as most metals, allow electrical current to flow through them •These are known as conductors •Materials that do not allow electrical current to flow through them are called insulators •Pure silicon, the base material of most transistors, is considered a semiconductor because its conductivity can be modulated by the introduction of impurities
    7. 7. Semiconductors   A material whose properties are such that it is not quite a conductor, not quite an insulator Some common semiconductors  elemental • Si - Silicon (most common) • Ge - Germanium  compound • GaAs - Gallium arsenide • GaP - Gallium phosphide • AlAs - Aluminum arsenide • AlP - Aluminum phosphide • InP - Indium Phosphide
    8. 8. Crystalline Solids  In a crystalline solid, the periodic arrangement of atoms is repeated over the entire crystal  Silicon crystal has a diamond lattice
    9. 9. Crystalline Nature of Silicon  Silicon as utilized in integrated circuits is crystalline in nature  As with all crystalline material, silicon consists of a repeating basic unit structure called a unit cell  For silicon, the unit cell consists of an atom surrounded by four equidistant nearest neighbors which lie at the corners of the tetrahedron
    10. 10. What’s so special about Silicon? Cheap and abundant Amazing mechanical, chemical and electronic properties The material is very well-known to mankind SiO2: sand, glass Si is column IV of the periodic table Similar to the carbon (C) and the germanium (Ge) Has 3s² and 3p² valence electrons
    11. 11. Semiconductor Crystalline Structure  Silicon atoms have 4 electrons in outer shell  inner electrons are very closely bound to atom  These electrons are shared with neighbor atoms on both sides to “fill” the shell resulting structure is very stable  electrons are fairly tightly bound no “loose” electrons  at room temperature, if battery applied, very little electric current flows 
    12. 12. Conduction in Crystal Lattices  Semiconductors (Si and Ge) have 4 electrons in their outer shell  2 in the s subshell  2 in the p subshell  As the distance between atoms decreases the discrete subshells spread out into bands  As the distance decreases further, the bands overlap and then separate  the subshell model doesn’t hold anymore, and the electrons can be thought of as being part of the crystal, not part of the atom  4 possible electrons in the lower band ( valence band)  4 possible electrons in the upper band ( conduction band)
    13. 13. Energy Bands in Semiconductors  The space between the bands is the energy gap, or forbidden band
    14. 14. Nature of Intrinsic Silicon  Silicon that is free of doping impurities is called intrinsic  Silicon has a valence of 4 and forms covalent bonds with neighboring silicon atoms four other
    15. 15. Insulators, Semiconductors, and Metals The separation of the valence and conduction bands determines the electrical properties of the material  Insulators have a large energy gap  electrons can’t jump from valence to conduction bands  no current flows  Conductors (metals) have a very small (or nonexistent) energy gap  electrons easily jump to conduction bands due to thermal excitation  current flows easily  Semiconductors have a moderate energy gap  only a few electrons can jump to the conduction band • leaving “holes”  only a little current can flow 
    16. 16. Insulators, Semiconductors, and Metals (continued) Conduction Band Valence Band Conductor Semiconductor Insulator
    17. 17. Hole - Electron Pairs   Sometimes thermal energy is enough to cause an electron to jump from the valence band to the conduction band  produces a hole - electron pair Electrons also “fall” back out of the conduction band into the valence band, combining with a hole pair elimination hole pair creation electron
    18. 18. Improving Conduction by Doping  To make semiconductors better conductors, add impurities (dopants) to contribute extra electrons or extra holes  elements with 5 outer electrons contribute an extra electron to the lattice (donor dopant)  elements with 3 outer electrons accept an electron from the silicon (acceptor dopant)
    19. 19. Improving Conduction by Doping (cont.) Phosphorus and arsenic are donor dopants  if phosphorus is introduced into the silicon lattice, there is an extra electron “free” to move around and contribute to electric current, very loosely bound to atom and can easily jump to conduction band  produces n type silicon  sometimes use + symbol to indicate heavier doping, so n+ silicon  phosphorus becomes positive ion after giving up electron
    20. 20. Improving Conduction by Doping (cont.)  Boron has 3 electrons in its outer shell, so it contributes a hole if it displaces a silicon atom  boron is an acceptor dopant   yields p type silicon Boronbecomes negative ion after accepting an electron
    21. 21. Diffusion of Dopants     It is also possible to introduce dopants into silicon by heating them so they diffuse into the silicon  no new silicon is added  high heat causes diffusion Can be done with constant concentration in atmosphere  close to straight line concentration gradient Or with constant number of atoms per unit area  predeposition  bell-shaped gradient Diffusion causes spreading of doped areas top side
    22. 22. Hole and Electron Concentrations  To produce reasonable levels of conduction doesn’t require much doping    silicon has about 5 x 1022 atoms/cm3 typical dopant levels are about 1015 atoms/cm3 In undoped (intrinsic) silicon, the number of holes and number of free electrons is equal, and their product equals a constant  actually, ni increases with increasing temperature np = ni2  This equation holds true for doped silicon as well, so increasing the number of free electrons decreases the number of holes
    23. 23. INTRINSIC (PURE) SILICON At 0 Kelvin Silicon density is 5x10²³ particles/cm³ Silicon has 4 valence electrons, it covalently bonds with four adjacent atoms in the crystal lattice Higher temperatures create free charge carriers. A “hole” is created in the absence of an electron. At 23ºC there are 10¹º particles/cm³ of free carriers
    24. 24. DOPING There are two types of doping N-type and P-type. The N in N-type stands for negative. A column V ion is inserted. The extra valence electron is free to move about the lattice The P in P-type stands for positive. A column III ion is inserted. Electrons from the surrounding Silicon move to fill the “hole.”
    25. 25. Energy-band Diagram       A very important concept in the study of semiconductors is the energy-band diagram It is used to represent the range of energy a valence electron can have For semiconductors the electrons can have any one value of a continuous range of energy levels while they occupy the valence shell of the atom That band of energy levels is called the valence band Within the same valence shell, but at a slightly higher energy level, is yet another band of continuously variable, allowed energy levels This is the conduction band
    26. 26. Band Gap  Between the valence and the conduction band is a range of energy levels where there are no allowed states for an electron  This is the band gap E G  In silicon at room temperature [in electron . volts]: E G = 11 eV  Electron volt is an atomic measurement unit, 1 eV energy is necessary to decrease of the potential of the electron with 1 V. 1eV = 1.602 × 10 −19 joule
    27. 27. Impurities  Silicon crystal in pure form is good insulator - all electrons are bonded to silicon atom  Replacement of Si atoms can alter electrical properties of semiconductor  Group number - indicates number of electrons in valence level (Si - Group IV)
    28. 28. Impurities   Replace Si atom in crystal with Group V atom  substitution of 5 electrons for 4 electrons in outer shell  extra electron not needed for crystal bonding structure • can move to other areas of semiconductor • current flows more easily - resistivity decreases • many extra electrons --> “donor” or n-type material Replace Si atom with Group III atom  substitution of 3 electrons for 4 electrons  extra electron now needed for crystal bonding structure • “hole” created (missing electron) • hole can move to other areas of semiconductor if electrons continually fill holes • again, current flows more easily - resistivity decreases • electrons needed --> “acceptor” or p-type material
    29. 29.  Intrinsic silicon:  DOPED SEMICONDUCTORS:
    30. 30. n-type material Donor (n-type) impurities: Dopant with 5 valence electrons (e.g. P, As, Sb) 4 electrons used for covalent bonds with surrounding Si atoms, one electron “left over”; left over electron is only loosely bound⇒ only small amount of energy needed to lift it into conduction band (0.05 eV in Si) ⇒ “n-type semiconductor”, has conduction electrons, no holes (apart from the few intrinsic holes)
    31. 31. p-type material Acceptor (p-type) impurities: dopant with 3 valence electrons (e.g. B, Al, Ga, In) ⇒ only 3 of the 4 covalent bonds filled ⇒ vacancy in the fourth covalent bond ⇒ hole. “p-type semiconductor”, has mobile holes, very few mobile electrons (only the intrinsic ones).
    32. 32. P-N Junction  Also known as a diode  One of the basics of semiconductor technology -  Created by placing n-type and p-type material in close contact  Diffusion - mobile charges (holes) in p-type combine with mobile charges (electrons) in n-type
    33. 33. P-N Junction  Region of charges left behind (dopants fixed in crystal lattice)    Group III in p-type (one less proton than Si- negative charge) Group IV in n-type (one more proton than Si - positive charge) Region is totally depleted of mobile charges - “depletion region”  Electric field forms due to fixed charges in the depletion region  Depletion region has high resistance due to lack of mobile charges
    34. 34. p-n JUNCTION: p-n junction = semiconductor in which impurity changes abruptly from p-type to n-type ; “diffusion” = movement due to difference in concentration, from higher to lower concentration; • in absence of electric field across the junction, holes “diffuse” towards and across boundary into n-type and capture electrons; • electrons diffuse across boundary, fall into holes (“recombination of majority carriers”); ⇒ formation of a “depletion region”(= region without free charge carriers) around the boundary; • charged ions are left behind (cannot move):    negative ions left on p-side ⇒ net negative charge on p-side of the junction; positive ions left on n-side ⇒ net positive charge on n-side of the junction ⇒ electric field across junction which prevents further diffusion.