1. SOLIDS AND SEMICONDUCTOR DEVICESElectrical 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. ELECTRICAL CONDUCTIVITY R = L/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. 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. 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. Band structure and conductivity 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. 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. Crystalline Solids In a crystalline solid, the periodic arrangement of atoms is repeated over the entire crystalSilicon crystal has a diamond lattice 9. Crystalline Nature of Silicon Siliconas utilized in integrated circuits is crystalline in nature Aswith all crystalline material, silicon consists of a repeating basic unit structure called a unit cell Forsilicon, the unit cell consists of an atom surrounded by four equidistant nearest neighbors which lie at the corners of the tetrahedron 10. Whats 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 3s and 3p valence electrons 11. Semiconductor Crystalline Structure Silicon atoms have 4 electrons in outer shell inner electrons are very closely bound to atomThese 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. Conduction in Crystal Lattices Semiconductors (Si and Ge) have 4 electrons in their outer shell 2 in the s subshell 2 in the p subshellAs the distance between atoms decreases the discrete subshells spread out into bandsAs the distance decreases further, the bands overlap and then separate the subshell model doesnt 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. Energy Bands in Semiconductors The space between the bands is the energy gap, or forbidden band 14. Nature of Intrinsic Silicon Siliconthat is free of doping impurities iscalled intrinsic Siliconhas a valence of 4 and formscovalentbondswithneighboring silicon atomsfourother 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 cant 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. Insulators, Semiconductors, and Metals (continued) Conduction BandValence Band ConductorSemiconductorInsulator 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 holepair eliminationholepair creationelectron 18. Improving Conduction by Doping Tomake 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. 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. 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. 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 areastopside 22. Hole and Electron Concentrations To produce reasonable levels of conduction doesnt require much doping silicon has about 5 x 1022 atoms/cm3 typical dopant levels are about 1015 atoms/cm3In 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 temperaturenp = ni2 This equation holds true for doped silicon as well, so increasing the number of free electrons decreases the number of holes 23. INTRINSIC (PURE) SILICON At 0 Kelvin Silicon density is 5x10 particles/cm 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 23C there are 10 particles/cm of free carriers 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. 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. Band Gap Betweenthe 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. Impurities Silicon crystal in pure form is good insulator - all electrons are bonded to silicon atomReplacement of Si atoms can alter electrical properties of semiconductorGroup number - indicates number of electrons in valence level (Si - Group IV) 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. Intrinsic silicon:DOPED SEMICONDUCTORS: 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. 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. P-N Junction Also known as a diodeOne of the basics of semiconductor technology -Created by placing n-type and p-type material in close contactDiffusion - mobile charges (holes) in p-type combine with mobile charges (electrons) in n-type 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 regionDepletion region has high resistance due to lack of mobile charges 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.