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Transcript
Chapter-5: Introduction to Semi-Conductors and Diode Circuits Conduction band 3p Energy gap Valence band 3s 2p 2s 1s bound electrons When atoms form a crystalline structure (metals, semiconductors, ) the valence electrons loose their attraction to a local atom and form an band of ~equivalent charges- valence band. The conduction band lies within or above the valence band. 1 Energy Bands Electron Energy Electron Energy Electron Energy Conduction Band Valence Band Overlapping bands - little energy is needed for conduction Energy Gap Energy Gap Conduction Band Conduction Band Valence Band Valence Band Insulator Conductor Semiconductor 2 IA VIIIA 2 4.002 1 1.008 H Hydroge n 3 6.939 Li IIA 4 Be Berylliu Lithiu m m 11 22.98 12 24.312 9 Mg K Ca IVA VA VIA VIIA B C N O F 6 Heliu m 5 10.81 6 12.01 7 14.00 8 15.99 9 18.99 10 20.18 1 1 7 9 8 3 9.01 2 Na IIIA He Ne Florin Boron Carbon Nitroge Oxygen Neon n e 26.98 28.08 30.97 32.06 35.45 13 14 15 16 17 18 39.94 1 6 4 4 3 8 Transition Metals Al Si P S Cl Ar Ga Ge As Se Br Kr IVB VB VIB VIIB VIIIB IB IIB Aluminum Silicon Phosphorus Sulfur Chlorin Argon Sodiu Magnesium IIIB e m 19 39.10 20 40.08 21 44.95 22 47.90 23 50.94 24 51.99 25 54.93 26 55.84 27 58.93 28 58.71 29 63.54 30 65.37 31 69.72 32 72.59 33 74.92 34 78.96 35 79.90 36 83.80 2 6 2 6 8 7 3 2 9 Sc Ti V Cr Mn Fe Co Cu Ni Zn Potassiu Calcium Scandium Titaniu Vanadium Chromiu Manganese Iron Nickel Zinc Cobalt Coppe Galliu Germaniu Arseni Seleniu Bromin Krypton m m m m r m c m e 37 85.47 38 87.62 39 88.90 40 91.22 41 92.90 42 95.94 43 99 44 101.0 45 102.91 46 106.4 47 107.8 48 112.4 49 114.8 50 118.6 51 121.7 52 127.6 53126.90 54 131.3 5 6 7 7 0 9 5 0 4 0 2 Rb Sr Y Zr Nb Tc Mo Ru Pd Rh Ag Cd Sn In Sb Te I Xe Rubidiu Strontium Yttrium Zirconium Niobium Molybde- TechnitiumRuthenium Rhodiu Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon num m m 1137.34 57138.91 72178.49 73180.95 74183.85 75 186.2 76 190.2 77 192.2 78195.09 79196.967 80200.59 81204.37 82207.19 83208.98 84 210 85 210 86 222 55132.90 56 Cs Ba La Hf Ta Cesium Barium Lanthanum Hafniu m 87 223 88 226 89 227 104 Fr Ra Francium Radium Ac Re W Ir Os Tantalum Tungsten Rhenium Osmium 105 Rf 106 Ha 107 Sg 108 Uns Pt Iridium Platinum 109 Uno Hg A Gold u Tl Pb Mercury Thallium At Bismuth Poloniu m Lead 110 Une Po Bi Rn Astatine Radon Nonmetals Uun Actiniu m Metalloids (semimetals) 58 140.1 59140.91 60 144.2 61 147 62 150.3 63 151.9 64 157.2 65 158.9 66 162.5 67 164.9 68 167.2 69 168.9 70 173.0 71 174.9 Lanthanides Ce 2 Cerium Th Nd 4 Praseodym- Neodym ium -ium 90232.04 91 Actinides Pr Pm Pa U 5 Eu 6 Gd 5 Prome- Samarium Europium Gadolinium thium 231 92 238.0 93 3 ProcatThorium inium Sm Tb Terbium 237 94 242 95 243 96 247 97 Np Pu Am Cm 2 Dy 0 Ho 3 Er 6 Tm 3 Yb 4 Lu 7 Dyspro- Holmium Erbium sium Thuliu Ytterbiu Lutetium m m 24 98 249 99 254 100 253 101 256 102 253 103 257 7 Bk Cf CaliforUranium Neptunium Plutoniu Americium Curium Berkelium nium m Es Einsteinium Fm Md Mendelev Fermium -ium No Nobeliu m Lr Lawrencium 3 Group IVA Elemental Semiconductors Carbon Silicon Germanium Tin Lead C Si Ge Sn Pb 6 • Very Expensive diamond • Band Gap Large: 6V • Difficult to produce without high contamination 14 • Cheap • Ultra High Purity • Oxide is amazingly perfect for IC applications 32 • High Mobility (good) • High Purity Material (good) • Oxide is porous to water/hydrogen (disasterous!) 50 82 • Only “White Tin” is semiconductor • Converts to metallic form under moderate heat • Only “White Lead” is semiconductor • Converts to metallic form under moderate heat 4 Covalent Bonding of Pure Silicon Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Silicon atoms share valence electrons to form insulator-like bonds. 5 Electrons in N-Type Silicon with Phosphorus Dopant Si Si Si Si Si Si Si Si P Si Si P Si Si Si Si Si Si P Si Si Si Si Si Si Excess electron (-) Phosphorus atom serves as n-type dopant Donor atoms provide excess electrons to form n-type silicon. 6 Holes in p-Type Silicon with Boron Dopant Si Si Si Si Si Si Si Si B Si + Hole Si B Si Si Si Boron atom serves as p-type dopant Si Si Si B Si Si Si Si Si Si Acceptor atoms provide a deficiency of electrons to form p-type silicon. 7 Silicon Dopants Acceptor Impurities Semiconductor Donor Impurities Group III (p-type) Group IV Group V (n-type) Boron Carbon 6 Silicon 14 Germanium 32 5 Aluminum 13 Nitrogen 7 Phosphorus 15 Gallium 31 Arsenic 33 Indium 49 Antimony 51 8 Conduction in n and p-Type Silicon Positive terminal from voltage supply Negative terminal from voltage supply ow e Fl l o H low F n tro Elec -Electrons flow toward positive terminal +Holes flow toward negative terminal 9 Silicon Resistivity Versus Dopant Concentration 1021 Dopant Concentration (atoms/cm3) 1020 1019 1018 1017 n-type 1016 p-type 1015 1014 1013 10-3 10-2 10-1 100 101 102 103 Electrical Resistivity (ohm-cm) 10 n- and p-Type Silicon Important Facts: The dominant Not shown are the silicon charge carrier atoms, which are present in in n-type Si is vastly greater numbers than the electron. either arsenic or boron. Typical doping concentrations are 1016 arsenic or boron 3. atoms per cm The dominant The concentration of silicon charge carrier 22 in p-type Si is atoms is about 10 atoms per cm3. the hole For every "dopant" atom, there are about a million silicon atoms. 11 Open-Circuit Condition of a p-n Junction Diode p-type Si } Depletion Region !x n-type Si Anode Cathode Diffusing electrons and holes create an electric field Eo and potential step at the junction. Eo Charge distribution of barrier voltage across a pn Junction. Metal contact Barrier voltage 0 Potential step 12 Forward-Biased PN Junction Diode p Eapp n Eo Hole flow Electron flow V Lamp In forward bias the applied E-field cancels internal electric field and charges begin flowing across the junction when Eapp>Eo or Vapp>~0.6V. 13 Reverse-Biased PN Junction Diode Eapp p 3V Open-circuit condition (high resistance) n Lamp In the reverse bias the external E-field increases the size of the depletion zone until all charge carries are near the contacts - fully depleted. 14 I-V Characteristic Curve This is the "characteristic" curve of a pn junction diode. It shows the slow, then abrupt, rise of current as the voltage is raised. Under reverse bias, even very large voltages will cause only very small currents, essentially constant reverse bias currents. Shockley Diode Law (Ideal) Reverse current exaggerated; typical reverse saturation current: IS ~10-(6-12)A. I = I S (e+ eVD /kT − 1) ~ I S e+ eVD /kT I S = reverse bias saturation current VD = diode voltage kT = 0.025eV at 300 o K 15 Turn-On Voltage A pn junction diode "turns on" at about 0.6 V, but that varies according to the doping concentration, diode type. Notice the sharp rise in current near the turn-on voltage. This behavior will be exploited later in the construction of the transistor amplifier. Use the Shockley Equation to det er min e the forward diode current VD = 1 V T = 300 0 K and I S = 10 pA. I = 10 pA exp(1eV / 0.025eV ) = 2.4e9A !! 16 Solving Diode Equation by Iteration ε − Vd − IR = 0 Kirchhoff ' s law ε − Vd I= = I S ( eeVd /nkT − 1) n ≡ diode realty factor n ~ 1.5 R ⎛ I ⎞ eVd = ln ⎜ + 1⎟ nkT ⎝ IS ⎠ ε I ⎛ ε − Vd ⎞ ⎛ n⎞ Vd = ⎜ ⎟ kT ln ⎜ + 1⎟ Solve by iteration ⎝ e⎠ ⎝ IS R ⎠ DIODE EQ. ITERATIVE SOLUTION 1.40 1.20 Is emf R n 2.00E-06 10 1000 1 Amps Volts Ohms 1.00 Vd (1) Vd (2) ⎛ ε − Vd (0) ⎞ ⎛ n⎞ = ⎜ ⎟ kT ln ⎜ + 1⎟ ⎝ e⎠ ⎝ IS R ⎠ ⎛ ε − Vd (1) ⎞ ⎛ n⎞ = ⎜ ⎟ kT ln ⎜ + 1⎟ ⎝ e⎠ ⎝ IS R ⎠ Vd . 0.80 Stop when Vd ( N +1) ~ Vd ( N ) 0.60 0.40 http://en.wikipedia.org/wiki/Diode_modelling#Shockley_diode_model 0.20 0.00 0 5 10 Iteration Number 15 20 17 A PN Junction Diode and Its Symbol Black band corresponds to the "point" in the diode symbol on the right. It points as "point" is spelled, with "p" first, and"n" later. 18 Basic Symbol and Structure of the pn Junction Diode pn junction diode Cathode Anode Heavily doped p region Heavily doped n region Metal contact p- Substrate 19 Light Emitting Diodes As electrons and holes pass thru the forward biased junction, some annihilate and release hf = eUo of activation energy in to light. - - - - hf Activation Energy Fermi level + Activation Voltage Uo Fermi level + + + + Forbidden zone E=hc/λ = 1240 eV-nm / λ(nm) Red light of 650nm would correspond to Uo = 1240/650 eV = 1.9 eV 20 Simple Diode Circuit ~0.6V zd R Vout V Vdiode ! 0.6V R 1 )V = ( )V ! (1− r / R) V ! V − Vdiode r+R 1+ r / R R = ( )V = 0 ∞+ R V > Vdiode zd = r and VOUT = ( V < Vdiode zd = ∞ and VOUT zd V R Vout ~0.6V 21 Half Wave Rectifier - AC-to-DC Conversion zd Vs Vout C R Vout AC - DC Converter Capacitor ch arg es to peak voltage VOUT ~ VS − 0.6V Voltage decays on down − cycle VOUT = VS e−t /RC VOUT (t) = VS (1− t / RC + ... ) t / RC << 1 VS t i ΔV = VOUT (t) − VS = = ripplevoltage R C f C r = ΔV / VOUT ripple fa ct or VOUT ≅ VS − 0.6V We have produced a DC voltage from AC input. 22 Full Wave Rectifier - AC-to-DC Conversion V2= (N2/N1)V1 120VAC CR Vout = 1 2 V2 1 i r= f C 2 ( ) Vout - In a Full Wave Rectifier two diodes act to rectify both positive and negative cycles. - A transformer is generally used to isolate the 120VAC (20A) from the secondary. - Less power wasted and a reduced ripple factor: 23 Cockroft -Walton Voltage Multipliers + + ++ ++ 4-stage Multiplier 1. On the positive halfcycle C1,C2,C,C4 charge to +Us thru D2 and D4. D1 and D3 are blocking. 2. On the negative halfcycle C1,C3, charge to -Us, but C2 and C4 can not discharge. 3. On the 2nd cycle C1,C2,C,C4 charge again to +Us thru D2 and D4. and C2 and C4 coming each chargeing to 2 Us. 4. The total potential difference across the output C2+C4 is Vout= +4Us. 5. Practically speaking Vout < +4Urms. 6. You can hear your digital camera charge up the capacitor banks for the flash. The cockroft walton concept can only supply a short burst of charge and must be modified for high DC current use. http://www.blazelabs.com/e-exp15.asp 24 Voltage Doubler Circuit D2 V=2Vo C Q C Q D1 Vo • • • Capacitors are charged to Q by Vo sin(wt) after a few cycles. Charge can not leak off due to diodes. T Total Voltage sum is V=2Vo (doubler)! 25 Zener Diode / Voltage regulator • A Zener works in reverse bias mode. It is be designed to break down at the precise voltage called VZener ~10-100V eg. • Point a in the circuit is fixed to the Zener voltage when Vin > Vz. • The Zener diode is used for tight DC voltage regulation. Zener breakdown Voltage ~ -VZ R a V -V Vz 0.6 V RL Vout +V Choose Rto limit the current thru the Zener diode (IZmax ≈ 50ma) I L = VZ / RL I = (V − VZ ) / R I Z = I − I L < I Z max (V − VZ ) / R − VZ / RL < I Z max (V − VZ ) / R < I Z max + VZ / RL (V − VZ )RL (10V − 5V )1000Ω R> > = 91Ω I Z max RL + VZ 0.005A ⋅1000Ω + 5V 26 Input/Output Protection with Diodes A diode placed in series with the positive side of the power supply is called a reverse protection diode. It ensures that current can only flow in the positive direction, and the power supply only applies a positive voltage to your circuit. 27 Input/Output Protection with Diodes • The 470Ω resistor in series with the LED limits the forward bias current flowing through the LED typically Ilimit< 20ma. • 9V - 470 I – 0.6V = 0 so I = 8.4V/470Ω =18ma (safe). • Id = 0 in most cases where point a is maintained with a positive voltage. • If someone attached the battery in the reverse directions the LED would blow out w/o the protection diode. The diode allows the current to bypass the LED! a id i 28 Load Protection with Diodes • Consider a circuit with a power supply and an inductive load (e.g. motor) on it. From the instant the switch is closed, the inductive load will accumulate stored energy. When the switch is opened this energy will generate a high reverse voltage and arc across the contacts of the switch. This could damage the switch, load and other circuit components. • A power diode placed across the inductive load will provide a path for the release of energy stored in the inductor while the inductive load voltage drops to zero. + - - + 29