Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
7/30/2007 CHAPTER 4: ELECTRICAL and ELECTRONIC PROPERTIES • How electrical conductance and resistance are characterized. • What physical phenomena distinguish conductors, semiconductors, and insulators? • For metals, how is conductivity affected by imperfections, T, and deformation? • For semiconductors, how is conductivity affected by impurities (doping) and T? 1 1 7/30/2007 Alkali Metals The Periodic Table Alkaline Earths Halogens Noble Gases Main Group Transition Metals Main Group Lanthanides and Actinides Periodic Properties of the Elements 2 7/30/2007 Electrical Conduction • Ohm's Law: ∆V = I R voltage drop (volts) resistance (Ohms) current (amps) A (cross sect. area) e- I ∆V L • Resistivity, ρ and Conductivity, σ: --geometry-independent forms of Ohm's Law E: electric field intensity ∆V I = ρ L A ρL L • Resistance: R = = A Aσ resistivity (Ohm-m) J: current density conductivity σ= I ρ 3 3 7/30/2007 Effect of Impurity on Electrical Conductivity Metals are fairly insensitive to impurities. It takes a lot of impurity to change the conductivity of a metal by as much as a factor of 10. Unlike semiconductors, metals become poorer conductors when impure. Semiconductors , on the other hand, are very sensitive to impurities. The conductivity of silicon or germanium can be increased by a factor of up to 106 by adding as little as 0.01% of an impurity. • Problem 12.2, p. 524, Callister 2e: e- Cu wire - 100m I = 2.5A + ∆V What is the minimum diameter (D) of the wire so that ∆V < 1.5V? 100m < 1.5V R= 2 πD 4 L ∆V = Aσ I 2.5A 7 -1 6.07 x 10 (Ohm-m) Solve to get D > 1.88 mm 5 4 7/30/2007 Conductivity Comparison -1 • Room T values (Ohm-m) METALS conductors Silver 6.8 x 107 Copper 6.0 x 107 Iron 1.0 x 107 CERAMICS Soda-lime glass 10 -10 Concrete 10-9 Aluminum oxide <10-13 SEMICONDUCTORS POLYMERS -4 Polystyrene <10-14 Silicon 4 x 10 Polyethylene 10-15-10 -17 Germanium 2 x 100 GaAs 10-6 insulators semiconductors Selected values from Tables 18.1, 18.2, and 18.3, Callister 6e. 4 Metals, Insulators and Semiconductors METALS: Electron density is such that valence band ( highest occupied band) is partially (say, half) filled. INSULATORS/SEMICONDUCTORS: Valence band (highest occupied band) is completely filled at low temperature. 5 7/30/2007 Energy Bands ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. The energy levels broaden into bands as the number of electrons grouped together increases. THE VALENCE BAND:The highest energy band that is occupied by one or more electrons is called the valence band. THE CONDUCTION BAND: The next higher vacant level is called the conduction band. Band Theory CONDUCTION BAND BAND GAP BAND GAP VALENCE BAND 6 7/30/2007 Bonding in Metals Electron sea model Nuclei in a sea of e- . Metallic lustre. Malleability. Force applied Bonding in Metals 14 7 7/30/2007 Bonding in Metals Band theory. According to band theory, the electrons in a crystal become free to move when they are excited to the unoccupied orbitals of a band. –N atoms give N orbitals that are closely spaced in energy. N/2 are filled. The valence band. N/2 are empty. The conduction band. Metals - Properties Crystal structure: Atoms arranged in a regular repeating structure Relatively strong Dense Malleable and ductile: high plasticity Resistant to fracture: tough Excellent conductors of electricity and heat Opaque to visible light Shiny appearance 8 7/30/2007 Metal Alloys Alloys are compounds consisting of more than one metal. Adding other metals can affect the density, strength, fracture toughness, plastic deformation, electrical conductivity and environmental degradation. Al-Fe = Iron will make it stronger. Cu-Sn = bronze, Cu-Zn = brass, Fe-C=steel, Fe-C-Cr = Stainless steel Pb-Sn = solder Conductivity of Metals and Alloys Mean free path - The average distance that electrons can move without being scattered by other atoms. Temperature Effect - When the temperature of a metal increases, thermal energy causes the atoms to vibrate Effect of Atomic Level Defects - Imperfections in crystal structures scatter electrons, reducing the mobility and conductivity of the metal Matthiessen’s rule - The resistivity of a metallic material is given by the addition of a base resistivity that accounts for the effect of temperature (ρT), and a temperature independent term that reflects the effect of atomic level defects, including impurities forming solid solutions (ρd). ρ= ρT + ρi + ρd Effect of Processing and Strengthening 9 7/30/2007 Metals: Resistivity vs T, Impurities,defects I) Imperfections increase resistivity 6 (10-8 Ohm-m) Resistivity, ρ --grain boundaries --dislocations --impurity atoms --vacancies 5 4 3 2 1 0 These act to scatter electrons so that they take a less direct path. Ni at% 2 3 . i +3 i Cu t%N at%N a 6 1 2 1.1 + 2. Cu u+ C ed Ni orm f at% e 2 d 1 + 1. Cu Cu re” “Pu -200 -100 0 2) Resistivity increases with: --temperature --wt% impurity --%defect ρ = ρ thermal + ρ imp + ρ def T (°C) from Fig. 18.8, Callister 6e 8 Metals: Resistivity vs T, Impurities,defects ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Movement of an electron through (a) a perfect crystal, (b) a crystal heated to a high temperature, and (c) a crystal containing atomic level defects. Scattering of the electrons reduces the mobility and conductivity. 10 7/30/2007 Estimating Conductivity --Estimate the electrical conductivity of a Cu-Ni alloy that has a yield strength of 125MPa. 180 160 140 120 100 21 wt%Ni 80 60 0 10 20 30 40 50 Resistivity, ρ (10-8 Ohm-m) Yield strength (MPa) • Question: wt. %Ni, (Concentration C) Adapted from Fig. 7.14(b), Callister 6e. 50 40 30 20 10 0 0 10 20 30 40 50 wt. %Ni, (Concentration C) ρ = 30x10 −8 Ohm − m σ= Adapted from Fig. 18.9, Callister 6e. 1 = 3.3x10 6 (Ohm − m) −1 ρ 9 Semi-conductors Semiconductors are materials which have a conductivity between conductors (generally metals) and nonconductors or insulators (such as most ceramics). Semiconductors have special electronic properties which allow them to be insulating or conducting depending on their composition. Semiconductors also have special optical properties when exposed to electricity or light. Several elements are semiconductors, the most important being silicon. Semiconductors can be pure elements, such as silicon or germanium, or compounds such as gallium arsenide or cadmium selenide. 11 7/30/2007 P-type N-type Semiconductor materials Elemental semiconductors : Group IV Elements Silicon and Germanium Compound semiconductors III-V Compounds (e.g. GaAs, InSb) II and VI Compounds (e.g. ZnS, CdSe) III IV V VI Al Si P S Zn Ga Ge As Se Cd In Sn Sb Te II Hg 12 7/30/2007 Properties of Semiconductors Made primarily from metalloids Regular arrangement of atoms (high quality crystals) amorphous silicon, exception, used for thin film transistor and solar cells Extremely controlled chemical purity Adjustable conductivity of electricity Opaque to visible light Shiny appearance Some have good plasticity, but others are fairly brittle. Some have an electrical response to light. Electrons In a Semiconductor electrons In semiconductors we are pimarily interested in the valence band and conduction band. A semiconductor has a “small” small” energy gap For most applications we are interested in what happens near the top of the valence band and the bottom of the conduction band. 13 7/30/2007 Types of Semiconductors in Terms of Band Theory Semiconductors:Holes Holes are empty states in the valence band created by electrons that have jumped to the conduction band It is common to view the conduction process in the valence band as a flow of positive holes toward the negative electrode applied to the semiconductor 14 7/30/2007 Current Process in Semiconductors An external voltage is supplied Electrons move toward the positive electrode Holes move toward the negative electrode There is a symmetrical current process in a semiconductor Doping in Semiconductors Doping is the adding of impurities to a semiconductor Generally about 1 impurity atom per 107 semiconductor atoms Doping changes both the band structure and the resistivity 15 7/30/2007 Intrinsic vs Extrinsic Conduction • Intrinsic: # electrons = # holes (n = p) example :pure Si • Extrinsic: # electrons ≠ # holes (n ≠ p) Example: when impurities are added with a different # valence electrons than the host (e.g., Si atoms) • N-type Extrinsic: (n >> p) • P-type Extrinsic: (p >> n) 12 Extrinsic Semiconductor alloys Extrinsic Semiconductors:Electrical behaviour is directed by the electronic structure of the impurity element Group V elements in Si: P,As,Sb in Si (5 valence e) Group III in Si: Al,B,Ga in Si (3 valence e) n type semiconductor p type semiconductor 16 7/30/2007 Conduction in Terms of Electron and Hole Migration • Concept of electrons and holes: valence electron + no applied electric field electron hole pair migration electron hole pair creation Si atom - + applied electric field applied electric field Adapted from Fig. 18.10, Callister 6e. 11 n-type Semiconductors Negative electrons are charge carriers and so called n-type. Donor atoms are doping materials that contain one more electron than the semiconductor material This creates an essentially free electron with an energy level in the energy gap, just below the conduction band Only a small amount of thermal energy is needed to cause this electron to move into the conduction band 17 7/30/2007 p-type Semiconductors There is a hole in the valence band to accept electron and so called p-type. Electron Acceptor atoms are doping materials that contain one less electron than the semiconductor material A hole is left where the missing electron would be The energy level of the hole lies in the energy gap, just above the valence band An electron from the valence band has enough thermal energy to fill this impurity level, leaving behind a hole in the valence band Pure Semiconductors: Conductivity vs T • Data for Pure Silicon: --σ increases with T --opposite to metals electrical conductivity,σ (Ohm-m)-1 102 101 100 10-1 pure (undoped) 10-2 50 100 Fig. 19.15, Callister 5e. 1000 T(K) −Egap / kT Energy empty band ? GAP filled states 104 103 σundoped ∝ e filled valence band electrons can cross gap at higher T filled band material Si Ge GaP CdS band gap (eV) 1.11 0.67 2.25 2.40 Table 18.2, Callister 6e. 10 18 7/30/2007 Electrical Conduction vs Temperature 1)Thermal energy puts many electrons into a higher energy state. Click to animate 2)Photon effect on conductivity Click to animate Conductivity vs T: Doped Semiconductors 0.0052at%B 102 101 doped 0.0013at%B 100 10-1 pure (undoped) 10-2 50 100 1000 T(K) Adapted from Fig. 19.15, Callister 5e. 1021/m3 of a n-type donor impurity (such as P). --for T < 100K: "freeze-out" thermal energy insufficient to excite electrons. --for 150K < T < 450K: "extrinsic" --for T >> 450K: "intrinsic" doped undoped 3 2 1 Adapted from Fig. 18.16, Callister 6e. intrinsic 104 103 extrinsic conduction... --extrinsic doping level: extrinsic electrical conductivity, σ (Ohm-m)-1 lower the activation energy to produce mobile electrons. • Comparison: intrinsic vs freeze-out --σ increases with doping --reason: imperfection sites conduction electron concentration (1021/m3) • Data for Doped Silicon: 0 0 200 400 600 T(K) 13 19 7/30/2007 Insulators The valence band is completely full of electrons A large band gap separates the valence and conduction bands A large amount of energy is needed for an electron to be able to jump from the valence to the conduction band The minimum required energy is Eg Energy empty band GAP filled states filled valence band filled band Semiconductors: summary of def’ns Intrinsic semiconductor - A semiconductor in which properties are controlled by the element or compound that makes the semiconductor and not by dopants or impurities. Extrinsic semiconductor - A semiconductor prepared by adding dopants, which determine the number and type of charge carriers. Doping - Deliberate addition of controlled amounts of other elements to increase the number of charge carriers in a semiconductor. Thermistor - A semiconductor device that is particularly sensitive to changes in temperature, permitting it to serve as an accurate measure of temperature. Radiative recombination - Recombination of holes and electrons that leads to emission of light; this occurs in direct bandgap materials. 20 7/30/2007 Applications of Semiconductors Diodes, transistors, lasers, and LEDs are made using semiconductors. Silicon is the workhorse of very large scale integrated (VLSI) circuits. Forward bias - Connecting a p-n junction device so that the pside is connected to positive. Enhanced diffusion occurs as the energy barrier is lowered, permitting a considerable amount of current can flow under forward bias(bias :meyil/eğilim). Reverse bias - Connecting a junction device so that the p-side is connected to a negative terminal; very little current flows through a p-n junction under reverse bias. Avalanche breakdown - The reverse-bias voltage that causes a large current flow in a lightly doped p-n junction. Transistor - A semiconductor device that can be used to amplify electrical signals. Semiconductor Devices Advantages of semiconductor devices( solid-state devices): 1. Small size 2. Low power consumption 3. No warmup time p-n Rectifying junction: A rectifier or diode is an electronic device that allows the current to flow in one direction only A diode transforms an alternating current into direct current Rectifying: düzeltmek, alternatif akımı doğru akıma çevirmek 21 7/30/2007 p-n Rectifying Junction • Processing: A single piece of semiconductoris doped so as to be n-type on one side and p-type on the other. Diffuse P(Gr VA) into one side of a B-doped (Gr.IIIA) crystal. p-type n-type + + + + + 1)No applied potential: no net current flow. 2)Forward bias: carrier flow through p-type and n-type regions; holes and electrons recombine at p-n junction; current flows. 3)Reverse bias: carrier flow away from p-n junction; carrier conc. greatly reduced at junction; little current flow. p-type + - + - - - - - + - n-type + + - - +- +p-type + + + + n-type - - - + 14 Behavior of a p-n junction device: (a) When no bias is applied electron and hole currents due to drift and diffusion cancel out and there is no net current (b) Under a forward bias causes the potential barrier to be reduced causing a current to flow. (c) Under reverse bias, the potential barrier increases and very little current flows. (Source: From Solid State Electronic Devices, Third Edition, by. B.G. Streetman, Fig. 5-10. Copyright © 2000 Prentice Hall. Reprinted by permission Pearson Education, Inc., Upper Saddle River, NJ.) Drift: yığılmak, birikmek, sürüklenmek. 22 7/30/2007 Applications of Semiconductors Semiconductors can be made into transistors which can store a digital (on or off) signal. Transistors are used in all electronics--computers, calculators, stop watches, amplifiers, etc. Semiconductors can also be made to emit light by exposing them to an electric field (light emitting diodes (LEDs) and diode lasers). Semiconductors are used in computers (45%), consumer products (23%), communications equipment (13%), manufacturing industries (12%), automobiles (5%), and by the military (2%). Semiconductors make up a huge, and growing, industry worldwide with billions of dollars in sales by semiconductor companies in North America, Europe, and the Asia-Pacific region. Advances in the field of electronics can continue to improve our lives. Photovoltaic Cells 23 7/30/2007 Moore's Law (1965 then 1975 ) the number of transistors on a chip doubles about every two years. Moore's Law (1975) In 1965, a single transistor cost more than a dollar. By 1975, the cost of a transistor had dropped to less than a penny, while transistor size allowed for almost 100,000 transistors on a single die. From 1979 to 1989, to 1999, processor performance went from about 1.5 million instructions per second (MIPS), to almost 50 MIPS on the i486™, to over 1,000 MIPS on the Intel® Pentium® III. Today's Intel® processors, some topping out at well above 1 billion transistors, run at 3.2 GHz and higher, deliver over 10,000 MIPS, and can be manufactured in high volumes with transistors that cost less than 1/10,000th of a cent. 24 7/30/2007 Moore's Law (1975) Teramac Computer,1999 25 7/30/2007 A current bit on a hard drive is 300 nm by 100 nm The short history of Silicon Transistors: In 1955, Texas Instruments work to develop silicon transistors for military use. It was recognized that the germanium transistors then available would not be able to perform at the high temperatures common to military applications. TI had invented the silicon transistor, the first of its kind. 26 7/30/2007 What is a transistor? CPU (Central Processing Unit) The "brain" of the computer that performs most computing tasks. In microcomputers, the entire CPU is on a single chip. Also called a processor/microprocessor. The on and off switches found inside every single piece of computer equipment. In a computer, this on/off ability is used to represent binary digits (bits). A CPU contains millions of transistors. www.pcstats.com/glossary.cfm An active solid-state device in which larger output current is obtained by small changes in the input current.www.sfxaudio.com/AudioSchool/glossary.asp Transistors are tiny electrical devices that can be found in everything from radios to robots. They have two key properties: 1) they can amplify an electrical signal and 2) they can switch on and off, letting current through or blocking it as necessary. www.pbs.org/transistor/glossary.html A device used to amplify a signal or open and close a circuit. In a computer, it functions as an electronic switch.www.synopsys.com/news/pr_kit/eda_glossary.html An electronic device that can regulate electricity and act as an on/off switch. www.pccomputernotes.com/pcterms/glossaryt.htm An active component of an electronic circuit consisting of a small block of semiconducting material. It may be used as an amplifier, detector, or switch. www.csa.com/hottopics/mems/gloss.php A small chip of semiconductor material that amplifies or switches electrical current. www.southerncompany.com/learningpower/glossary.asp Transistors made from purified silicon. There are also polymer transistors. www.rolltronics.com/links/glossary.html View of an Integrated Circuit • Scanning electron microscope images of an IC: Al Si (doped) (d) (a) 45µm 0.5mm • A dot map showing location of Si (a semiconductor): --Si shows up as light regions. (b) • A dot map showing location of Al (a conductor): --Al shows up as light regions. Fig. (d) from Fig. 18.25, Callister 6e. (Fig. 18.25 is courtesy Nick Gonzales, National Semiconductor Corp., West Jordan, UT.) (c) Fig. (a), (b), (c) from Fig. 18.0, Callister 6e. 2 27 7/30/2007 Microelectronic Circuit SEM of a small region of a microprocessing chip( 0.5 MB)Narrow white regions Al wiring,the gray regions doped Si ( x2000) Optical Photomicrograph of a part of a circuit to test microprocessing chips( 0.5 MB)Narrow light regions Al wiring,the white squares are test semiconductor devices ( x50) 28 7/30/2007 The Transistor Capable of two primary types of function can amplify an electrical signal serve as switching devices in computers for the processing and storage of information •Two major types 1)The junction(bipolar)transistor 2)Metal-Oxide-Semiconductor Field-Effect Transistor ( MOSFET) The Junction(bipolar)transistor Two p-n junctions arranged back to back oA very thin n type base is sandwiched in between p type emitter and p type collector. oEmitter p is forward bias, so that large number of holes enter n-type base region. oThese holes are minorty type carriers in n-type base,some will combine with electrons. oMost of them will be pass across junction 2 to collector. oThe holes become a part of the emittercollector circuit. oA small increase in input voltage produces a large increase in output voltage. 29 7/30/2007 The Junction(bipolar)transistor Bipolar transistors find important uses as amplifiers, drivers of other devices, and certain high speed digital circuits. Metal-Oxide-Semiconductor Field-Effect Transistor ( MOSFET) oTwo Two small islands of pp-type semiconductors in n-type silicon. silicon. 30 7/30/2007 Insulators and Dielectric Properties Materials used to insulate an electric field from its surroundings are required in a large number of electrical and electronic applications. Electrical insulators obviously must have a very low conductivity, or high resistivity, to prevent the flow of current. Porcelain, alumina, mica, and some glasses and plastics are used as insulators. Dielectric Materials,Polarization A dielectric material is electrically insulating and may exhibit a dipole structure. Dielectric materials are used in capacitors and insulators. eg.glass,porcelain,mica, TiO2 and BaTiO3 Polarization:The process of dipole alignment. Types: Electronic,ionic,orientation Figure shows an electric dipole generated by two electric charges separated by distance d P is polarization vector 31 7/30/2007 Polarization in Dielectrics Capacitor - A microelectronic device, constructed from alternating layers of a dielectric and a conductor, that is capable of storing a charge.These can be single layer or multilayer devices. Dielectric strength - The maximum electric field that can be maintained between two conductor plates without causing a breakdown. (a) a charge can be stored at the conductor plates in a vacuum (b) when a dielectric is placed between the plates the dielectric polarizes and additional charge is stored. 32 7/30/2007 Ferroelectric Materials,Piezoelectric Materials Ferroelectrics are dielectric material which exhibit continuous polarity even in the absence of external electric field. Have extremely high dielectric costants Ferroelectric capacitors are smaller than the other capacitors. eg. Pb(ZrO3 )(TiO3), Bismuth tungstate Piezoelectricity is the phenomenon where polarization is induced in a material by the application of external forces. Piezoelectrics - Materials that develop voltage upon the application of a stress and develop strain when an electric field is applied. Application: pickups, microphones, sonar detectors etc. PbZrO3 , quartz etc. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. The (a) direct and (b) converse piezoelectric effect. In the direct piezoelectric effect, applied stress causes a voltage to appear. In the converse effect (b), an applied voltage leads to development of strain. 33 7/30/2007 34