Historical Timelines Properties Related To Band Theory-Books Pdf

Historical Timelines Properties Related to Band Theory
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China Semiconductor Consumption The Electronics Ecosystem. China to grow faster than the world at,CAGR Compound Annual Growth Rate of Estimate. 17 from 2001 2005 reaching 27B in 2001 2004,2005 Electronic End Equipment 879B 990B. 25 27 0 Semiconductor 139B 218B,China currently s,produces only 1. Billions US,Semiconductor,15 of every 4 chips SEMI Equipment 28B 46B. 10 12 8 it consumes MEMBERSHIP,Temperature Dependence of the Electrical.
Three Types of Solid Materials Conductivity of Metals and Semiconductors. Based on Electrical Conductivity Isolators,alloy increasing. resistivity,resistivity below Tc 0,insulators semiconductors metals. diamond germanium copper,fused glass silicon iron decreasing. silica isolator resistivity,10 24 10 20 10 16 10 12 10 8 10 4 100 10 4 10 8. Conductivity 1cm 1, Electrical Conductivity Creation of Carriers in Intrinsic.
Semiconductors by Thermal Excitation,Thermally induced electrical. conductivity,Conduction band empty,Valence band completely filled. No electrical conductivity,The thermal energy is responsible for. the promotion of electrons to the, Conductivity of metals decreases with conduction band. temperature as atomic vibrations scatter free,electrons Creation of electron hole pairs.
Conductivity of semiconductors increases with carriers. temperature as the number of carriers increases electrical conductivity. Semiconductors,Experimental Observation, Conductivity of Semiconductors Intrinsic Semiconductors If a semiconductor. crystal contains no impurities the only charge, Semiconductor block carriers present are thus produced by thermal. connected to the terminals of breakdown of the covalent bonds The. a battery conducting properties are thus characteristic. No conductivity observed at of the pure semiconductor Such a crystal is. low or room temperature or termed an intrinsic semiconductor. in the dark Extrinsic Semiconductors If a semiconductor. crystal contains n type or p type impurities,When we increase the. the conducting properties are chiefly due to,temperature or expose the. the impurities Such a crystal is termed an,semiconductor to light we.
extrinsic semiconductor,observe that it starts,conduction. Conductivity of Intrinsic Semiconductors,the Response of Equilibrium to. The valence band of semiconductors is, completely filled However the band gap Temperature. between the valence and conduction,bands is small and electrons can be. promoted to the conduction band, In semiconductors only the electrons The van t Hoff.
promoted to the conduction band and the equation, holes created in the valence band will be d ln K H 0. The smaller the gap the easier to,promote electrons to the conduction band. At the same temperature smaller gap d ln K P H o,semiconductors will show a larger. conductivity Eg,The higher the temperature the larger. the number of carriers Ce 2K BT,ln K 2 ln K 1,Conductivity increases with temperature 1 T2 1 T1.
Temperature Effects,Carrier Mobility,The intrinsic carrier. Intrinsic semiconductors mobility is defined,Concentration of holes and as the drift velocity. free electrons increase with per unit electric,temperature Because field. increasing thermal energy will VD,excite more e across the band. Ge has a greater charge,concentration than Si,Because Ge has a smaller.
Similar to metals charge carriers in,band gap than Si 0 67 vs 1 11. semiconductors lose mobility with increasing,dopant concentration. Carrier Mobility,Delocalized Bonding Model,Conduction band. Valence band,Temperature also affects carrier mobility Note. regardless of dopant concentration high temperatures. reduce mobility,Semiconductors and Acid Base Analogy.
Bonding Picture of Silicon,Chemical Equilibrium in Solution. H 1014 ions cm3,Chemical Equilibrium in Solid,Delocalized bonding picture. Si crystal h e,h 1 5x1010cm,mobile holes acid species 3. electrons basic species,Extrinsic Semiconductors,Donor States n type Semiconductors. Donor States n type Semiconductors,If an atom in the lattice is.
substituted by an atom of a,different element with more. valence electrons once the,impurity is accommodated to. the lattice and the new bonds,are formed there will be a. remaining negative charge,Example Pentavalent Sb,impurity in a silicon crystal Valence Band. tetrahedrally coordinated,Extrinsic Semiconductors.
Acceptor States p type Semiconductors,Acceptor States p type Semiconductors. If an atom in the lattice is,substituted by an atom of a. different element with less,valence electrons once the. impurity is accommodated to,the lattice and the new bonds. are formed there will be a,remaining positive charge.
Example Trivalent boron B,impurity in a silicon crystal. Valence Band,tetrahedrally coordinated, Intrinsic vs Extrinsic Extrinsic Doped Semiconductors. We can enhance the electrical, Extrinsic doped properties of a semiconductor n type p type. Intrinsic by adding impurities to it The semiconductors semiconducto. n type p type addition of impurities is called Addition of rs Addition of. doping and the doped donor states acceptor, Conduction Conduction Conduction semiconductor is called states. e Band Band Band extrinsic,Example the addition of 1.
Boron atom every 105 Silicon,atoms enhance the,conductivity of Silicon by a. h factor of 103 at room,Valence Valence Valence temperature. Band Band Band Extrinsic semiconductors are Examples Examples. the basic materials in the P As or Sb B Al Ga or, Silicon Si P Si Al electronics technology Great impurities In. importance in current in Si or Ge impurities,technology lasers solar cells in Si or Ge. rectifiers transistors,Extrinsic n type, Band Diagram n and p type Temperature Effects Semiconductors.
Ed Low Temperatures,Thermal energy is,n insufficient to excite. type electrons from the donor,Donor Level in Band, Electrons can jump from Gap Intermediate Temperatures. P atom to Conduction,Band e s from donor state are. excited into the conduction,band e concentration,equal to dopant. p type concentration,High Temperatures, Acceptor level in Band p types behave similarly Enough thermal energy to.
Electrons can excite an effective amount,jump to Al atom Gap with temperature. of valence e s into the,conduction band,Analogy Between pH and Fermi Level Ef. Fermi Level,undoped semiconductor,Ef Ef The pH of,solutions and the. Fermi level in, p type semiconductor n type semiconductor semiconductors. play analogous,Ef roles in,Ed determining the,ionization in the.
Extent of Ionization, Weak Acid Acceptor Analogy Energy Levels for Impurities in Silicon. Semiconductor,Acid base system A h A e,pK a log K a Acceptors. When pH pKa h,HA A Ea acceptor energy level,When Ef Ea A. Optical Properties of Semiconductors,Interaction of Light and Electrons. Absorption Emission,absorption,spontaneous,stimulated.
longest wavelength absorption to promote e,corresponds to Eg. Eg is energy between HOMO of valence band,and LUMO of conduction band. Band Gap Energy and Color Semiconductor Glossary, Color that Apparent color Direct Bandgap Semiconductor semiconductor in. corresponds to,band gap energy,of material, unabsorbed light which the bottom of the conduction band and the top. of the valence band occur at the momentum k 0 in,this case energy released during band to band.
ultraviolet colorless electron recombination with a hole is converted. primarily into radiation radiant recombination,Bandgap energy eV. wavelength of emitted radiation is determined by the. yellow energy gap of semiconductor,green orange e g GaAs InP etc. Indirect Bandgap Semiconductor semiconductor in, which bottom of the conduction band does not occur. 1 infrared black at effective momentum k 0 i e is shifted with respect. to the top of the valence band which occurs at k 0. energy released during electron recombination with a. hole is converted primarily into phonon e g Si Ge, Bandstructure in Three Dimensions Bandstructure in Three Dimensions. An important property of direct semiconductors is that In indirect semiconductors the bottom of the conduction band. electrons may easily drop from the conduction band to the and the top of the valence band occur at different points in. valence band by emitting a photon k space, This process is known as electron hole recombination since An electron cannot therefore drop from the conduction band to.
the valence band just by emitting a photon since this would violate. the electron drops to occupy a hole state in the valence band momentum conservation. the energy of the photon emitted by the semiconductor is instead the electron must simultaneously emit a photon and. determined by the size of its energy gap exchange momentum with the crystal lattice. recombination is therefore analogous to the level the probability of this double process occurring is very small so. transitions that occur in atomic systems indirect semiconductors turn out to be much poorer emitters of. light than direct ones,Electron hole Recombination in a direct. E semiconductor such as GaAs E Electron hole recombination in an indirect. An electron drops from the conduction Semiconductor. band to the valence band and its excess In order to conserve energy and momentum. energy is emitted in the form of a photon an electron must drop to the valence band by. PHOTON Note that in the figure shown here the emitting a photon and exchanging momentum. initial and final wavevector states are the,PHOTON with the crystal. same this is an important property of Because this process has a low probability. k k indirect semiconductors such as Si or Ge cannot. direct semiconductors,be used in optoelectronic applications as light. Bandstructure in Three Dimensions Luminescence, The opposite process to recombination is electron hole. generation in which an electron is excited from the valence. band into the conduction band by absorbing a photon. Since this process also must conserve momentum the. electron is excited into a state with the same k value as the. initial valence band state, Both direct and indirect semiconductors may therefore.
be used as photodetectors to detect electromagnetic. The absorption of these materials strongly increases once. the photon energyE exceeds the direct Eband gap, absorption of light by direct left and indirect right. semiconductors,What s Luminescence,Solid line,Dotted line. Matched system to,reduce the strain,effect and epitaxial. growth defects,The spontaneous emission of light upon electronic. excitation is called luminescence,Absorption and Luminance.
p n Junction,What happens if we bring a,p type semiconductor in. contact with a n type,semiconductor,Electrons close to the. junction diffuse across the e,junction into the p type. region Holes are filled by,recombination, Equilibrium is established resulting in a potential. difference,If the two regions are connected in a circuit a.
variety of applications are possible,Biasing the p n Junction. Majority Carrier and Current Flow,in p type Silicon. Biasing introduction,of a voltage into the circuit. containing the p n junction,Forward bias negative,voltage is applied to n type. side Decreases energy,n p p Type Silicon,barrier for electrons and.
holes to flow through the Hole Flow,Reverse bias positive Current Flow. voltage applied to n type,side Raises energy barrier. for current flow,Majority Carrier and Current Flow. the p n Junction,in n type Silicon,n Type Silicon Hole Diffusion. Electron Flow Electron Diffusion,Current Flow,Holes and Electrons.
Recombine at the, A Depletion Zone D and a Barrier Field Forward Bias of a p n Junction. Forms at the p n Junction,Barrier p n,Field Volts,p D n Current. Acceptor Ions Donor, Hole Diffusion Ions Applied voltage reduces the barrier field. Holes and electrons are pushed toward the,junction and the depletion zone shrinks in size. The Barrier Field Opposes Carriers are swept across the junction and the. Further Diffusion depletion zone, Equilibrium Condition There is a net carrier flow in both the p and n sides.
current flow,Reverse Bias of a p n Junction,Forward Bias. Holes and free,p D n electrons flow together,Volts and recombine at the. Current Volts junction Current flows,Applied voltage adds to the barrier field. Holes and electrons are pulled toward the Reverse Bias. terminals increasing the size of the depletion zone Holes and free. The depletion zone becomes in effect an insulator electrons flow away. for majority carriers from each other The, Only a very small current can flow due to a small center of the diode. number of minority carriers randomly crossing D quickly becomes a. reverse saturation current dead zone with no,charge carriers.
Current is reduced,Applications of p n Junction Diode. Energy diagram of p n Junction, When p type and n type semiconductors touch Why call it p n Junction as a Diode. the Fermi levels do not align until equilibrium is. reached Rectifier,Photodetectors,solar cells,Optoelectronics. diode lasers,Simple Application Rectifier,PN junction. One of the most important uses of a diode is,rectification The normal p n junction diode is.
well suited for this purpose as it conducts very,heavily when forward biased low resistance. direction and only slightly when reverse biased, high resistance direction If we place this diode in. series with a source of ac power the diode will be. forward and reverse biased every cycle Since in this. situation current flows more easily in one direction. than the other rectification is accomplished,p n Rectifying Junction Optoelectronics. In optoelectronic applications of semiconductor, devices the basic idea is that the device is used to. either detect or to emit electromagnetic radiation. In detection the incident light is converted into a. measurable electrical signal by exploiting internal. carrier processes within the device Examples of, such devices include photodetectors and solar cells.
In emission on the other hand the internal, processes allow the conversion of an electrical signal. into detectable light and examples of such devices. A diode s properties can be seen when the voltage include LEDs and lasers. is examined,Photodetector LED Light Emitting Diodes. LEDs are p n junction devices constructed of gallium. Photodetector converts optical energy into electrical arsenide GaAs gallium arsenide phosphide GaAsP. energy thus making possible data reading in the or gallium phosphide GaP Silicon and germanium. are not suitable because those junctions produce heat. optical storage systems such as CD or DVD drives and no appreciable IR or visible light. Modern photodetectors are typically semiconductor The junction in an LED is forward biased and when. photodiodes electrons cross the junction from the n to the p type. so called reverse bias p n photodiode with the material the electron hole recombination process. carriers flowing away from the p n junction thus produces some photons in the IR or visible in a process. called electroluminescence An exposed semiconductor. creates a depletion region There is very little current surface can then emit light. flowing through this junction until the light illuminates. the surface of the photodiode Then the absorbed, photons create pairs of electrons and holes mostly. in the depletion area Those new carriers move quickly. in opposite directions and moving electrons create.


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