Semiconduct Historical timelines Properties Related to Band Theory History of Semiconductors + Pn unction a Photoelectricity Semiconductor Products are Proliferating The Next Big Thing . A Lot of Little Things 中画 aaIn China Semiconductor Consumption The Electronics Ecosystem China to grow faster than the world at CagR (Compound Annual Growth Rate)of 17% from very 4 chips it
2005-11-11 1 Properties Related to Band Theory History of Semiconductors Color Conductivity Photoluminescence P-n Junction Photoelectricity Semiconductors Historical Timelines Semiconductor Products are Proliferating LANs WANs Routers Hubs Switches Workstations Internet Servers Video Games Voice Over IP Digital Cameras Wireless Handsets PDAs Storage PCs Systems Set-Top Boxes Internet Browsers Scanners Digital Copiers Internet The Next Big Thing… A Lot of Little Things $27.0 $12.8 0 5 10 15 20 25 30 2000 2005 Billions US$ China Semiconductor Consumption China to grow faster than the world at CAGR (Compound Annual Growth Rate) of 17% from 2001-2005, reaching $27B in 2005 China currently produces only 1 of every 4 chips it consumes The Electronics Ecosystem Materials Semiconductor Equipment Semiconductors Electronic End Equipment SEMI MEMBERSHIP $990 B 2001 Estimate 2004 $21B $28B $139 B $879 B $28B $46B $218 B
Band gap energy and color Bandgap and Conductivity in group 4A Element Unit cell A insulator ultraviolet colorless conductor as Unit Cell increases, band gap energy o E decreases cmajor factor is lattices, the band splitting also increase eases in tighter rlap; as it in intrared Band Gap and Periodic Properties Delocalized Bonding Model ·· electrons 7L08270■A0elw holes elso, E increases as the unit cell size decreases Electrical Conductivit Bonding Picture of Silicon Delocalized bonding picture e Conductivity of metals decreases with temperature as atomic vibrations scatter free electron a Conductivity of semiconductors increases w emperature as the number of carriers increa
2005-11-11 2 Band Gap Energy and Color . Bandgap energy (eV) Color that corresponds to band gap energy Apparent color of material (unabsorbed light) 4 3 2 1 red yellow green blue violet colorless black yellow orange ultraviolet infrared red Bandgap and Conductivity in Group 4A as Unit Cell increases, band gap energy (Eg ) decreases major factor is orbital overlap; as it increases in tighter lattices, the band splitting also increases Element Unit Cell(Å) Eg (eV) l (nm) C 3.57 5.5 230 insulator Si 5.43 1.1 1100 semiconductor Ge 5.66 0.66 1900 semiconductor a-Sn 6.49 <0.1 12000 conductor Band Gap and Periodic Properties Note that Eg increases as the Pauling electronegativity difference, Dc, increases (the compound gets more polar). Also, Eg increases as the unit cell size decreases. Material Unit Cell (Å) D c Eg ( e V) l (nm) Color G e 5.66 0 0.66 1900 black GaAs 5.65 0.6 1.42 890 black ZnSe 5.67 0.8 2.70 460 yellow CuBr 5.69 0.9 2.91 430 white Delocalized Bonding Model energy Conduction band Valence band electrons holes Bonding Picture of Silicon Delocalized bonding picture Electrical Conductivity Conductivity of metals decreases with temperature as atomic vibrations scatter free electrons. Conductivity of semiconductors increases with temperature as the number of carriers increases
Three Types of Solid Materials Temperature Dependence of the Electrical Based on Electrical Conductivity Conductivity of Metals and Semiconductors (Isolators) lenr -, resistivity below T=0!1 semiconductors metals olator /-decreasing resistivity Conductivity @-lcm.l) Semiconductors by Thermal Excitati jon Experimental observatic onductivity of Semiconductors =0K d Semiconductor block connecte onduction band empty to the terminals of a battery lence band completely filled No electrical conductivity a No conductivity observed at low or room temperature or in the >>0K dark The thern gy is responsible for the e When we increase the or expose the observe that it starts conduction aElectrical conductivity Semiconductors Conductivity of Intrinsic Semiconductors The valence band of semiconductors b Intrinsic Semiconductors: If a semiconductor mplen the valence and conduction bands bonds. T ducting properties are thus characteristic of the pure onductors. only the a crystal holes created in the valence ba he smaller the gap, the e ucting properties are chiefly due to the At the same temperature, smaller gap The higher the temperature, the larger the a=Ce Conductivity increases with temperature
2005-11-11 3 Three Types of Solid Materials ¾¾ Based on Electrical Conductivity 10 -20 -16 -12 -8 -4 4 8 10 10 10 10 10 10 10 glass diamond fused silica silicon germanium iron copper insulators semiconductors metals -24 0 10 Conductivity (W -1cm-1 ) = isolator = alloy ® increasing resistivity ® resistivity below Tc = 0 !! ® decreasing resistivity Temperature Dependence of the Electrical Conductivity of Metals and Semiconductors (Isolators) Creation of Carriers in Intrinsic Semiconductors by Thermal Excitation Thermally induced electrical conductivity T=0 K Conduction band empty Valence band completely filled No electrical conductivity T>>0 K The thermal energy is responsible for the promotion of electrons to the conduction band. Creation of electron-hole pairs: carriers Electrical conductivity Experimental Observation: Conductivity of Semiconductors Semiconductor block connected to the terminals of a battery No conductivity observed at low or room temperature or in the dark. When we increase the temperature or expose the semiconductor to light, we observe that it starts conduction. Semiconductors Intrinsic Semiconductors: If a semiconductor crystal contains no impurities, the only charge carriers present are thus produced by thermal breakdown of the covalent bonds. The conducting properties are thus characteristic of the pure semiconductor. Such a crystal is termed an intrinsic semiconductor. Extrinsic Semiconductors: If a semiconductor crystal contains n-type or p-type impurities, the conducting properties are chiefly due to the impurities. Such a crystal is termed an extrinsic semiconductor. Conductivity of Intrinsic Semiconductors ßThe valence band of semiconductors is completely filled. However, the band gap between the valence and conduction bands is small, and electrons can be promoted to the conduction band. ßIn semiconductors, only the electrons promoted to the conduction band and the holes created in the valence band will be carriers. ßThe smaller the gap, the easier to promote electrons to the conduction band. ßAt the same temperature, smaller gap semiconductors will show a larger conductivity. ßThe higher the temperature, the larger the number of carriers. ßConductivity increases with temperature 2K T E B g Ce - s =
the Response of Equilibrium to Temperature Effects Temperature Intrinsic semiconductors The vant Hoff equation b Concentration of holes and mperature. Because ncreasing thermal energy will xcite more e across the ban 息 soncentars ti gn thtr sa rgeca Inky-InK,- aller band gap than Si(0.67w1.11) Carrier mobility arrier Mobility The intrinsic carrier Similar tals, charge carriers in onductors th increasing dopa entration Temperature also affects carrier mobility. Note regardless of dopant concentration, high temperatures reduce mobility. Semiconductors and Acid- Base analog Donor States: n-ty Se emiconductors Chemical Equilibrium in Solution aIf an atom in the lattice is H3O→H++OH substituted by an atom of a K=[H+OHI H+]≈101 ions/en3 lence electrons, once th impurity is accommodated to N-Type Chemical Equilibrium in Solid the lattice and the new bonds are formed. there will be a remaining negative charge Example: Pentavalent s Skervstal"→ht+e purity in a silicon cry mobile holes acid species K=lh*lleI=pn (tetrahedrally coordinated) electrons:basic species
2005-11-11 4 the Response of Equilibrium to Temperature The van’t Hoff equation 2 1 0 2 1 2 0 1 1 1 / T /T H /R lnK ln K RT H dT dlnK R H d( / T) d(lnK) o P - D - = - D = D = - Temperature Effects Intrinsic semiconductors Concentration of holes and free electrons increase with temperature. Because increasing thermal energy will excite more e- across the band gap. Ge has a greater charge concentration than Si. Because Ge has a smaller band gap than Si (0.67 vs 1.11) Carrier Mobility Similar to metals, charge carriers in semiconductors lose mobility with increasing dopant concentration. The intrinsic carrier mobility is defined as the drift velocity per unit electric field. Carrier Mobility Temperature also affects carrier mobility. Note regardless of dopant concentration, high temperatures reduce mobility. mobile holes:acid species electrons:basic species Semiconductors and Acid-Base Analogy Chemical Equilibrium in Solution H2O ® H++OHKw=[H+][OH- ] [H+]»1014 ions/cm3 Chemical Equilibrium in Solid Si(crystal)®h++eK=[h+][e- ]=p•n [h+]»1.5x1010cm-3 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 (tetrahedrally coordinated)
Effect of Doping on Conductivity Acceptor States: p-type Semiconductors TIf an atom in the lattice is substituted by an atom of a 置, valence electrons. once the purity is accommodated to P-Type the e formed. there will be a remaining positive charge. ole: Trivalent (B) n a silicon tetrahedrally coord Intrinsic vs. extrinsi Effect of Doping on Conductivity Conduction Conduction Electron vacancies domine ato 曹曹鲁曹 valence valen Valence Band Silicon Extrinsic (Doped) Semiconductors Band Diagram(n and p type) wWe can enhance the electrical n-type Addition of donor Addition of oping and the doped m}a出e Donor Level in Band Gap alert ron cond jutop amdP a factor of 10 electronics technology.Great p type m跟镏hm如xamB Si or Ge Electrons ea to Al atom
2005-11-11 5 Effect of Doping on Conductivity Valence Band 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 (tetrahedrally coordinated) Effect of Doping on Conductivity Valence Band Intrinsic vs. Extrinsic Valence Band Conduction Band Intrinsic Extrinsic (doped) Conduction Band Valence Band h+ e- Conduction Band Valence Band n-type p-type Silicon Si:P Si:Al Extrinsic (Doped) Semiconductors We can enhance the electrical properties of a semiconductor by adding impurities to it. The addition of impurities is called doping and the doped semiconductor is called extrinsic. Example: the addition of 1 boron atom every 105 silicon atoms enhance the conductivity of silicon by a factor of 103 at room temperature. Extrinsic semiconductors are the basic materials in the electronics technology. Great importance in current technology: lasers, solar cells, rectifiers, transistors, ... p-type semiconductors Addition of acceptor states n-type semiconductors Addition of donor states Examples: P, As, or Sb impurities in Si or Ge. Examples: B, Al, Ga, or In impurities in Si or Ge. Band Diagram (n and p type) Electrons can jump to Al atom Electrons can jump from P atom to Conduction Band n type p type Acceptor level in Band Gap Ea Donor Level in Band Gap Ed
Temperature Effects Semiconductors Fermi level doped semiconductor =‖ from the donor state ntermediate Temperatures to dopant concentration qual/ d Enough thermal energy to E, Altav-Er xcite an effective amount of alence e-s into the p-types behave similarly nduction band with temperatur Analogy Between pH and Fermi Level E) Extent of ionization Weak Acid- Acceptor Analogy A台h"+A lutions and the HA←H+A [H+IA-I play analogous HAl oles in pK,=-log K in the two media When pH=pk. E,=acceptor energy leve/ s When Er- ea LAJ- IA-I Energy Levels for Impurities in Silicon Interaction of light and Electrons shallow 一Pm040一n·8pm aDsorption Donors 0s一0怕 Spontaneous Acceptors oM emIssIo shallow
2005-11-11 6 Temperature Effects undoped Extrinsic n-type Semiconductors Low Temperatures Thermal energy is insufficient to excite electrons from the donor state Intermediate Temperatures e -’s from donor state are excited into the conduction band. e- concentration equal to dopant concentration. High Temperatures Enough thermal energy to excite an effective amount of valence e-’s into the p-types behave similarly conduction band with temperature Fermi Level Ef metal Ef undoped semiconductor Ef Ef p-type semiconductor n-type semiconductor Ed Ea The pH of aqueous solutions and the Fermi level in semiconductors play analogous roles in determining the extent of ionization in the two media. Analogy Between pH and Fermi Level (Ef ) Extent of Ionization: Weak Acid - Acceptor Analogy When pH = pKa [HA] = [A- ] Acid-base system Semiconductor When Ef ~ Ea [A] ~ [A- ] Ea = acceptor energy level a a a pK log K [HA] [H ][A ] K HA H A = - = « + + - + - + - A « h + A Energy Levels for Impurities in Silicon Donors Acceptors e - h+ shallow shallow deep Interaction of Light and Electrons absorption Spontaneous emission Stimulated emission
Optical Properties of Semiconductors Semiconductor Glossary mission Direct Bandgap Semiconductor: semiconductor in which the ottom of the conduction band and the top of the valence se, energy ased during band-to-band electron recombination with g GaAs, InP, ete. Indirect Bandgap Semiconductor semiconductor in which m longest wavelength absorption to promote e corresponds to E a上=1、 i hifted with respect to the top of the electron recombina th a hole is converted primarily TE is energy between"HOMO"of valence band and into phonon 'e. g. Si, Ge, GaP LUMo"of conduction band Bandstructure in Three dimensions Bandstructure in Three dimensions easily drop from the conduction band to the valence band by reet semiconductor we have seen that the bottom of the n band and the top of th ce this would violate momen to t by the size of its energy gap E instead the electron must simultaneously emit a photon and exchange recombination is therefore analogous to the level transitions that ng is very small, so indireet In atomic systems rn out to be much poorer emitters of light than direct one ELectronhole Recombination in a direct semiconductor such as GaAs nElectrorrhole recombination in an indireet a An electron drops from the conduction band g and momentum. an aNote that in the figure shown here the initial his is an important property of direct 地四山Su semiconductors Bandstructure in Three dimensions Luminescence from the valence band into then cegs to recombination is (GaAs) alence-band stat Both direct and indirect se luctors may therefore be used as The absorption of these materials strongly absorptionof light by direct (left)and indirect (right) semiconductor
2005-11-11 7 Optical Properties of Semiconductors longest wavelength absorption to promote ecorresponds to Eg Eg is energy between “HOMO ”of valence band and “LUMO”of conduction band Eg Absorption Emission Eg Semiconductor Glossary Direct Bandgap Semiconductor: semiconductor in 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 electron recombination with a hole is converted primarily into radiation (radiant recombination); wavelength of emitted radiation is determined by the energy gap of semiconductor. e.g. GaAs , InP, etc. Indirect Bandgap Semiconductor: semiconductor in which bottom of the conduction band does not occur 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, GaP. An important property of direct semiconductors is that electrons may easily drop from the conduction band to the valence band by emitting a photon This process is known as electron-hole recombination since the electron drops to occupy a hole state in the valence band Þ the energy of the photon emitted by the semiconductor is determined by the size of its energy gap Þ recombination is therefore analogous to the level transitions that occur in atomic systems Bandstructure in Three Dimensions E k PHOTON Electron-hole Recombination in a direct semiconductor such as GaAs An electron drops from the conduction band to the valence band and its excess energy is emitted in the form of a photon Note that in the figure shown here the initial and final wavevector states are the same … this is an important property of direct semiconductors In indirect semiconductors, we have seen that the bottom of the conduction band and the top of the valence band occur at different points in k-space An electron cannot therefore drop from the conduction band to the valence band just by emitting a photon since this would violate momentum conservation Þ instead the electron must simultaneously emit a photon and exchange momentum with the crystal lattice Þ the probability of this double process occurring is very small, so indirect semiconductors turn out to be much poorer emitters of light than direct ones Bandstructure in Three Dimensions E k PHOTON Electron-hole recombination in an indirect Semiconductor In order to conserve energy and momentum, an electron must drop to the valence band by emitting a photon and exchanging momentum with the crystal Because this process has a low probability, indirect semiconductors such as Si or Ge cannot be used in optoelectronic applications as light emitters 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 Þ Both direct and indirect semiconductors may therefore be used as photodetectors to detect electromagnetic radiation Þ The absorption of these materials strongly increases once the photon energy exceeds the direct band gap Bandstructure in Three Dimensions E k PHOTON absorptionof light by direct (left) and indirect (right) semiconductors E k PHOTON Luminescence
What's Luminescence? terials Dotted line. bandgap materials Matched system to growth defects! The spontaneous emission of light upon electronic excitation is d luminescence Absorption and Luminance p-n Junction a What happens if we bring a n semiconductor? p e Electrons close to the junction diffuse across the junction into the p-type region. Holes filled by recombination pt this larg n Equilibrium is established resulting in a potential aIf the two regions are connected in a circuit a variety of applications are possible. Energy-diagram of p-n Junction Biasing the p-n Junction When p-type a Biasing- introduction of Fermi levels do not align until equilibrium is reached. Forward bi Equilibriun for electrons and holes to flow through the junction positive oltage applied to n-type side. HYpe hYpe Raises energy barrier for current
2005-11-11 8 Solid line: direct bandgap materials Dotted line: indirect bandgap materials Matched system to reduce the strain effect and epitaxial growth defects! What's Luminescence? 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 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. n p - - - - e - + + + + h+ Energy- diagram of p-n Junction When p-type and n-type semiconductors touch, the Fermi levels do not align until equilibrium is reached. Biasing the p-n Junction Biasing ¾¾ introduction of a voltage into the circuit containing the p-n junction. Forward bias ¾¾ negative voltage is applied to n-type side. Decreases energy barrier for electrons and holes to flow through the junction. Reverse bias ¾¾ positive voltage applied to n-type side. Raises energy barrier for current flow. n p V — + e -
Majority Carrier"and Current Majority Carrier" and Curren Flow in p-type Silicon Flow in n-type Silicon Hole flow Electron flow Current flow Current flow A Depletion Zone (D)and a Barrier The p-n Junction Field Forms at the p-n Junction 0 Volts Hole diffusion Acceptor Electron Diffusion The Barrier Field Opposes Further diffusion Recombine at the unction Forward Bias"of a p-n Junction Reverse Bias of a p-n Junction 4 volts volts Current holes an ons are"pushed" toward the junction increasing the size of the depletion zone. wept across the junction and the The depletion zone becomes, in effect, an ins There is a net carrier flow in both the P and N sides= majority car Only a very small current can flow, due to a small urrent tlo number of minority carriers randomly crossing D( reverse saturation current)
2005-11-11 9 “Majority Carrier”and Current Flow in p-type Silicon + p Type Silicon - Hole Flow Current Flow “Majority Carrier”and Current Flow in n-type Silicon Electron Flow + n Type Silicon - Current Flow The p-n Junction p n 0 Volts Hole Diffusion Electron Diffusion Holes and Electrons “Recombine”at the Junction A Depletion Zone (D) and a Barrier Field Forms at the p-n Junction The Barrier Field Opposes Further Diffusion (Equilibrium Condition) p -- ++ n 0 Volts Hole (+) Diffusion Electron (-) Diffusion D Barrier Field Acceptor Ions Donor Ions “Forward Bias”of a p-n Junction •Applied voltage reduces the barrier field •Holes and electrons are “pushed”toward the junction and the depletion zone shrinks in size •Carriers are swept across the junction and the depletion zone •There is a net carrier flow in both the P and N sides = current flow! p - + n + Volts - Volts Current “Reverse Bias”of a p-n Junction p --- +++ n - Volts D + Volts Current •Applied voltage adds to the barrier field •Holes and electrons are “pulled”toward the terminals, increasing the size of the depletion zone. •The depletion zone becomes, in effect, an insulator for majority carriers. •Only a very small current can flow, due to a small number of minority carriers randomly crossing D (= reverse saturation current)
Applications of p-n Junction Diode Whie Diode Why call it p-n Junction ● Rectifier ● Photodetectors lar cells ●LEDs diode lasers When we brne ptpe Ntype together a lepton zone is crewed around re junior Ths produce a bemer, becking charge Simple Application: Rectifier p-n Rectifying Junction Forward Bias AAA combine at the junction. Current flows Une ot the most important uses of a diode is rectification. The normal PN junction diode is well-suited for this Reverse bias purpose as it conducts very heavily when forward biased 6「 Holes and free electrons Qow-resistance direction) and only slightly when reverse w away from each biased(high-resistance direction). If we place this diode in series with a source of ac power, the diode will be forward ead zone with no charge current flows more easily in one direction than the other Current is reduced p-n Rectifying Junction Optoelectronics in optoelectronic applications of semiconductor devices, he basic idea is that the device is used to either detect or to 4n detection the incident light is converted into a processes within the device. Examples of such devices include photodetectors and solar cells Gin emission on the other hand the internal proe allow the conversion of an electrical signal into detectable light and examples of such devices include LEDs and lasers A diodes properties can be seen when the voltage is examine
2005-11-11 10 Applications of p-n Junction Diode Rectifier Photodetectors solar cells LEDs diode lasers Optoelectronics Why call it p-n Junction as a Diode? Simple Application: Rectifier One of the most important uses of a diode is rectification. The normal PN 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. PN junction p-n Rectifying Junction Forward Bias Holes and free electrons flow together and recombine at the junction. Current flows. Reverse Bias Holes and free electrons flow away from each other. The center of the diode quickly becomes a dead zone with no charge carriers. Current is reduced. p-n Rectifying Junction A diode’s properties can be seen when the voltage is examined 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. Þ 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 include LEDs and lasers