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