EECS143 Microfabrication Technology Professor Ali Javey Introduction to Materials Lecture 1 Announcements The first HW set is due next Tuesday at the beginning of the class Please make sure you are signed up for one of the lab section The labs started from this week Evolution of Devices Yesterday s Transistor 1947 Today s Transistor 2006 Why Semiconductors Conductors e g Metals Insulators e g Sand SiO2 Semiconductors conductivity between conductors and insulators Generally crystalline in structure In recent years non crystalline semiconductors have become commercially very important Polycrystalline amorphous crystalline What are semiconductors Elements Si Ge C Binary GaAs InSb SiC CdSe etc Ternary AlGaAs InGaAs etc Electrons and Holes in Semiconductors Silicon Crystal Structure Unit cell of silicon crystal is cubic Each Si atom has 4 nearest neighbors 5 43 Silicon Wafers and Crystal Planes z z z y 100 x y y x 011 111 x 100 plane 011 flat Si 111 plane The standard notation for crystal planes is based on the cubic unit cell Silicon wafers are usually cut along the 100 plane with a flat or notch to help orient the wafer during IC fabrication Bond Model of Electrons and Holes Intrinsic Si Si Si Si Si Si Si Si Si Si Silicon crystal in a two dimensional representation Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si When an electron breaks loose and becomes a conduction electron a hole is also created Dopants in Silicon Si Si Si Si Si Si Si As Si Si B Si Si Si Si Si Si Si N type Si P type Si As Arsenic a Group V element introduces conduction electrons and creates N type silicon and is called a donor B Boron a Group III element introduces holes and creates P type silicon and is called an acceptor Donors and acceptors are known as dopants Types of charges in semiconductors Hole Electron Ionized Donor Ionized Acceptor Mobile Charge Carriers they contribute to current flow with electric field is applied Immobile Charges they DO NOT contribute to current flow with electric field is applied However they affect the local electric field EE143 Vivek Subramanian Slide 1 10 Doped Si and Charge What is the net charge of your Si when it is electron and hole doped GaAs III V Compound Semiconductors and Their Dopants Ga As Ga As Ga As Ga As Ga As Ga GaAs has the same crystal structure as Si GaAs GaP GaN are III V compound semiconductors important for optoelectronics Which group of elements are candidates for donors acceptors From Atoms to Crystals conduction band Energy p s valence band isolated atoms lattice spacing Decreasing atomic separation Energy states of Si atom a expand into energy bands of Si crystal b The lower bands are filled and higher bands are empty in a semiconductor The highest filled band is the valence band The lowest empty band is the conduction band Energy Band Diagram Conduction band Ec Eg Band gap Ev Valence band Energy band diagram shows the bottom edge of conduction band Ec and top edge of valence band Ev Ec and Ev are separated by the band gap energy Eg Measuring the Band Gap Energy by Light Absorption electron Ec photons Eg photon energy h v E g Ev hole Eg can be determined from the minimum energy h of photons that are absorbed by the semiconductor Bandgap energies of selected semiconductors Material E g eV PbTe Ge Si GaAs GaP Diamond 0 31 0 67 1 12 1 42 2 25 6 0 Semiconductors Insulators and Conductors Ec Top of conduction band Ec E g 9 eV empty E g 1 1 eV Ev Ev Si Semiconductor SiO Insulator 2 filled Ec Conductor Totally filled bands and totally empty bands do not allow current flow Just as there is no motion of liquid in a totally empty bottle totally filled or Metal conduction band is half filled Semiconductors have lower Eg s than insulators and can be doped Donor and Acceptor Levels in the Band Model Conduction Band Ed Donor Level Ec Donor ionization energy Acceptor ionization energy Acceptor Level Ea Valence Band Ev Ionization energy of selected donors and acceptors in silicon Donors Dopant Sb Ionization energy E c E d or E a E v meV 39 Hydrogen E ion P 44 m0 q4 8 02h2 Acceptors As 54 B 45 13 6 eV Al 57 In 160 Dopants and Free Carriers Donors n type Acceptors p type Dopant ionization energy 50meV very low General Effects of Doping on n and p Charge neutrality n Na p Nd 0 Na Nd number of ionized acceptors cm3 number of ionized donors cm3 Assuming total ionization of acceptors and donors n Na p Nd Na Nd number of ionized acceptors cm3 number of ionized donors cm3 0 Density of States E gc Ec Ec g E Ev Ev gv gc E number of states in E 1 3 E volume eV cm mn 2mn E Ec gc E 2 h3 gv E m p 2m p Ev E 2 h3 Thermal Equilibrium Thermal Equilibrium An Analogy for Thermal Equilibrium Sand particles Dish Vibrating Table There is a certain probability for the electrons in the conduction band to occupy high energy states under the agitation of thermal energy vibrating atoms etc At E EF f E 1 2 Effect of T on f E T 0K Question If f E is the probability of a state being occupied by an electron what is the probability of a state being occupied by a hole Nc is called the effective density of states of the conduction band Nv is called the effective density of states of the valence band Intrinsic Semiconductor Extremely pure semiconductor sample containing an insignificant amount of impurity atoms n p ni Ef lies in the middle of the band gap Material Ge Si GaAs Eg eV 0 67 1 12 1 42 ni 1 cm3 2 x 1013 1 x 1010 2 x 106
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