1Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 7Lecture 7: Properties of Materials for Integrated CircuitsProf. J. S. SmithDepartment of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 7 Prof. J. S. SmithContextz Over the last two weeks, we reviewed:– Basic passive components• Capacitors• Resistors• Inductors– Linear circuit models– Phasor notation– Transfer functions– Bode plots2Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 7 Prof. J. S. SmithContextz Over the next couple of weeks, we will cover:– The relevant basic physics of materials– How electrons move through materials– How semiconductor devices are made– Models of semiconductor devicesDepartment of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 7 Prof. J. S. SmithContextz In this lecture, we will cover:z How atoms join up to form materialsz The differences between:– Metals– Insulators– Semiconductorsz How the properties of semiconductors can be modified to form electronic devices3Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 7 Prof. J. S. SmithIntegrated circuitsz An integrated circuit is mostly an arrangement of just a few elements:– Silicon– Aluminum– Oxygen– Nitrogenz And traces of:– Boron– Phosphorus– Arsenic– Etc.Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 7 Prof. J. S. SmithWhat gives these materials their properties?z Atoms consist of:– A nucleus containing protons and neutrons– A cloud of electronsz The difference between different atoms:– the number of protons, – and consequently the number of electrons needed to maintain neutrality4Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 7 Prof. J. S. SmithFermi exclusionz Since no two electrons can be doing exactly the same thing, and there are two types of “spin” each spatial orbital can only hold two electrons.Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 7 Prof. J. S. SmithOrbitalsz As electrons are added to the potential well of a positively charged nucleus, they go into orbitals which are similar to the resonances of a drum head. The rate of change of phase e–iωt corresponds to the energy of the orbitalz E=ħω5Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 7 Prof. J. S. SmithEnergy levelsz We can diagram the levels as a function of their energy:Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 7 Prof. J. S. SmithMoleculesz If two or more nuclei are close together, electrons around them will have new orbitals, with new shapes and different energies6Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 7 Prof. J. S. SmithBondingz The orbitals close to each of the nuclei don’t change much, but the outer orbitals join together and have different shapes. If electrons occupy these orbitals, and they have a lower energy when the molecules are close together, then it requires energy to separate the atoms.z This is called bondingDepartment of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 7 Prof. J. S. SmithElementsz The configurations of bonds that will form between atoms with different numbers of protons in their nuclei depend on the blending of their outer orbitals into molecular orbitals. The pattern of the filling of orbitals of different angular shapes yields a pattern of similarity of the types of bonds that will be observed between atoms. This pattern is made explicit in the periodic table7Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 7 Prof. J. S. SmithPeriodic Table of ElementsDepartment of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 7 Prof. J. S. SmithGroups of atomsz As atoms come close together:– the number of electrons stays the same (to maintain neutrality)– The number of orbitals stay the same– Each orbital from each atom is transformed and blended into a new orbital, over many atoms8Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 7 Prof. J. S. SmithLots of atoms close together: solidz As more and more atoms come together in a clump, the orbitals from the atoms become nearly continuous bands of statesz In general, there will be many bands of states, but the lower ones will be all filled completely with electrons, and gridlocked, and the higher ones will be empty.Band b {} Band gapBand a {Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 7 Prof. J. S. SmithElectronic states in crystalsz The electrons are in states which extend through the solid as wavesz We model them as nicely as thisBut they mostly look like this→9Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 7 Prof. J. S. SmithMetalsz So we will mostly be interested in the top few states which are occupied, and the bottom few of those that are unoccupied. z If the electrons fill the lower states up to the middle of a band, the material will be a metalz The energy that the electrons are filled to is called the Fermi energyz Since the electrons can change their energies and orbitals a little bit at a time, they can move freely and the material is a good conductor of heat and electricity, and their strength comes from metallic bondingBand b {← Electrons up to hereBand a {Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 7 Prof. J. S. SmithInsulators and Semiconductorsz If the electrons fill the bands up to a gap, and the states above are empty, the material will not conductz If the material is very uniform and pure, if there are a few electrons in the band above the gap, they will be able to move freely. In this case, the material is called a semiconductorBand b {← Electrons up to hereBand a {10Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 7 Prof. J. S. SmithN type Semiconductorz If we add a few atoms with an extra proton, then the orbitals will be the same, but there will be a few more electrons, and they will go into the upper band.z These few electrons will have lots of elbow room, and so the material will conduct, not as well as a metal, but pretty wellConduction band {← A few Electrons hereValance band {←
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