EECS 105 Spring 2004 Lecture 7 Lecture 7 Properties of Materials for Integrated Circuits Prof J S Smith Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 7 Prof J S Smith Context z Over the last two weeks we reviewed Basic passive components Capacitors Resistors Inductors Linear circuit models Phasor notation Transfer functions Bode plots Department of EECS University of California Berkeley 1 EECS 105 Spring 2004 Lecture 7 Prof J S Smith Context z 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 devices Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 7 Prof J S Smith Context z z z In this lecture we will cover How atoms join up to form materials The differences between z Metals Insulators Semiconductors How the properties of semiconductors can be modified to form electronic devices Department of EECS University of California Berkeley 2 EECS 105 Spring 2004 Lecture 7 Prof J S Smith Integrated circuits z An integrated circuit is mostly an arrangement of just a few elements z Silicon Aluminum Oxygen Nitrogen And traces of Boron Phosphorus Arsenic Etc Department of EECS University of California Berkeley What gives these materials their properties EECS 105 Spring 2004 Lecture 7 z Atoms consist of z Prof J S Smith A nucleus containing protons and neutrons A cloud of electrons The difference between different atoms the number of protons and consequently the number of electrons needed to maintain neutrality Department of EECS University of California Berkeley 3 EECS 105 Spring 2004 Lecture 7 Prof J S Smith Fermi exclusion z 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 Berkeley EECS 105 Spring 2004 Lecture 7 Prof J S Smith Orbitals z z 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 i t corresponds to the energy of the orbital E Department of EECS University of California Berkeley 4 EECS 105 Spring 2004 Lecture 7 Prof J S Smith Energy levels z We can diagram the levels as a function of their energy Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 7 Prof J S Smith Molecules z If two or more nuclei are close together electrons around them will have new orbitals with new shapes and different energies Department of EECS University of California Berkeley 5 EECS 105 Spring 2004 Lecture 7 Prof J S Smith Bonding z z 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 This is called bonding Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 7 Prof J S Smith Elements z 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 table Department of EECS University of California Berkeley 6 EECS 105 Spring 2004 Lecture 7 Prof J S Smith Periodic Table of Elements Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 7 Prof J S Smith Groups of atoms z 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 atoms Department of EECS University of California Berkeley 7 EECS 105 Spring 2004 Lecture 7 Prof J S Smith Lots of atoms close together solid z As more and more atoms come together in a clump the orbitals from the atoms become nearly continuous bands of states Band b Band gap Band a z 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 Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 7 Prof J S Smith Electronic states in crystals z z The electrons are in states which extend through the solid as waves We model them as nicely as this But they mostly look like this Department of EECS University of California Berkeley 8 EECS 105 Spring 2004 Lecture 7 Prof J S Smith Metals z So we will mostly be interested in the top few states which are occupied and the bottom few of those that are unoccupied Band b Band a z z z Electrons up to here If the electrons fill the lower states up to the middle of a band the material will be a metal The energy that the electrons are filled to is called the Fermi energy 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 bonding Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 7 Prof J S Smith Insulators and Semiconductors z If the electrons fill the bands up to a gap and the states above are empty the material will not conduct Band b Electrons up to here Band a z 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 semiconductor Department of EECS University of California Berkeley 9 EECS 105 Spring 2004 Lecture 7 Prof J S Smith N type Semiconductor z 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 Conduction band Valance band z A few Electrons here Still completely full These few electrons will have lots of elbow room and so the material will conduct not as well as a metal but pretty well Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 7 Prof J S Smith P type Semiconductor z If we add a few atoms with one less proton each then the orbitals will be the same but there will be a few
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