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Semiconductor Fundamentals OUTLINE General material properties Crystal structure Crystallographic notation Read: Chapter 1

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Spring 2007EE130 Lecture 1, Slide 2 What is a Semiconductor? Low resistivity => “conductor” High resistivity => “insulator” Intermediate resistivity => “semiconductor” –conductivity lies between that of conductors and insulators –generally crystalline in structure for IC devices In recent years, however, non-crystalline semiconductors have become commercially very important polycrystallineamorphous crystalline

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Spring 2007EE130 Lecture 1, Slide 3 Semiconductor Materials Elemental: Compound: Alloy:

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Spring 2007EE130 Lecture 1, Slide 4 From Hydrogen to Silicon

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Spring 2007EE130 Lecture 1, Slide 5 The Silicon Atom 14 electrons occupying the 1st 3 energy levels: –1s, 2s, 2p orbitals filled by 10 electrons –3s, 3p orbitals filled by 4 electrons To minimize the overall energy, the 3s and 3p orbitals hybridize to form 4 tetrahedral 3sp orbitals Each has one electron and is capable of forming a bond with a neighboring atom

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Spring 2007EE130 Lecture 1, Slide 6 “diamond cubic” lattice The Si Crystal Each Si atom has 4 nearest neighbors lattice constant = 5.431Å

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Spring 2007EE130 Lecture 1, Slide 7 How Many Silicon Atoms per cm -3 ? Number of atoms in a unit cell: 4 atoms completely inside cell Each of the 8 atoms on corners are shared among cells count as 1 atom inside cell Each of the 6 atoms on the faces are shared among 2 cells count as 3 atoms inside cell Total number inside the cell = 4 + 1 + 3 = 8 Cell volume: (.543 nm) 3 = 1.6 x 10 -22 cm 3 Density of silicon atoms = (8 atoms) / (cell volume) = 5 x 10 22 atoms/cm 3

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Spring 2007EE130 Lecture 1, Slide 8 Compound Semiconductors “zincblende” structure III-V compound semiconductors: GaAs, GaP, GaN, etc. important for optoelectronics and high-speed ICs

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Spring 2007EE130 Lecture 1, Slide 9 Crystallographic Notation NotationInterpretation ( h k l ) crystal plane { h k l } equivalent planes [ h k l ] crystal direction equivalent directions h: inverse x-intercept of plane k: inverse y-intercept of plane l: inverse z-intercept of plane (Intercept values are in multiples of the lattice constant; h, k and l are reduced to 3 integers having the same ratio.) Miller Indices:

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Spring 2007EE130 Lecture 1, Slide 10 Crystallographic Planes and Si Wafers Silicon wafers are usually cut along a {100} plane with a flat or notch to orient the wafer during IC fabrication:

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Spring 2007EE130 Lecture 1, Slide 11 Unit cell: View in direction Crystallographic Planes in Si View in direction

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Spring 2007EE130 Lecture 1, Slide 12 Summary Crystalline Si: –4 valence electrons per atom –diamond lattice each atom has 4 nearest neighbors –5 x 10 22 atoms/cm 3 Crystallographic notation –Miller indices are used to designate planes and directions within a crystalline lattice

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