536_Lecture_2_Semiconductor_and_device_lectures_v1

536_Lecture_2_Semiconductor_and_device_lectures_v1 - 5360...

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5360 Nanofabrication Review of basic semiconductor concepts 1
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2 Semiconductors are the class of material where the conductivity of the material can be controlled to vary a large orders of magnitude. – Elemental semiconductor: Si, Ge – Compound semiconductor (fixed composition): SiC, GaAs, GaN – Alloy: Si 1-x Ge x , Al 1-x Ga x As, Hg 1-x Cd x Te, etc. Together with the ability to have two types of carriers makes this material highly versatile
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Goals • Calculate the conductivity in a semiconductor material as a function of physical properties • Engineer and fabricate materials with a designed conductivity – N-type – P-type – Semi-insulating • Predict how the conductivity varies as a function of temperature 3
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Key Physical Concepts • Band model • Doping • Density of states • Fermi Statistics • Fermi level 4
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5 Actual 3D Structure of a Silicon Crystal Lattice covalent bonds Each Silicon atom is surrounded by 4 other Silicon atoms in a tetrahedral configuration. Silicon atomic density = 5x10 22 cm -3
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6 We ignore the transition metals here (things with d orbital in the valence band). They are surely important, just more complex. The Simple Periodic Table
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7 Intrinsic Si with Thermally Generated Electrons and Holes This picture is known as the bond model… There is another model, which is quantitatively more elegant, called the band model. Intrinsic (undoped): n = p n : electron concentration (cm -3 ) p : hole concentration (cm -3 ).
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8 Extrinsic Semiconductors (Intentional Doping: Donors from Group V for N-Type) For Si, Group V elements are donors: P, As, and Sb , Bi (not N) Electrons are only weakly confined to the ionized donor N D + : ionized donor conc. (cm -3 ) Fig. 3.43
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9 Extrinsic Semiconductors (Intentional Doping: Acceptors from Group III for P Type) For Si, Group III elements are acceptors: B and In (not Al or Ga) Holes are only weakly confined to the ionized acceptor N A - : ionized acceptor conc.
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536_Lecture_2_Semiconductor_and_device_lectures_v1 - 5360...

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