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semi1 - ELEC 101 Semiconductor 1 1 Conductor and Insulator...

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Unformatted text preview: ELEC 101 Semiconductor 1 1 Conductor and Insulator a Conductor The outer shells of most metal atoms are partially filled. Hence the outer electrons are loosely bounded and can easily escape from the metal atoms. b + - + + + + - - + - - + - - + + + + - - + Free electron Insulator When combined as a compound (for example: NaCl) due to ionic exchange of 1 electron, the outer orbits of the atoms are completely filled. The compound has closed shell configurations with no free electrons to participate in conduction . Ionic bond Na + The positive metal ions are immersed in a “sea of free electrons” . The electrons are free to move under the influence of an electric field. Hence metals are good conductors. Positive metal ion 2009/10 Spring 2 Cl - Semiconductor and Silicon Semiconductors are a group of materials having electrical conductivities between metals and insulators. Small amount of dopants can be used to vary the conductivities of semiconductors over wide range. - ELEC 101 Semiconductor 2 Semiconductors can be classified as elemental or compound semiconductors. The most important semiconductor today is Silicon (Si). The atoms are arranged in a tetrahedral lattice (diamond structure), and held in place by covalent bonds. 2009/10 Spring Si Si covalent bond Tetrahedral lattice Si Si Si a Other semiconductors Elemental: Ge, C Semiconductor Elements in the Periodic Table (with atomic number and atomic weight) III V +3 II IV +4 +5 N NITROGEN 14.008 13 Al 14 Si 15 P ALUMINUM SILICON PHOSPHORUS 26.97 28.09 31.02 As Ge 33 31 Ga 32 ARSENIC GERMANIUM GALLIUM 74.91 72.60 69.72 49 In 50 Sn 51 Sb INDIUM TIN ANTIMONY 114.8 118.7 121.8 5 B BORON 10.82 Zn Cd IV-IV Compound: SiC, SiGe 6 C VI 7 CARBON 12.01 S Se Te III-V Compound: GaP, GaN, GaAs, InP, InAs, InSb, etc (mainly for emission, absorption, detection and modulation of light ) II-VI Compound: CdS, CdSc, CdTc, ZnS, ZnSc, ZnTc, etc (mainly as florescent and detector materials) The electronic and optical properties of semiconductor materials are strongly affected by dopants (or impurities). ELEC 101 3 Semiconductor 3 2009/10 Spring Intrinsic (or pure) Silicon At 0 K , Si atoms have zero kinetic energy, lattice is not vibrating, there are no free electrons and Si is an insulator. Free Electron +4 +4 +4 Valence electrons o +4 +4 +4 +4 +4 +4 +4 +4 +4 Covalent bond +4 +4 +4 +4 +4 Hole (broken covalent bond) +4 As temperature increases, the thermal energy in the lattice is high enough to break some covalent bonds. Some electrons are free to move through the lattice. The broken bond is missing an electron and is called a hole. electron density n (number of free electrons / volume) = hole density p (number of free holes / volume) = a constant ( ni ) ELEC 101 Semiconductor 4 4 Extrinsic Silicon : N-Si and P-Si a N-Si (or N type Si) N-Si is formed by replacing Si atoms by donor atoms (group V or penta-valent atoms such as Arsenic). The extra electron is a free electron. +4 Pentavalent (donor) atom +4 +4 +5 +4 Extra free electron b 2009/10 Spring P-Si (or P type Si) P-Si is formed by replacing Si atoms by acceptor atoms (group III or tri-valent atoms such as Boron). The unfilled bond is an extra hole. +4 +4 +4 +4 +3 +4 +4 +4 +4 +4 +4 +4 Trivalent (acceptor) atom Extra hole (unfilled bond) +4 N-Si has much more free electrons than pure Si P-Si has much more holes than pure Si electron density n donor density ND (number of donor atoms/volume) hole density p acceptor density NA (number of acceptor atoms/volume) ELEC 101 1 Semiconductor 5 2009/10 Spring P-N Junction Diode Acceptor ion Donor ion Donor ion electron diffusion current Junction E Acceptor ion electron hole hole Electron (majority carrier) Carriers diffuse from high concentration to low concentration. Hence holes in the P-Si diffuse to the NSi, and electrons in the N-Si diffuse to the P-Si. This leaves acceptor ions (negative charge) in the P-Si , and donor ions (positive charge) in the N-Si. The region has no free carriers and is called depletion region. Hole (majority carrier) The ions set up an electric field E and cause a drift current (opposes the diffusion current). At equilibrium, the diffusion and drift currents cancel. ...
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