A semiconductor is an almost insulating material, in which by contamination (doping) positive or negative charge carriers can be introduced.

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1 Semiconductor A semiconductor is an almost insulating material, in which by contamination (doping) positive or negative charge carriers can be introduced. Page 2

2 Semiconductor materials Page 3

3 Energy levels Electrons can only have discrete values of energy Electrons in the outermost shell, called Valence Electrons. With large number of atoms in solid, energy levels form bands Important bands are the valence band, conduction band and energy gap Page 4

4 Electronic properties of materials general case. Insulators. Conductors. Semi-conductors. Page 5

5 Energy diagrams of three categories material Page 6

6 Semiconductor The energy band gap between the conduction band and valance band determines the conductive properties of the materials Metal Negligible band gap Conduction Band Valence Band Al, Au, Pt, Ni, Co, etc. Insulator Large band gap, >6eV Conduction Band Band Gap Valence Band SiO 2, Si 3 N 4, Al 2 O 3, HfO 2, etc. Semiconductor Medium band gap, 0~4 ev Conduction Band Band Gap Valence Band Si, Ge, GaAs, GaN, SiC, ZnO etc. Page 7

7 Covalence Bonds Atoms of solid materials form crystals, which are 3D structures held together by strong bonds Between atoms covalence Bonds like Silicon. Silicon forms a covalent crystal. The shared electrons are not mobile. Therefore there is a energy gap between the valence band and the conduction band. 8 Page 8

8 Holes An electron hole is the conceptual and mathematical opposite of an electron Holes do not travel like electrons Only electrons can move from atom to atom Page 9

9 Doping in Silicon Pure silicon wafers are called intrinsic silicon Intrinsic silicon doesn t have enough free electrons for current conduction. So impurities are introduced to increase conductivity The introduction of these impurities is called doping Page 10

10 Donors and Acceptors Donors are dopant atoms that added to a semiconductor to provide electrons Acceptors are dopant atoms that provide holes Page 11

11 Type of doping N (negative) type: doped with donors and has extra free electrons. P (positive) type: doped with acceptors and has extra holes. Page 12

12 Electronic properties of doped silicon qualitative picture. Page 13

13 Methods for doping (2) Diffusion Diffusion impurities move by a difference in concentration gradients Conducted at very high temperatures Gas sources are the most common but liquids and solids are also used The sources react with silicon to form dopant oxide which then diffuses into the rest of the substrate by the increase of temperature Page 14

14 Methods for doping (2) Ion Implantatoin Ion implantation: Alternative to high temperature diffusion A beam of highly energetic dopant ions is aimed at the semiconductor surface Collision with ion distorts the crystal structure Annealing has to be performed to correct the damage Page 15

15 Electronic properties: Silicon in general. E F E G = 1.12 ev Boltzman constant: k = ev/k Fundamental materials property: n N c e ( E E c F )/ kt Where n = concentration of negative (electron) carriers (typically in cm - 3 ) E c is the energy level of the conduction band E F is the Fermi level. N c is the intrinsic density of states in the conduction band (cm -3 ). Similarly, p N V e ( E E F E C E V V )/ kt Where p = concentration of positive (hole) carriers (typically in cm -3 ) E V is the energy level of the valence band N V is the intrinsic density of states in the valence band (cm -3 ). Page 16

16 Electronic properties: intrinsic (undoped) silicon. Density of states in conduction band, N C (cm -3 ) 3.22E+19 Density of states in valence band, N V (cm -3 ) 1.83E19 Note: without doping, n = p n i where n i is the intrinsic carrier concentration. For pure silicon, then n 2 i N c N Thus n i = 1 x cm -3 V exp( E G / kt) Similarly the Fermi level for the intrinsic silicon is, E i E V ( EC EV ) / 2 (1/ 2) kt ln( NV / NC ) Where we have used E i to indicate intrinsic Fermi level for Si. Page 17

17 Consider doping with n-type (or electron donating) dopant (such as Arsenic). Then n N D where N D is the arsenic doping concentration. The injection of negative (electron) carriers dramatically alters the Fermi level of the system since there are now a significant sea of negative carriers available. We can determine the new Fermi level as well as the resulting change in positive carriers. n 2 i pn N c N V exp( E G / kt) Thus p=n i2 /N D. And E F = E i + kt ln(n D /n i ) Page 18

18 Correspondingly, for p-type (acceptor) dopants such at Boron: Thus n=n i2 /N A. p n i e ( E E i F )/ kt And E F = E i -kt ln(n A /n i ) n n i e ( E E F i )/ kt Resistivity q( n n 1 p p ) Where q is electron charge and are mobilities. Page 19

19 Equations to remember Note: Our interest was in determining n and p. Free carriers strongly influence the properties of semiconductors. 20 Page 20

20 Example 1 (a) Consider Si doped with cm 3 boron atoms. Calculate the carrier concentration (n and p) at 300 K. (b) Determine the position of the Fermi level and plot the band diagram. 21 Page 21

21 Example 2 Consider a Si sample doped with cm 3 of phosphorous (P) atoms and cm 3 of boron (B) atoms. (a) Is the semiconductor n-type or p-type? (b) Determine the free carrier concentration (hole and electron concentrations, or p and n) at 300K. (c) Determine the position of the Fermi level and draw the band diagram. 22 Page 22

22 MICROCHIP MANUFACTURING by S. Wolf Chapter 2: SEMICONDUCTORS and BASIC MATERIALS SCIENCE 2004 by LATTICE PRESS

23 Chapter 2: Semiconductors and Basic Materials Science CHAPTER CONTENTS Atomic Structure & Periodic Table Solids, Liquids, and Gases Electrical Conductivity of Solids Conductors Insulators Semiconductors Chemical Bonding Model Energy-Band Model Semiconductor Silicon Other Semiconductor Materials MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 2-2

24 Chapter 2: Semiconductors and Basic Materials Science Atomic Structure and The Periodic Table The Structure of the Atom The electronic structure of several atomic elements The Periodic Table MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 2-3

25 Chapter 2: Semiconductors and Basic Materials Science Molecules and Compounds Solids, Liquid, & Gases MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 2-4

26 Chapter 2: Semiconductors & Basic Materials Science Classifying Solids by Their Electrical Properties - Conductors - Insulators - Semiconductors The resistance (in ohms - W) of a solid made from a specific material having a resistivity (r ), and a particular geometric-form Is found from: R (W) = r (l /A) MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 2-5

27 Chapter 2: Semiconductors and Basic Materials Science CHEMICAL BONDING MODEL OF CONDUCTIVITY IN SOLIDS Ionic Bonding Covalent Bonding Covalent Bonds in Silicon MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 2-6

28 Chapter 2: Semiconductors & Basic Materials Science ENERGY-BAND MODEL OF CONDUCTIVITY IN SOLIDS Correlating the Chemical-Bonding Model and the Energy-Band Model MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 2-7

29 Chapter 2: Semiconductors & Basic Materials Science SEMICONDUCTOR SILICON Intrinsic (Pure) Extrinsic (Doped) n-type (Donors) p-type (Acceptors) Carrier generation(electrons and holes) in intrinsic-si Intrinsic- Si Si with: n-type dopant (Phosphorus) with p-type dopant (Boron) MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 2-8

30 Chapter 2: Semiconductors and Basic Materials Science ELECTRON & HOLE CONDUCTION IN SILICON Electrons from donor-atoms in Si Holes from acceptor-atoms in Si Concept of (a) electron-conduction; and (b) hole-conduction in silicon MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 2-9

31 Chapter 2: Semiconductors & Basic Materials Science DEPENDENCE OF RESISTIVITY ON DOPING CONCEN- TRATION IN SILICON Resistivity vs. doping for n-type and p-type silicon MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 2-10

32 Chapter 2: Semiconductors and Basic Materials Science OTHER SEMICONDUCTOR MATERIALS Germanium (Ge) III-V Compounds Gallium Arsenide (GaAs) - High frequency devices Indium Phosphide (InP) - High frequency devices Gallium Phosphide (GaP) - LEDs (Red, Green) Gallium Nitride (GaN) - LEDs (Blue, White) II-VI Compounds Mercury-Cadmium Teluride (HgCdTe) - IR Detectors Zinc Selenide (ZnSe) - Optoelectronic devices MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 2-11

33 Chapter 2: Semiconductors and Basic Materials Science SUMMARY OF KEY CONCEPTS The Atomic Structure of Matter is the Basis of its Material Properties (Periodic Table) Solids can be Electrically Classified as: Conductors, Insulators, or Semiconductors Electrical Conductivity can be explained using: Chemical Bonding Model, or Energy-Band Model Semiconductors have two carrier types: Electrons and Holes Carrier concentration is controlled by adding extremely small quantities of dopants (n-type or p-type) to Si. Selective-doping is the basis Si-device fabrication! MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 2-12

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