1 Characteristics of Semiconductor Materials Chapter 2 : Semiconductor Manufacturing Technology by M. Quirk and J. Serda Chapter 3.1 and 3.2 : Semiconductor Science by Tudor E. Jenkins Saroj Kumar Patra, Department of Electronics and Telecommunication, Norwegian University of Science and Technology ( NTNU ) TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 2 Electron Shells in Atoms Q=2 P = 10 O = 32 N = 32 M = 18 L=8 K=2 Figure 2.2 Quirk & Serda TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 3 Elementary Model of Carbon Atom Proton (positive charge) - Electron (negative charge) Atomic number (number of protons) Nucleus (center of atom; contains protons and neutrons) C 6 Orbital shell - Neutron (neutral charge) + N + N +N + N N + N + - - Valence electron Valence shell (outer shell of atom) Carbon atom: The nucleus contains an equal number of protons (+) and neutrons (6 each). Six electrons (-) orbit around the nucleus. Figure 2.1 Quirk & Serda TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 4 Group IV A Elemental semiconductors Semiconductors Group IVA C, Carbon 6 Si, Silicon 14 Ge, Germanium 32 Sn, Tin 50 Pb, Lead 82 Figure 2.18 Quirk & Serda TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 5 Covalent Bonding of Pure Silicon Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Silicon atoms share valence electrons to form insulator-like bonds. Figure 2.19 Quirk & Serda TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 6 Crystal Structure of Si and GaAs TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 7 Crystal Structure TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 8 Classifying Materials • Conductors • Insulators • Semiconductors TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 9 Energy Bandgaps Electron Energy Electron Energy Electron Energy Conduction Band Valence Band Overlapping bands - little energy is needed for conduction Energy Gap Energy Gap Conduction Band Conduction Band Valence Band Valence Band Insulator Conductor Semiconductor Figure 2.4 Quirk & Serda TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 10 Energy Band Diagram for III-V Semiconductor TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 11 Covalent Bonding in Pure Silicon Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Silicon atoms share valence electrons to form insulator-like bonds. Figure 2.19 Quirk & Serda TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 12 Doping of Silicon Deposition Step Drive-in & Diffusion Step dopant dispenser wafer substrate Activation Step dopant layer diffusion of dopant atoms through silicon wafer Si Si PP PP Si Si Si Si P Figure 2.21 Quirk & Serda TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 13 Silicon Dopants Acceptor Impurities Semiconductor Donor Impurities Group III (p-type) Group IV Group V (n-type) Boron 5 Carbon 6 Nitrogen 7 Aluminum 13 Silicon 14 Phosphorus 15 Gallium 31 Germanium 32 Arsenic 33 Indium 49 Tin 50 Antimony 51 * Items underlined are the most commonly used in silicon-based IC manufacturing. Figure 2.22 Quirk & Serda TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 14 Electrons in n-type Silicon with Phosphorus Dopants Si Si Si Si Si Si Si Si P Si Si P Si Si Si Si Si Si P Si Si Si Si Si Si Excess electron (-) Phosphorus atom serves as n-type dopant Donor atoms provide excess electrons to form n-type silicon. Figure 2.23 Quirk & Serda TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 15 Conduction in n-type Silicon Positive terminal from power supply Negative terminal from power supply Free electrons flow toward positive terminal. Figure 2.24 Quirk & Serda TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 16 Holes in p-type Silicon with Boron Dopant Si Si Si Si Si Si Si Si B Si + Hole Si B Si Si Si Boron atom serves as p-type dopant Si Si Si B Si Si Si Si Si Si Acceptor atoms provide a deficiency of electrons to form p-type silicon. Figure 2.25 Quirk & Serda TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 17 Conduction in p-type Silicon Positive terminal from voltage supply Negative terminal from voltage supply -Electrons flow toward positive terminal +Holes flow toward negative terminal Figure 2.26 Quirk & Serda TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 18 Impurity States n-type p-type TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 19 Density of Impurity States (for amorphous semiconductor) TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 20 Density of Electrons and Hole in a Semiconductor TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 21 How Size affects Resistance Low Resistance R= High Resistance L A Figure 2.12 Quirk & Serda TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 22 Electrical Conductivity and Mobility • • The simplest picture of electrical conductivity and mobility can be understood by considering the electrons in a semiconductor as a classical gas in the body of the material. Then using the Maxwell-Boltzmann distribution function we can get the relation, • At a temperature of 300 K and using free electron mass for ‘m’, we have • • The random nature of electron velocities means that the time average current that flows is zero. On application of an electric field to the semiconductor, the electrons will drift in the opposite direction of the field, so that there is now a net flow of charge and hence a flow of current. Therefore the equation of motion using drift velocity vd can be written as: τ TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 23 Electrical Conductivity and Mobility(continued) • • τ The second term in the equation is introduced to prevent ever increasing electron velocity with the electric field on. Therefore electrons are accelerated until a time τ and then suffer scattering within the system and their velocity randomized. In the absence of Electric field the equation becomes: τ • The solution can be written as: τ • • The relaxation time represents the tendency of scattering in the semiconductor to return the electron distribution to thermal equilibrium. Therefore in the steady state the ‘d/dt’ term will not be there. τ TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 24 Electrical Conductivity and Mobility(continued) • The current density ‘J’ is therefore given as: τ • Where ‘n’ is the number of electrons per unit volume of the semiconductor. This is Ohm’s Law τ σ Or • τ σ Electrical Conductivity The mobility is expressed as : μ | | | | τ Mobility TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 25 1021 Dopant Concentration (atoms/cm3) Silicon Resistivity vs. Dopant Concentration 1020 1019 1018 1017 n-type 1016 p-type 1015 1014 1013 10-3 10-2 10-1 100 101 102 103 Electrical Resistivity (ohm-cm) Redrawn from VLSI Fabrication Principles, Silicon and Gallium Arsenide, John Wiley & Sons, Inc. Figure 2.27 Quirk & Serda TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 26 Cross-section of planar pn-junction p-type Si n-type Si Figure 2.28 Quirk & Serda TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 27 Flow of Free Electrons in Copper One electron in the valence ring Copper atom - - - - - - - - - Cu 29 - - - - - - - K - L - M - - - Figure 2.11 Quirk & Serda - Maximum # Actual # Shell # e- per shell e- per shell 2 2 K 8 8 L 18 18 M 32 1 N 60 29 Total # - N TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 28 Alternative Semiconductor Materials Comparison of Some Physical Properties for Semiconductor Materials Property Si Ge GaAs SiO2 Melting point 1700 1412 937 1238 (C) (approx.) Atomic Weight 28.09 72.60 144.63 60.08 Atomic Density 4.99 x1022 4.42x1022 2.21x1022 2.3x1022 3 (atoms/cm ) Energy Band 8 1.11 0.67 1.40 Gap (eV) (approx.) Table 2.3 Quirk & Serda TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 29 Resistivity vs. Impurity Concentration for Si and GaAs TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 30 Electronic Properties of some Semiconductors TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 31 Physical Constants TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials 32 g{tÇ~ lÉâ TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials