02_Device=) - (Gan 2010/11) Semiconductor Physics •...

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Unformatted text preview: (Gan 2010/11) Semiconductor Physics • Revision from MS2008 – Silicon – Intrinsic semiconductor – Extrinsic semiconductor – Carrier transport Readings: Plummer sections 1.3 (Gan 2010/11) Why Silicon? 1. Si is abundant in the form of silica (sand). 2. Availability of simple techniques to refine and grow single Si crystals. 3. Si is an elemental semiconductor. 4. Si has a wider energy band gap, leading to its operation at higher temperature, i.e. >150°C compared to 100°C for Ge. 5. Good thermally grown SiO 2 for Si/SiO 2 interface of MOS devices. ∴ More than 95% of semiconductor devices are Si-based. (Gan 2010/11) Si Semiconductor • Group IV intrinsic semiconductors • 4 valence electrons (Gan 2010/11) Si Crystal Structure Diamond lattice Face Centered Cubic (FCC) Two merged FCC cell, offset by a/4 in all 3 directions 4 atoms in second FCC cell lie within the first FCC cell (Gan 2010/11) Si Crystal Structure Coordination number = 4 a = 5.43 Å Distance between 2 neighbouring atoms = = 2.35Å Radius of Si atom = 1.18Å Packing density = 34% ( ) a 4 / 3 Diamond lattice Diamond lattice subcell (Gan 2010/11) Si Intrinsic Semiconductor • Above 0 K, thermal energy can break Si bonds • Creates free electron – hole pair • At room temperature, intrinsic carrier concentration, n i ~ 1.45 x 10 10 cm-3 (Note: Si atomic density = 5 x 10 22 cm-3 ) (Gan 2010/11) Band Model of Si • E G = bandgap energy = 1.12 eV (for Si) • E C = conduction band energy (free electrons) • E V = valence band energy (bound electrons) (Gan 2010/11) Si Extrinsic Semiconductor • Doped through diffusion or ion implantation process • Doped p-type with Group III elements, e.g. B • Doped n-type with Group V elements, e.g. P or As • Precise control of dopant concentration possible (Gan 2010/11) • E D/A = donor/acceptor energy level • E.g. Extra 5 th electron in As, low binding energy (E C - E D ) • As → As + + e- • “Shallow” donors usually all activated/ionized at room temp. Band Model of Extrinsic Si (Gan 2010/11) Band Model of Extrinsic Si • “Shallow” B acceptors • B → B- + h + • Smaller bandgap semiconductors have higher intrinsic conc. Material E G at room temp n i at room temp Germanium 0.66 eV 2.5 x 10 13 cm-3 Silicon 1.12 eV 1.45 x 10 10 cm-3 GaAs 1.42 eV 2 x 10 6 cm-3 (Gan 2010/11) Conductivity / Resistivity of Si p q n q p n μ μ σ + = • In N-doped or P-doped, n >> p or vice versa • Electron & hole mobilities, μ n μ p , vary with temperature, doping and electric field in semiconductors Conductivity , σ ρ 1 = Resistivity, (Gan 2010/11) Mobility vs Temperature • μ n μ p , decrease with temperature • At room temperature with low temperature and low field, μ n ~ 1500 cm 2 V-1 s-1 , μ p ~ 500 cm 2 V-1 s-1 (Gan 2010/11) Mobility vs Doping Concentration • Decrease in mobility due to ionized impurity scattering (Gan 2010/11) Mobility vs Electric Field • Drift velocity, • Saturation at high electric field E v d μ = (Gan 2010/11)...
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02_Device=) - (Gan 2010/11) Semiconductor Physics •...

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