EML4450L11

EML4450L11 - Sustainable Energy Science and Engineering...

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S ustainable E nergy S cience and E ngineering C enter Essentials of Quantum Physics Continued References Direct energy conversion by S.W. Angrist, Ch 3. (out of print text book) Essential Quantum Physics by Peter Landshoff, Allen Metherell and Gareth Rees, 1997, Cambridge University Press.
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S ustainable E nergy S cience and E ngineering C enter Review - Quantum Physics 4 numbers needed to specify wave function and energy level of electron orbitals Principal quantum number (n), orbital quantum number (L), magnetic quantum number (m L ), and spin quantum number (m s ) Pauli Exclusion Principle - no two electrons can have the same quantum description -> lowest energy levels filled first Electron energies are restricted to ‘energy bands’ Valence Band and Conduction Band separated by a band gap (forbidden zone) conductors -> small band gap, insulators -> large band gap semiconductors have medium band gaps
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S ustainable E nergy S cience and E ngineering C enter Review - Quantum Physics Electrons gain energy through photon interaction Jump from one energy band to another is only possible if the photons possess the proper amount of energy Frequency affects electron energy, intensity affects the number that escape over a given time
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S ustainable E nergy S cience and E ngineering C enter +5 1s (n=1, L=0) 2s (n=2, L=0) 2p (n=2, L=1) Electron Energy Potential Energy Curve for Electrons Review - Quantum Physics ε = 0 Valence Orbit Energy Level Scheme for Boron Ionization Voltage 8.30 5.98 6.00 14.54 10.55 9.81 Element Boron (5) Aluminum (13) Gallium (31) Nitrogen (7) Phosphorus (15) Arsenic (33)
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S ustainable E nergy S cience and E ngineering C enter Review - Semiconductors Semiconductors fall in between conductors and insulators Electrical characteristics can be changed by doping n- and p-type semiconductors generated this way Can function as switches and photovoltaic devices Si Si Si Si Si P Si Si Si Si B Si Si Si Si undoped n-doped p-doped
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S ustainable E nergy S cience and E ngineering C enter Review - Band Gaps The band gap of a semiconductor, measured in electron volts [eV], is the difference between the valence band and the conduction band potentials . Each type of semiconductor has a unique band gap, most of which fall in the range 1.0 to 2.6 eV. Semiconductor Band Gap [eV] Silicon 1.1 Gallium Arsenide 1.34 Copper Indium Delenide 1.0 Germanium 0.72 Indium antimonide 0.18 Cadium Sulfide 2.45 Zinc Oxide 3.3 The point to note here is that a photovoltaic material can only capture those photons which have an energy greater than or equal to the band gap of that material . Silicon, for example, will be transparent to photons with an energy of less than 1.1 eV. It might seem therefore, that the thing to do is use a very low band gap material, but the strength of the electric field created by the conjunction of n-type and p-type material is also dependent upon the band gap. One has to make a tradeoff between photon energy and field strength.
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S ustainable E nergy S cience and E ngineering C enter Semiconductor Junction
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EML4450L11 - Sustainable Energy Science and Engineering...

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