chap6 - ' $ EE432 Optoelectronic Devices Chapter 6...

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Unformatted text preview: ' $ EE432 Optoelectronic Devices Chapter 6 & Patrick Roblin T.H.E OHIO S ATE T UNIVERSITY 0 The Ohio State University % ' $ Lecture # 20 Photo-diode Solar cell LED Laser & Patrick Roblin T.H.E OHIO S ATE T UNIVERSITY 1 The Ohio State University % ' $ Photodiode + − P I N Ec E F Ev V h ω > Eg Optical Generation Rate E F & − + T.H.E The diode leakage is controled by the light. Patrick Roblin 2 OHIO S ATE T UNIVERSITY The Ohio State University % ' $ Diode Leakage Current + − P I N Ec E F Ev V h ω > Eg Optical Generation Rate E F − + I = Ith eqV=(kT ) ; 1 ; Iopt with 0 1 @ Lp p + Ln n A with I = qA th p n n p Derivation of di usion region: & 0 1 @ Lp pn + Ln npA Iopt = qA Patrick Roblin p Iopt = qAgopt(Lp + Ln + W ) n 3 with 8 > p =g < n opt p > : np = gopt n T.H.E OHIO S ATE T UNIVERSITY The Ohio State University % ' $ Choice of Materials α InSb GaAs GaP Si CdSe CdS SiC Ge ZnS Eg (eV) 0 Eg & Patrick Roblin E =h ω 1 2 3 4 λ (µ m) 5 Infrared 1 0.5 Visible 0.35 Ultraviolet T.H.E OHIO S ATE T UNIVERSITY 4 The Ohio State University % ' $ Depletion Layer Photodiode: PIN Diode metal front conctact + − N i P back metal contact A faster photodiode is obtained by maximizing the EHP generation in the depletion region where there are swept by the electric eld. OHIO PIN diode facilitates this process by de ning the depletion region S ATE T & Patrick Roblin T.H.E UNIVERSITY 5 The Ohio State University % ' $ Photodiode Optimization I V Provide gain by using avalanche (increase sensitivity but noisy): Avalanche Photodiode Separate carrier generation and multiplication to increase sensitivity while keeping noise low (useful for infrared/narrow bandgap applications) Use of a resonator (realized with Bragg re ectors) to increase the strength of the optical eld (equilalent to multiple pass) OHIO S ATE T & Patrick Roblin T.H.E UNIVERSITY 6 The Ohio State University % ' $ Solar Cell I Vmp 11111 00000 11111 00000 11111 00000 11111 Imp 00000 10000 01111 11111 00000 Voc V Isc 3rd quadrant & Power is generated. Fill factor (ratio of IscVoc=(Imp Vmp)) Patrick Roblin T.H.E OHIO S ATE T UNIVERSITY 7 The Ohio State University % ' $ Solar Cell Layout metal front conctact − + & Patrick Roblin N P T.H.E back metal contact OHIO S ATE T UNIVERSITY 8 The Ohio State University % ' $ Solar Cell Optimization Junction depth must be smaller that the di usion length to avoid EHP recombination and light absorption. Max voltage Voc generated is on the order of the contact potential V0 which can approach the bandgap Eg for strong doping. Low ohmic resistance to avoid waisting power Silicon (T < 100o C) but compound can be operated at higher temperature & Patrick Roblin T.H.E OHIO S ATE T UNIVERSITY 9 The Ohio State University % ' $ Typical Performance Power: 100 to 1000 W/m2 E ciency: 10 to 20 % Various types of cell: Si crytaline, Si polycrystaline, Compound & Patrick Roblin T.H.E OHIO S ATE T UNIVERSITY 10 The Ohio State University % ' $ LED and LASER LED: light emitting diode LASER light ampli cation by stimulated emission of radiation & Patrick Roblin T.H.E OHIO S ATE T UNIVERSITY 11 The Ohio State University % ' $ Coherent (Classical) and Non-Coherent Light 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 11111 00000 & Non −Coherent Light Patrick Roblin Coherent Light (laser) T.H.E OHIO S ATE T UNIVERSITY 12 The Ohio State University % ' $ Quantum Treament of Maxwell Equation: Photon Energy h ω(7+1/2) h ω(6+1/2) h ω(5+1/2) h ω(4+1/2) h ω(3+1/2) h ω(2+1/2) h ω(1+1/2) h ω(0+1/2) Eigenstates |7> |6> |5> |4> |3> |2> |1> |0> & Patrick Roblin Eigenvalues Potential Energy T.H.E OHIO S ATE T UNIVERSITY 13 The Ohio State University % ' $ Stimulated Emission h ω = E2 − E 1 E2 absorption Stimulated emission Spontaneous emission E1 In steady state the emission and absorption rates are balanced: B12 n1{z (!12) = B21 n2{z (!12) + A21 n2} | {z | }| } Absorption Stimulated emission Spontaneous emission with the photon eld strength (/ Nphoton), h!12 = E2 ; E1, and A21, B12 and B21 the Einstein coe cients. & Patrick Roblin T.H.E OHIO S ATE T UNIVERSITY 14 The Ohio State University % ' $ Radiation and Lasing Condition Condition for the radiation to be coherent (laser) Stimulated emission rate = B21 (! ) Spontaneous emission rate A21 12 We need a large photon eld. This can be achieved with a resonator (feedback). Condition for radiation: Stimulated emission rate B21 n2 = Absorption rate B12 n1 In thermal equilibrium the ratio of the population is given by: n2 = e;(E2;E1)=(kT ) < 1 & n1 We need a population inversion (e ective negative temperature): T.H.E n2 > n1 Patrick Roblin OHIO S ATE T UNIVERSITY 15 The Ohio State University % ' $ Population Inversion in a Seminconductor Laser Population inversion 1111111 0000000 1111111 0000000 1111111 0000000 1111111 0000000 Ec Fn Ev EF Filled States EF Ec F p Empty States Ev Filled States P N need a direct bandgap semiconductor requires degenerated doped semiconductors with & Fn ; Fp > h! ' Eg Patrick Roblin T.H.E OHIO S ATE T UNIVERSITY 16 The Ohio State University % ' $ Bias Dependence of Population Inversion Large population inversion Small population inversion Empty States Empty States Ec Ev EF Filled States EF 111 000 111 000 111 000 Empty States Filled States Ec Ec 111111 000000 111111 000000 111111 000000 111111 000000 111111 000000 P Ev Ev EF Ec Ev Filled States N P N Higher Bias Intensity Patrick Roblin Filled States 111111 000000 111111 Empty States 000000 111111 000000 111111 000000 Lower Bias & 1111111 0000000 1111111 0000000 1111111 0000000 1111111 0000000 Intensity Intensity hω hω hω T.H.E OHIO S ATE T UNIVERSITY 17 The Ohio State University % ' & $ Heterojunctions Quantum con nement is used to achieve population inversion atOHIO S lower threshold current to allow for room temperature operation.TATE Patrick Roblin T.H.E UNIVERSITY 18 The Ohio State University % ' $ Most Recently Developed: the Green and Blue Lasers & Patrick Roblin T.H.E OHIO S ATE T UNIVERSITY 19 The Ohio State University % ...
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This note was uploaded on 01/11/2012 for the course ECE 432 taught by Professor Lu during the Fall '08 term at Ohio State.

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