High Brightness LEDs

Chapter5

Chapter6

屠嫚琳

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Contents Bandgap Energies Band Offsets Quantum Well Devices Substrate Absorption Distributed Bragg Reflector LEDs

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Bandgap Energies

At room-temperature the direct-indirect crossover occurs for an alloy composition of x =0.53,corresponding to an emission wavelength of

555 nm. The (AlxGa1-x)0.5In0.5P alloy is direct from 555 to 650 nm.

Exx (eV)

EXxx (eV)

The direct() bandgap and indirect(X) bandgap vari-ation with composition is given by

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Band Offsets

A least squares fit to these data points results in expressions for the conduc-tion band offsets given by

ΔEc(x) = 0.369x (eV) x 0.53≦

ΔEc(x) = 0.285-0.157x (eV) x 0.53≦

and valence band offsets given by

ΔEv(x) = 0.241x (eV)

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Quantum Well Devices Quantum wells employed in (AlxGa1-x)0.5In0.5P have numero

us improvements in device performance,including: Reduced threshold currents Shortened wavelength cw operation Increased characteristic, operating temperatures and increased pow

er conversion efficiencies “thin” QW LED devices offer the potential of higher inject

ed carrier densities, and hence more efficient radiative recombination.

Quantum well structures utilizing quantum size effects(QSE) to shorten the emission wavelength.

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Substrate Absorption (cont.) The (AlxGa1-x)0.5In0.5P alloy lattice-matched to GaAs is the

material of choice for visible AlGaInP LED.However, the GaAs substrate absorbs at least half of the light emitted by the active layer.

So, in order to circumvent absorption of light by the GaAs substrate, many attempts have been made to grow the AlGaInP alloy lattice-mismatched on transparent substrates. Such as GaP.

But the cost of transparent substrate(TS) is higher than absorbing substrate(AS), we can grow distributed bragg reflector between GaAs substrate and active layer.

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Substrate Absorption

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Distributed Bragg Reflector LEDs The DBR LED structure is well-suited to minimize the abs

orption of light by the GaAs substrate in high-efficiency (AlxGa1-x)0.5In0.5P visible LEDs.

A highly reflective DBR mirror is generally formed from a repeated periodic stack of alternating high and low index quarter-wavelength layers.

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Applications for High-Brightness LEDs

Automotive signal lighting Automotive interior lighting Traffic signal lights Large-area displays Liquid -crystal display backlighting Fiber-optic LED backlighting techiniques

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Fiber-optic LED backlighting techiniques

Using LED lamps to backlight an LCD display is to pipelight into a bundle of plastic fiber optics.

The fiber-optic bundle is formed into a flat sheet behind the diffuser-transflector film.

Then the fiber-optic sheet to evenly backlight the LCD display.

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Fiber-optic liquid display backlighting module.