Lab4-S08-LED - Lab 4: Light Emitting Diodes Laboratory...

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Lab 4: Light Emitting Diodes Laboratory Goals In this week’s lab you will: • fabricate a circuit containing a light emitting diode (LED) • investigate the effect of chemical composition and temperature on the emission properties of LEDs. Introduction Light-emitting diodes (LEDs) are used in a wide range of applications as compact and efficient light sources. Unlike incandescent sources, which emit a broad range of visible and infrared frequencies according to the spectrum of a blackbody at the high temperature of the glowing filament, LEDs emit light only over a narrow spectrum corresponding to the band gap of the semiconductor material that is used as the active component. Accordingly, LEDs emit useful visible light much more efficiently than is possible for ordinary light bulbs. In this laboratory, you will construct a circuit that can be used to drive a set of LEDs with different semiconductor compositions. The effects of the composition and temperature on the emission spectrum of the LEDs will be observed and recorded visually and using the Ocean Optics spectrograph/CCD detector system. The wavelength of light emitted is related to the band gap. Changes in the band gap, due to changes in orbital overlap from composition and structure, will then result in changes in the emitted light. Background Reading There is a good section on the band theory for materials in Kotz, Treichel and Weaver, supplemental section on materials (pages 643-655), which may be helpful to read prior to coming to the laboratory. Section 3 in Chapter 1 of What Science Is and How It Works by Derry describes the efforts to understand the motion of electrons in solids using quantum mechanical models that ultimately led to the discovery of band structure in solids. Below is a partial summary of the process. Theory As shown in some of the earlier laboratory exercises in this course, molecules and atoms emit light at wavelengths corresponding to the transition energies between a pair of energy levels. These energy levels arise from the molecular or atomic orbitals that are resident on single molecules or atoms. In crystals, however, the energy levels arise from a vast number of interacting atoms, which are arranged in repeating sequences according to the structure of the crystal. In a real sense, a single crystal of NaCl can be considered as an enormous single molecule. The overlapping atomic orbitals in a crystal lead to delocalized orbitals that extend over the entire crystal. These delocalized molecular orbitals tend to congregate around certain energies and in practice merge into bands . Whereas the energy levels of an atom or molecule are sharp, being at discrete energies, 4-1
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the bands of a crystal are a continuum of energies (see Figure 1). These bands are the origin of the high electrical conductivity of metals. The electrons in a metal are only weakly associated with the nuclei; in a way, bonding in metals can be described as a sea of delocalized and mobile valence electrons that serve to hold the positively charged
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Lab4-S08-LED - Lab 4: Light Emitting Diodes Laboratory...

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