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L4.Optoelectronic Materials

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  ME  –  421 G Alexandru Damoc, Muhamad Amru Eldan, Yazan Alshawa Optoelectronic Materials   -report- Purpose of the experiment -   interaction with some materials used in optoelectronics   -   measurement of the characteristics of some optoelectronic devices (luminescent diodes (LEDs), phototransistor diodes)   -   applications of optoelectronic devices with the aid of a test board EasyPIC 4  Theoretical background   The term „optoelectronics” refers to those devices based on semiconductors, where the recombination processes emit light. This radiation process is called „spontaneous e mission of light” because it involves nothing but electrons and holes.  Semiconductor materials present an energetic band structure and based on the positioning of the maximum on the VB compared to the minimum on the CB, they can be of 2 types: a)   DIRECT materials in which the maximum on the VB(valence band) coincides with the minimum on the CB(conduction band) b)   INDIRECT materials in which the maximum on the VB does NOT coincide with the minimum on the CB Fig.1 Band structure for semiconductors with a)direct b)indirect band Wavelength of the emitted radiation If light is produced through band-band recombination we can assume : C V  h E E       and using the relation : mat  C             c mat    = speed of light in the material = c o  /n    c o   = speed of light in vacuum    n  = the refraction index we obtain 0 h cn     .    ME  –  421 G Alexandru Damoc, Muhamad Amru Eldan, Yazan Alshawa LEDs and Phototransistors LEDs, or light-emitting diodes are semiconductor diodes which emit light during direct polarization of p-n junctions (they convert electric energy into light energy) Fig.2 Structure of a semiconductor diode If the diode is directly polarized (LEDs, semicondutor lasers), the luminous radiation increases exponentially with the voltage applied and is influenced by the temperature. Fig.3 Electron-hole recombination. Generation of photons The electron-hole recombination process : the direct current injects electrons in the band gap, which is poor in carriers, and there they recombine with the holes in a radiative or a non-radiative way. Therefore, the non- radiative recombinations “consume” the electrons for the radiative recombinations and so, the efficiency of the process is affected. The recombination electron-hole releases a quanta of energy, a photon. Therefore, in order to get the semiconductor to radiate, we have to sustain the electron-hole recombination process. There are two types of recombination  –  non-radiative, release energy as heat  –    ME  –  421 G Alexandru Damoc, Muhamad Amru Eldan, Yazan Alshawa radiative, release energy as light (another specter domain). Both types of recombination take place in a diode, but when the majority of them are radiative, those diodes are called LEDs. Just as the diodes, the transistors  are light sensitive. Phototransistors  are constructed especially to take advantage of this, by amplifying the input light signal variations. Description of laboratory proceedings: This experiment contains some of the usual optoelectronic devices used in electric circuits: photodiodes, phototransistors. At each optoelectronic device are installed terminals in order to connect them to the measurement device or the step-signal generator.    In order to determine the shape of the GT signals and the correlation between them,we: -   adjusted the oscilloscope with the given instructions -   connected both channels of the oscilloscope to the step-signal generator( which is in visualize mode) in order to see the G16 and G8 signals on the same grid of the oscilloscope -   re-adjusted the oscilloscope for a better image of the signals -   filled in the data tables for for both signals    In order to determine the I D =f(U D ) characteristics for electroluminescent diodes, we: -   connected the step-signal generator (which is now in measure mode) to the temrinals of each optoelectronic device on the EasyPIC4 test board -   in parallel with the step-signal generator at the same terminals, we connedted a multimeter to measure the U x  and U y  for each diode in both G16 and G8 modes -   the data we collected is introduced in the data tables corresponding to this part of the experiment  ME  –  421 G Alexandru Damoc, Muhamad Amru Eldan, Yazan Alshawa Experimental Measurements: -   Determination of the GT signals graph and the relation between them (G16 and G8) o   G16 Step 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 U(V) 0 0.44 0.86 1.31 1.5 2.21 2.63 3.08 3.52 3.97 4.39 4.84 5.29 5.74 6.16 6.61 o   G8 Step 1 2 3 4 5 6 7 8 U(V) 0 0.83 1.66 2.5 3.26 4.09 4.93 5.76 o   GRAPH
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