Determination of the Energy Gap in Semiconductors Part I: Prequel: Understanding Band Gap Through LED's 1. Purpose of th

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Determination Of The Energy Gap In Semiconductors Part I Prequel Understanding Band Gap Through Led S 1 Purpose Of Th 1 (98.35 KiB) Viewed 40 times

Determination of the Energy Gap in Semiconductors Part I: Prequel: Understanding Band Gap Through LED's 1. Purpose of the Experiment The purpose of the prequel is to get a physical understanding of what an electrical band gap is. By using common examples and easy calculations, an intuitive understanding of band gap can be developed. 2. Theoretical and Experimental Background Requirements Students are expected to have a basic understanding of what a band gap is, and how it effects electrical conduction in materials. Basic understanding of the visible light spectrum, wavelengths and energies of primary light colors, and voltage. As you should know from class discussions about band gap, the band gap of electrical conduction refers to the minimum energy required to promote electrons from valence electrons to the conduction band, where they are free to move to applied electric fields. A crude example of this is to imagine an atom, Silicon (atomic number 14, band gap 1.14eV) for example, with 14 valence electrons. By applying a 1.14eV electric potential to the silicon atom, it will eject one of its electrons, causing the electron to be "free" for conducting electricity Within a Light Emitting Diode (LED), voltage is applied to the terminals and light is produced. It can be shown that the light produced is monochromatic, and has an energy equal to the bandgap of the material the LED is made of A=hc/E=1.24um/E(ev) From the above discussion, one can conclude that LED's which require different applied voltages will produce different colours of light. Higher energy photons require a higher applied voltage than lower energy photons, which will be the focus of this prequel. 3. Description of Experimental Setup This experiment requires an adjustable DC power supply, and LEDs of varying colours. The power supply can be connected directly to each LED. 4. Performance of Experiment - Connect the DC power supply to one of the LEDs. Ensure the positive of the power supply is on the positive leg of the LED (typically the longer leg) - Turn the power to supply to ov. Slowly increase the voltage of the power supply until light is produced from the LED. - DO NOT EXCED AN APPLIED VOLTAGE OF SV - Once light is seen, record the applied voltage and colour produced. - Repeat for all LED's 5. Evaluation of Results (1) Using equation (1), calculate the photon energy for red, green and blue light. (ii) Knowing that LEDs produce photons with energy equal to their bandgap, use your results from (1), determine which colour of light requires the highest applied voltage, and which colour requires the lowest voltage (ii) Compare your predictions to what was found experimentally. Briefly comment on any discrepancies. (iv) (Bonus) What voltage was used to produce white light? How does this compare to the applied voltages of the other colours? What can you conclude about how white light is produced within an LED? You may need to research properties of white lights, and how to produce white light.