EMS172L_Lab04_Semiconductors_LEDS

EMS172L_Lab04_Semiconductors_LEDS - UC Davis EMS 107L...

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UC Davis – EMS 107L Electronic, Magnetic, and Optical Properties Laboratory – Prof. Ricardo Castro 1 UNIVERSITY OF CALIFORNIA, DAVIS Department of Chemical Engineering and Materials Science EMS 172L: Electronic, Magnetic, and Optical Properties Laboratory Laboratory Class IV – Semiconductors The number of electrons and holes when using an intrinsic semiconductor is considerably low. This leads to high band gaps and limits the practical application for these materials in electronic devices. To overcome this problem and expand possibilities of applications, the number of charge carries can be increased by using dopants in the so called extrinsic semiconductors. These dopants form holes or provide extra electrons to lower the band gap of the semiconductor. For instance, when adding Group V elements (such as phosphorous at 0.0001%) into a silicon crystal (Group IV), since phosphorous has 5 valence electrons and silicon just 4, there will be 1 electron that is loosely attached to the structure. This electron can easily be excited to the conduction band. Considering the band theory discussed in the last lab class, we could say that we are introducing a donator level much closer to the conduction band, virtually leading to a decrease of the band. In the phosphorous example, the new band gap is 0.045eV, which is lower when compared to pure Si 1.1eV. Since these electrons can be excited with the energy provided by the room temperature, the electrons can be moved if an electrical field is applied. Since the main carriers are the n egative electrons (no associated holes as in the intrinsic semiconductor), materials of this type are called n-type semiconductors. A similar approach can be used to increase the number of holes instead. That is, if impurities of
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EMS172L_Lab04_Semiconductors_LEDS - UC Davis EMS 107L...

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