ENGRI111_HW1_ans_2006 - ENGRI 111 Homework#1 Q1 Using...

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ENGRI 111 Homework #1 Q1. Using the "chemist's approach" of electrons in covalent bonds, explain how the substitution of a small amount of aluminum increases the conductivity of silicon. A1. In pure silicon (Group IVA element; has 4 valence electrons), the covalent bonding of the atoms satisfies the octet rule. Since all valence electrons are involved in covalent bonding, the application of a voltage fails to move them in any preferred direction. Without charge carriers, there can be no current. The addition of aluminum increases the conductivity of silicon due to the introduction of holes into the semiconductor. The covalent bonding of aluminum atoms (Group IIIA element; has 3 valence electrons) to neighboring silicon atoms involves only 7 electrons (out of 8 to satisfy the octet rule). The missing electron, known as a hole, is the charge carrier in this type of semiconductor (p-type). The introduction of a voltage allows a nearby electron to hop into the hole, which in turn leaves another hole; the process continues with more hopping events. The mobility of these electrons is responsible for the increased conductivity. The figure above illustrates this point using boron, another trivalent element, as the dopant.
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