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a) b - École Polytechnique de Montréal

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Equivalently, it is the inverse of the electrical resistivity, with units in Siemens per meter.<br />

Therefore, it can be written as:<br />

Equation 2-42 <br />

where R is resistance, is resistivity, l is length, and A is cross sectional area.<br />

Devices ma<strong>de</strong> from semiconductor materials are the foundation of mo<strong>de</strong>rn electronics, including<br />

radio, computers, telephones, and many other <strong>de</strong>vices. Semiconductor <strong>de</strong>vices inclu<strong>de</strong><br />

transistors, solar cells, many kinds of dio<strong>de</strong>s including the light-emitting dio<strong>de</strong>, the silicon<br />

controlled rectifier, and digital and analog integrated circuits. In semiconductors, electrical<br />

conductivity is controlled over a wi<strong>de</strong> range, either dynamically or permanently. These materials<br />

generally have an intermediate conductivity (Figure 2-17) which varies un<strong>de</strong>r different<br />

conditions such as specific frequencies in AC, light, temperature and electric field. Insulators are<br />

non-conducting materials as they lack mobile charges.<br />

Figure 2-17. Logarithmic conductivity lad<strong>de</strong>r locating some metals and conducting<br />

polymers(Pratt, 2001)<br />

<br />

44

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