Carbon, Silicon, and Germanium are the three semiconducting materials listed as "pure," their impurities do supply a free charge
20.3 Resistance and Resistivity detailed Summary:
This chapter was over Resistance and Resistivity. The resistance of an object depends on its shape and the material. It also decreases with a larger cross-sectional area. Conductors have the smallest resistivities, insulators have the largest, and semiconductors have intermediate resistivites. For example, aluminum has a larger diameter than copper, because copper is a better conductor. Therefore, coppers diameter is smaller than aluminums diameter. The resistivity of ALL materials depends on temperature. Over relatively small temperature changes (about 100˚ C or less), resistivity p varies with temperature change ΔT, expressed as p=p_0 (1+aΔT). α is positive for metals, but is negative for semiconductors. They become better conductors at high temperature.
The longer the cylinder, the greater its resistance. The longer its area, the smaller its resistance. The greater the diameter of the cylinder, the more current the object can carry.
A great example that is most common is a thermometer (a semiconductor), the device is small, but as soon as it touches a person, it quickly turns into a thermal equilibrium.
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