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Name: Mark
Status: student
Grade: other
Location: Outside U.S.
Country: USA
Date: Winter 2011-2012


Question:
Is it true that generally electrical conductivity of metals first increases with temperature, then decreases? (one of my teachers said that mobility of electrons increases rapidly and thus conductivity increases but with the increasing temperature, their movement is restricted by vibrating atoms, so the conductivity decreases.. )

Replies:
Mark

From the following Wikipedia article:

http://en.wikipedia.org/wiki/Electrical_resistance_and_
conductance#Temperature_dependence

"Near room temperature, the resistivity of metals typically increases as temperature is increased, while the resistivity of semiconductors typically decreases as temperature is increased."

What this means is that current is reduced as temperature and resistivity increase. Please note the word "typically." That means in most situations as there are a few situations where this is not true as this article discusses.

Sincere regards, Mike Stewart


Mark,

In general, this is not true for metals as can be seen in the Handbook of Chemistry and Physics. This reference book gives resistivity (1/conductivity) for a wide range of metals and over a range from almost absolute zero to 900 Kelvin. In general, conductivity decreases with increases in temperature from low temperature to high. That said, we can explain this general trend by noting that good conductors (metals) have many electrons that are free to move in the material. These are called "conduction" electrons and act much like they are a flowing like water in a pipe. Occasionally, these electrons will collide with the fixed atoms of the metals (nuclei) and be deflected, or "scatter." They are slowed in this process and have to return to the overall flow of electrons resulting in what we see as "resistance" to electron flow or a decrease in conductance. Increasing temperature increases the agitation of this flow, the electron collisions, and the scattering amount resulting in a decrease in conductance over temperature. What your teacher probably meant to say (or you were meant to hear) is that the conductance of semiconductors will increase initially with temperature then decrease. Electrons in a semiconductor are more tightly bound to their respective atoms and are not free to roam the material and participate in current flow. Agitation of the atoms by increasing the temperature will cause electrons bound to the atoms to escape, roam the material and participate in current flow. Increased temperature increases the number of these available electrons and causes conductivity to decrease. As temperature increases, scattering from the vibration of the atoms will take over, and the semiconductor will act much like a metal--the conductivity will decrease over temperature.

Kyle J. Bunch


There may be a mix of misunderstanding and / or incomplete explanation. The comparison you make describe the behavior of both metals and semiconductors. The resistivity of a METAL increases with increasing temperature. In contrast the resistivity of a SEMICONDUCTOR decreases with increasing temperature. The "reason" for this is the different mechanisms for the transport of electrons. But this is an apples and oranges comparison. I suppose it is possible that in a temperature range a substance behaves like a METAL and in another temperature range behaves like a semiconductor, but I don't know of an example. In any case it would be misleading to describe such a substance as both a metal and semiconductor. Physicists would consider these as different PHASES of the substance. An analogy would be the resistivity of a solid phase and liquid phase of a given substance. The mechanism for electron transport in those two phases is different. You can see details at the website:

http://en.wikipedia.org/wiki/Electrical_resistivity_and_
conductivity#Semiconductors

Vince Calder


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