|The band structure described
on the previous page seems to have no room for semiconductors. Conductors
have half-filled valence bands, so electrons move freely. Insulators
have full valence bands, making it difficult for electrons to move.
But if an electron in an insulator can gain enough energy to jump to the
band, available states abound. Materials with properties
we now associate with semiconductors were first identified in the early
1800s, but they remained little more than a scientific curiosity until
the 1900s. Over time, scientists discovered that they could control
the conductivity of certain materials, turning a good insulator into a
decent conductor by changing certain attributes, such as the temperature
of the substance or the amount of impurities found in it. These materials
that could conduct upon demand were called semiconductors.
Semiconductors made of one material (such as silicon) with no impurities
are called pure,
Click on the Semiconductor image to the right to open up an animation of conduction in semiconductors.
The difference between conduction in semiconductors and conduction in conductors is evident in the effect of temperature on resistance. You may have learned in a physics class that resistance increases (and conductivity decreases) as a resistor gets hot. Heating any device results in more atomic vibrations. If the atomic cores are vibrating more, electrons will have decreased mobility. Voila - increased resistance. This is the end of the story for conductors, but the resistance of semiconductors depends upon temperature in an additional manner. Increasing the temperature of intrinsic semiconductors provides more thermal energy for electrons to absorb, and thus will increase the number of conduction electrons. Voila - decreased resistance. This second effect in semiconductors is much greater than the effect of atomic vibrations, so increasing the temperature of a semiconductor ends up decreasing its resistance.
1 Numbers taken from The Quantum Dot,
by Richard Turton, a very good book that has unfortunately gone out of
Go to the next page to find out!
Copyright © 2003 Doris Jeanne Wagner and Rensselaer Polytechnic Institute. All Rights Reserved.