You have learned that electrons in a semiconductor which gain the correct amount of energy jump to the conduction band, where they can move freely.  But the action in a semiconductor is not limited to the conduction band.  Look at the valence band in the image at the left.  The valence band is no longer full, but contains a few empty energy states.  One of those empty states has been circled in the diagram.  An electron can move into that energy state from one of the adjacent atoms.  This electron, however, can go no further.  The only empty state around it in its new location is the state from which it came.  It still looks like the valence band doesn't significantly contribute to conduction.  But appearances can be deceiving.  Individual electrons don't move large distances through the valence band; instead, "holes" do.  Once our original electron has moved into the empty energy state next door, another electron can move into the energy state left vacant by the original electron.  This second electron leaves a vacant state that can in turn be occupied by another electron, and so on.  Click on the image to the left to open up an animation of this process.  The net effect of all this shuffling is that an empty energy state, called a hole, moves through the material.  This hole motion is depicted by the second part of the animation.

In a pure semiconductor, every electron moving through the conduction band has left behind it a hole moving through the valence band, doubling the conduction.  (Of course, 2 charge carriers per 10¹³ atoms is not much better than 1 per 10¹³ atoms, particularly when compared to a conductor that generally has a charge carrier associated with every atom.  This is why pure silicon is such a poor conductor.)  Close the previous animation, and click here to open a different animation.  This new animation depicts the flow of charges through intrinsic semiconductors with both electrons and holes considered.  The first part of the animation shows random motion in the absence of a voltage.  The second part shows the motion in the presence of a voltage.  We can visualize a voltage as a "tilting" of the wire, using gravity as an analogy for the electric force on electrons.  Note that when we do this, the holes move uphill.  This inclination of holes to move opposite the direction of electrons will become important as we expand our understanding of semiconductor devices.

The skeptical student will be wondering what the significance of holes is, beyond a visual tool.  After all, a hole is really the absence of an electron, not a physical particle.  The electrons are what move, even if individual electrons don't move very far.  But scientists and engineers who study the properties of semiconductors and devices made from them have found the hole to behave just like a particle itself.  One can measure an "effective mass" of holes in a material, which can in turn be used just like the mass of more tangible particles when predicting physical effects.  In general, the hole behaves like a positive electron.  If a voltage is applied, the hole moves toward the negative terminal, opposite the direction that an electron would move.  As we move into the dicussion of doped semiconductors and the devices built out of them, we will find the concept of holes essential to our understanding of the observed behaviors.
 

What is a doped semiconductor? Go to the next page to find out!