Semiconductors Defined

Semiconductors Defined - 35

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 (empty) conduction 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, or intrinsic, semiconductors.

Click on the Semiconductor image to the right to open up an animation of conduction in semiconductors.

Electrons in semiconductors fill the valence band, which is separated by a narrow band gap from the conduction band.

Band diagram of a Semiconductor

Another Word of Warning: As was emphasized before, the motion of electrons in solids is complex. Not only do the diagrams and animations included in these materials display an infintesimal percentage of the number of electrons actually present in a physical device and show motion only in one dimension, they also exaggerate the number of conduction electrons. Pure Silicon at room temperature has perhaps one conduction electron for every 10¹³ (that's ten trillion) atoms.¹ The author begs your forgiveness for not drawing all 10¹³ atoms for each conduction electron but feels the important physics is better demonstrated by the simpler, albeit somewhat inaccurate, diagrams and animations.

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 print.

What exactly is the relationship between the number of conduction electrons and the familiar concepts of resistance and resistivity?
Go to the next page to find out!