We said that the Conduction Band is at a higher energy than the Valence Band, and that all electrons in the Conduction Band have more energy than electrons in the Valence Band. Thus, in order to get an electron to jump from the Valence Band up into the Conduction Band, we have to give it some more energy. Specifically, we have to give it an amount of energy at least equal to, in magnitude, the size of the bandgap.
This energy can come in many forms. Heat is one example; if the sample is heated, thermal energy will excite some electrons up into the Conduction Band. This is called Thermal Excitation
or Thermal Generation
. Light energy is another example. If we illuminate our silicon sample with light of the right wavelength, the electrons will absorb the energy from the light, and jump up into the Conduction Band. This process is known as Optical Excitation
or Optical Generation
This example of Optical Generation is perhaps the clearest way of picturing the process. Consider the following: A small packet of light, called a Photon
, travels and strikes the silicon material. The Photon strikes an electron that is trapped in a bond. When the collision occurs, the electron is accelerated by the fast-traveling Photon. If the Photon had sufficient energy, the electron will be smashed out of the bond, and up into the Conduction Band. Once there, it is free to float about the material, and participate in electrical conduction. The diagram below illustrates.
http://media.hardwareanalysis.com/articles/small/10639.gif" alt="Semiconductor Physics">Fig. 8 - A diagram showing electrons being excited into the Conduction Band (broken free of their bonds) due to Optical Generation.
Given the above processes, we can realize that Silicon at room temperature will have a small number of electrons in its Conduction Band due to Thermal Generation. If we were to increase the temperature of the sample, more would jump into the Conduction Band, and the sample would conduct better. As we’ll see in Part 2 of this series, we can use other techniques such as Doping
to dramatically change the number of electrons in the Conduction Band, thus drastically altering the electrical properties of the silicon.