From the beginning, we’ve said that semiconductors are so useful because we can easily and significantly alter their conductive properties. Now that we know that conductivity relates to free charge carriers, we can change that statement slightly, to say that they’re useful because we can easily and significantly alter the number of charge carriers available.
We saw two means of increasing charge carriers in Part 1, namely Thermal Generation and Optical Generation. Both of those methods impart energy to the electrons, causing them to break from their bonds and jump into the conduction band. The result is the creation of an Electron-Hole Pair
. Both the newly created free electron and hole are available to aid in conduction of current through the material. However, when the heat or light is removed, the electrons, wanting to return to their lower energy state, will tend to drop back down into the Valence band, into one of the holes left when they were excited into the Conduction Band. This process is known as Recombination
. A recombination event requires one hole and one free electron, and both are destroyed in the process (the electron isn’t actually destroyed, but once trapped back in the bond, it is no longer useful for conduction). Just as energy was required to get the electron to jump up, the electron gives off energy when it jumps back down. This can be in many forms; common ones are heat, and light (the latter is the basic premise of Light Emitting Diodes). If all sources of heat and light were removed, every single electron would eventually return to the Valence Band, recombining with all the holes, and leaving the material we began with before the heat or light application.
http://media.hardwareanalysis.com/articles/small/10652.gif" alt="Semiconductor Physics">Fig. 6 - An example of carrier generation by optical excitation. If the source of energy (light, in this case) is removed, the electrons will eventually drop back down into the valence band, recombining with a hole, and shedding their excess energy in the process, typically in the form of heat or light.
Our examples thus far have consisted of the exact same number of free electrons and holes in all cases, and at all times. They have to; every time we create a free electron, we also create a hole, and every time we lose a free electron, we lose a hole as well. Materials with the same number of electrons and holes are known as intrinsic
semiconductors. Recombination is always occurring, but the application of heat, for example, keeps generating new carriers, resulting in a certain number of carriers available at any time. Remove that heat, and they’ll all eventually recombine. So keeping the device around room temperature would ensure that we always had a certain number of charge carriers available, but for pure intrinsic silicon, as we’ve been considering, the number of charge carriers available due to thermal excitation, even well above room temperature, is minimal.
No, we need another method of increasing the number of charge carriers available; one that can increase the number of carriers a thousand-, if not a million-fold versus intrinsic silicon at room temperature. That method is called Doping