1.3 P-n junction: part i

There are a couple of things wrong with . First of all, one of the rules regarding the Fermi level is thatwhen you have a system at equilibrium (that is, when it is a rest, and is not being influenced by externalforces such as thermal gradients, electrical potentials etc.), the Fermi level must be the same everywhere. Secondly, we havea big bunch of holes on the right and a big bunch of electrons on the left, and so we would expect, that in the absence of someforce to keep them this way, they will start to spread out until their distribution is more or less equal everywhere. Finally,we remember that a hole is just an absence of an electron, and since an electron in the conduction band can lower the systemenergy by falling down into one of the empty hole states, it seems likely that this will happen. This process is called recombination . The place where this is most likely to occur, of course, would be right at the junction between then and p regions. This is shown in .

Now is might seem that this recombination effect might just go on and on, until there are no carriers left in the sample. Thisis not the case however. In order to see what brings everything to a halt, we need yet another diagram. is more physical than what we have been looking at so far. It is a picture of the actual p-n junction, showing both the holes andthe electrons. We also need to put in the donors and acceptors however, if we want to see what goes on. The fixed (can't move around)charges of the donors and acceptors are represented by simple "+" and "-" signs. They are arranged in a nice lattice-likearrangement to remind us that they are stuck to the crystal lattice. (In reality however, even though they are stuck in thecrystal lattice, there are so few of them compared to the silicon atoms that their distribution would be quite random.)For the mobile holes and electrons, we will stay with the little circles with charge signs in them. These are randomlydistributed, to remind us that they are free to move about the crystal.

We will now have to allow some of the holes and electrons (again near the junction) to recombine. Remember, when an electron anda hole recombine, they both are annihilated and disappear. Note that this process conserves charge (and if we could calculateit) momentum as well. There is obviously some energy lost, but this will simply show up as vibrations, or heat, within the crystallattice. Or, in the case of an LED, as light emitted from the device. See, already we know enough about semiconductors to understand (somewhat) how an actual device works. Light comming from an LED is simply the energy which is realeased when an electron and hole recombine. We will take a look at this in more detail later. Let's allow some recombination to occur, as shown in .