Moving back out to the molecular level and returning to the silicon crystalline structure, we return to the discussion of semiconductor dopants using Phosphorous and Boron. We now have discussed that introduction of either of these elements would accomplish the same task…so if that’s ultimately the case, what was the purpose for developing them both? Wouldn’t either method be sufficient on its own?
Yes, unless you wanted to do more with your silicon than just allow it to conduct electricity. Being conductive makes silicon about as useful as copper wire, but since silicon is so much more expensive, it would be silly to use this as its primary function. Rather, the primary advantage that semiconductors provide is when the two types of doped silicon are placed next to each other, like the image below.
Although in reality, the dopants would never be this densely populated, I’ve made the arrangement in this scaled version in order to more clearly illustrate what’s going on. In the image above, the atoms have been “just placed,” as in they have been arranged into the crystalline lattice, but their electrons haven’t evened out yet. With unpaired electrons just floating around the Phosphorous in such close proximity to the Boron and its missing electron, that weird ionic exception I mentioned before starts coming into effect.
The extra electrons from the Phosphorous are pulled and attracted by the electron slots open in the Boron, strong enough that all of the electrons are pulled away from the Phosphorous side of the crystal. Once on the Boron side, they bond with the nearby atoms and “fill the slots” in such a way that all locations that can accept an electron have an electron, and there are little to no remaining extra electrons or electron slots in the area close to where the two layers meet.
This connection point where two oppositely-doped layers come into contact with one another is called a “PN Junction,” and the area in which the electrons swap positions very close to the PN junction is a thin layer called the “depletion region.” When the electrons are allowed to even themselves out and move across the PN junction in order to find places for them to bond, there exists a slight net charge in the overall area very close to the junction itself that occurs from the ionic charge left behind when the electrons shifted positions.
This approximate area of ionic charge is what defines the boundary of the depletion region, as the term’s name was created to describe the area of crystalline material that “depletes” all of its conduction-band electrons due to this swap. It’s important to understand that only the doping atoms that are very close to the PN junction are depleted in such a way, as doping atoms elsewhere in the crystalline structure are too far away to be affected by ionic pull from the other side. In this way, only the current-conducting electrons immediately near the PN junction (and within the depletion region) will be swapped and bound, the rest will be allowed to continue their normal existence, helping to allow current to easily flow through the silicon material.
However, it’s this residual ionic charge that’s left behind in the depletion region that starts to have some rather interesting effects on the electrical current flowing through the material. For example, in the current state as shown above, current is only able to flow in one direction across the PN junction and not the other. In the image below, I’ve faded the crystalline structure to focus more on the electron movement through the depletion region. For simplicity’s sake, lets discuss two examples of electron flow, the first is flowing from left to right, the other flowing backwards, from right to left. Remember, this is electron flow, not current flow.
If an electron enters from the Phosphorous side (left side), traveling across the image, as it begins to feel the effects of the positive ionic field, it accelerates. As the electron nears the PN junction, it picks up speed pulled by the ionic charge of the missing electrons that have been swapped to the Boron side already. By the time the electron reaches the PN junction, where the Phosphorous changes to Boron, it’s traveling so quickly that the negative ionic charge on the Boron side of the junction only slows the electron a little bit, allowing it to pass through and continue on its way to the rest of the circuit path.
Alternatively, if an electron enters from the Boron side (right side), traveling the other direction (toward the left), then a different outcome occurs. As the electron nears the depletion region, it encounters a negative ionic charge. This negative charge slows the electron down, reducing its speed more quickly the closer it gets to the PN junction. Since opposites attract and like-charges repel each other, the closer the electron approaches the PN-junction (where the ionic field is strongest), the more force it has to fight to keep moving. Rather than sail across the junction like the previous example, an electron trying to cross from the other direction never reaches the PN junction at all, trapping the electrons on the Boron-side of the junction. Since electrons must be flowing in order to have an electric current, the current also stops and will cease to flow as long as something is trying to force it in that direction (as in a battery or some voltage source).
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