By Robert Hazen, Ph.D., George Mason University
Most types of semiconductors begin with a crystal of the element silicon; that’s element 14 on the periodic table. Silicon, when it’s electrically neutral, has 14 protons and 14 electrons, so it has a half-filled outer shell. Four of the eight possible electrons are present in the third shell. Therefore, silicon’s strategy is to find four more atoms.
Structure of Silicon
In a silicon crystal, each silicon shares four electrons with four neighboring silicon atoms. Every atom sees the full complement of 18 electrons. This situation is very much like the covalent bonding of carbon, which lies just above silicon. Silicon is a very poor conductor of electricity: since all the electrons are locally shared, since each silicon sees those 18 electrons, there’s not much incentive for an electron to go traveling off as an electric current.
Think about what happens now if you substitute an atom of phosphorus. Look at the periodic table; that’s element 15. Phosphorus has 15 electrons instead of 14 electrons. That’s one more than you need to give a filled shell in the silicon crystal. So this extra electron becomes de-localized, just like electrons in a metal. What you do is add a small fraction of phosphorus impurity to a silicon crystal—no more than about one atom in a million.
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The Different Types of Semiconductors
Engineers add a controlled number of these negatively charged electrons, effectively, to a silicon crystal, and the result is an n-type semiconductor. That’s a semiconductor with a few mobile negative charges, a few electrons. By the way, that minor impurity, the phosphorus, is called a dopant, and this process is called doping of the silicon crystal.
Now, apply the same logic to a silicon crystal in which a single atom of aluminum—element 13—substitutes for a silicon atom. In the periodic table, aluminum is right next door to silicon; element 13. With 13 electrons, including three electrons in the outer shell, you have one too few electrons; there is essentially a missing electron in this silicon crystal now. A missing electron is called a hole in the terminology of electronics.
A hole is essentially the absence of an electron; it carries a positive charge. By adding a small fraction of aluminum atoms to a silicon crystal then, you’re adding a controlled number of positively charged holes, which can move around the crystal. The result is a p-type semiconductor.
Learn more about atoms.
Features of Different Types of Semiconductors
One important point: n- and p-type semiconductors, in and of themselves, are lousy conductors of electricity. They don’t really have any significant use by themselves. In order to think about n- and p-type semiconductors, Think about a huge, crowded parking lot; say, on the 24th of December, just before Christmas, when everybody’s out shopping, looking for a place to park to visit the mall.
N-type semiconductors are very analogous to a parking lot that’s completely full, with a few extra cars driving back and forth in the rows, trying to find a parking place. Those cars just keep moving about. They’re like the electrons, they can’t seem to find a parking place in an n-type semiconductor.
A p-type semiconductor with holes is a situation where you have a very crowded parking lot, but there may be just a few empty parking places. Now you have to imagine that the shoppers, eager to get as close as possible to the store they want to visit, are constantly sitting in their cars, waiting to dash out of one place to get to a place that’s slightly closer to the store they want to shop at.
Learn more about why some materials are useful because of distinct physical properties.
Types of Semiconductors from a Materials-Fabrication Point of View
Individual cars darting around—but you can model that mathematically more easily, not as cars moving but as holes moving. Indeed, when traffic engineers model a traffic jam on an interstate, they do exactly that. You could keep track of thousands of cars that are creeping along in one direction, but mathematically it’s easier to keep track of the few holes that open up and move backward through the traffic jam.
From a materials-fabrication point of view, these n-and p-type semiconductors are quite remarkable. Most metals, most insulators, and other materials in our world are manufactured in bulk, in big vats, or big refineries, and so forth; these things that hold large amounts of material, and you manufacture large quantities of material that have exactly the same properties.
But, semiconductors are manufactured atom by atom under exquisitely controlled conditions. You have high-tech, ultra-clean laboratories, you have engineers wearing clean suits from head to toe, and you have vacuum chambers where various quantities of atoms are deposited, sputtered off one by one—controlled amounts of phosphorus and aluminum is added to the silicon.
Common Questions about the Structure of Silicon & Various Types of Semiconductors
N-type is among the types of semiconductors, and it’s produced when a controlled number of negatively charged electrons are added effectively to a silicon crystal. This type of semiconductor has only a few mobile negative charges/electrons.
P-type is a type of semiconductor produced when aluminum atoms are met with silicone crystals. Engineers add a controlled number of aluminum atoms to a silicon crystal to create positively charged holes that result in p-type semiconductors.
Different types of semiconductors can be compared to a parking lot. N-type semiconductors are like a crowded parking lot where there is no parking space. On the other hand, the p-type semiconductor can be compared to a parking lot with many parking spaces.