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Semiconductors

The best way to think of a semiconductor is as part way between a full conductor (like copper and other metals) and an insulator (like rubber and hydrogen). Metals make good conductors of electricity because they have very loosely attached electrons in their outer orbits. These electrons can be easily detached and then “flow” through the metal. This flow is electricity.

What sets a semiconductor apart from an insulator is the energy needed to detach an electron so that it can flow. In semiconductors, the energy is relatively low at about 4 electronVolts. In insulators, the level is substantially higher, generally beyond any practical limit that can be achieved. Hence, semiconductors have electrons that can flow easily, just not quite as easily as those in conductors.

Silicon

Silicon is the 14 element on the periodic table, sitting between aluminum and phosphorus and having a structure that is quite similar to that of carbon. Silicon is the main element is sand and the primary component of quartz.

In its pure form, silicon actually forms crystal structures that have nearly perfect covalent bonds. This means that there are no electrons free for detachment and silicon, when pure, is thus a very good insulator. What turns silicon into a semiconductor is the process of doping.

It is worth noting that silicon has very diverse uses. It can be found in everything from concrete to bricks to ceramics. It is also used to may glass and to form industrial abrasives as well as in the production of steel and aluminum. Less than 10% of silicon goes into semiconductors.

Doping

Doping is a process of making a pure product impure. Of course, it is more technical than that because the impurity must be a specific material. Many materials can be “doped” to create semiconductors, but silicon is by far the most common. In the case of silicon, two types of doping occur: N-type and P type.

In N-type doping, an impurity is added to the silicon that will give it a net NEGATIVE charge (hence the term N-type). Usually the substance that is added is phosphorus or arsenic. Each of these materials has 5 electrons in its outer orbit while silicon has only 4. When they combine, 8 electrons are used to complete and orbit, but that leaves on electron out. That single electron becomes much like the free electron in a metal. It is weakly bound to the silicon-phosphorus or silicon-arsenic crystal, but can be easily displaced to create a flow of electricity. N-type silicon, unlike pure silicon, is an excellent conductor.

In P-type doping the impurity is added to create a net POSITIVE charge. The impurity is usually boron or gallium. These materials have one less electron than silicon so the combination gives only 7 electrons total. This means the orbit almost complete, but not quite. The “hole” left by the missing 8th electron can accept an electron.

The N-type and P-type silicon are brought into close contact, though they are not allowed to touch. When sunlight is added, it excites the electrons in the N-type silicon and releases them from their bonds. They are free to flow at this point and attempt to reach the P-type silicon. When they cannot reach the P-type, a voltage is set up. By connecting the N and P-type silicon through an external circuit the electrons are allowed to flow and can be used to do work.
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