Semiconducter+Material


 * Note this information comes from [|How Stuff Works.com]

Semiconductors have had a monumental impact on our society. You find semiconductors at the heart of [|microprocessor chips] as well as transistors. Anything that's computerized or uses [|radio waves] depends on semiconductors. Today, most semiconductor chips and transistors are created with silicon. You may have heard expressions like "Silicon Valley" and the "silicon economy," and that's why -- silicon is the heart of any electronic device. A diode is the simplest possible semiconductor device, and is therefore an excellent beginning point if you want to understand how semiconductors work. In this article, you'll learn what a semiconductor is, how doping works and how a diode can be created using semiconductors. But first, let's take a close look at silicon. Silicon is a very common element -- for example, it is the main element in sand and quartz. If you look "silicon" up in the [|periodic table], you will find that it sits next to aluminum, below carbon and above germanium. Carbon, silicon and germanium (germanium, like silicon, is also a semiconductor) have a unique property in their electron structure -- each has four electrons in its outer orbital. This allows them to form nice crystals. The four electrons form perfect covalent bonds with four neighboring [|atoms], creating a lattice. In carbon, we know the crystalline form as [|diamond]. In silicon, the crystalline form is a silvery, metallic-looking substance. Metals tend to be good conductors of electricity because they usually have "free electrons" that can move easily between atoms, and electricity involves the flow of electrons. While silicon crystals look metallic, they are not, in fact, metals. All of the outer electrons in a silicon crystal are involved in perfect covalent bonds, so they can't move around. A pure silicon crystal is nearly an insulator -- very little electricity will flow through it. But you can change all this through a process called doping.
 * [[image:http://static.howstuffworks.com/gif/solid-state1.jpg align="center" caption="silicon"]]Clockwise from top: A chip, an [|LED] and a transistor are all made from semiconductor material. ||
 * [[image:http://static.howstuffworks.com/gif/diode-periodic.gif align="center" caption="silicon periodic table"]]Silicon sits next to aluminum and below carbon in the periodic table. ||
 * [[image:http://static.howstuffworks.com/gif/diode-silicon-lattice.gif align="center" caption="silicon lattice"]]In a silicon lattice, all silicon atoms bond perfectly to four neighbors, leaving no free electrons to conduct electric current. This makes a silicon crystal an insulator rather than a conductor. ||

=Doping Silicon= You can change the behavior of silicon and turn it into a conductor by doping it. In doping, you mix a small amount of an impurity into the silicon crystal. There are two types of impurities: A minute amount of either N-type or P-type doping turns a silicon crystal from a good insulator into a viable (but not great) conductor -- hence the name "semiconductor." N-type and P-type silicon are not that amazing by themselves; but when you put them together, you get some very interesting behavior at the junction. That's what happens in a diode.
 * [[image:http://static.howstuffworks.com/gif/diode-periodic.gif align="center" caption="silicon periodic table"]] ||
 * N-type - In N-type doping, [|phosphorus] or [|arsenic] is added to the silicon in small quantities. Phosphorus and arsenic each have five outer electrons, so they're out of place when they get into the silicon lattice. The fifth electron has nothing to bond to, so it's free to move around. It takes only a very small quantity of the impurity to create enough free electrons to allow an electric current to flow through the silicon. N-type silicon is a good conductor. Electrons have a __n__egative charge, hence the name N-type.
 * P-type - In P-type doping, [|boron] or [|gallium] is the dopant. Boron and gallium each have only three outer electrons. When mixed into the silicon lattice, they form "holes" in the lattice where a silicon electron has nothing to bond to. The absence of an electron creates the effect of a __p__ositive charge, hence the name P-type. Holes can conduct current. A hole happily accepts an electron from a neighbor, moving the hole over a space. P-type silicon is a good conductor.