Silicon

A Silicon atom has 14 electrons around the nucleus, and of these, there are 4 valence electrons on the outermost orbital. When this is made into a single crystal, it can be used as a material for semiconductor products.

When it crystalizes, the nuclei share electrons and they bond with 8 electrons around each nucleus. Electricity for the most part does not conduct in this pure monocrystalline silicon state. Dropping silicon with other impurities changes it so it is conductive. The semiconductor is categorized as a P-type or N-type depending on the type of impurities that are doped. Junctions based on the P-types and N-types are integrated into one chip in order to use it as an electronic component.

Today, most semiconductor chips and transistors are created with silicon since it 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 semiconductor is, how doping works and how a diode can be created using semiconductors.

Silicon is a very common element – for example, it is the main element in sand and quartz. If you look Si up in the periodic table, you will find that it sits next to aluminum (Al), blow carbon and above germanium (Ge).

Carbon, silicon and germanium 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 substances.

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 SiO2 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 more around. A pure single crystal is nearly an insulator – very little electricity will flow through it.

But you can change all this through a process called Doping.

You can change the behavior of silicon and turn it into a conductor by doping it. In doping, you mix a small amount of impurity into the single crystal. There are two types of impurities:

• 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 negative 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 positive 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.

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”. A diode is the simplest possible semiconductor device. A diode allows current to flow in one direction but not the other. It is a one-way turnstile for electrons.