P-Type vs. N-Type Semiconductors
Difference Between P-Type And N-Type Semiconductors
Semiconductor materials used in semiconductor devices are in the form of single crystals. This means that the position of every atom is fixed relative to every other atom in a three-dimensional pattern. This pattern is called the crystalline structure.
Most solid materials are not perfect crystals. They have defects, such as misplaced atoms or impurities, that change their physical properties. For example, defects can have a great effect on the ability of a material to conduct electricity. When scientists carefully control defects, they can make materials with a wide range of useful properties.
One type of defect is especially important in silicon and other semiconductors. In a perfect crystal of silicon at room temperature, no electric current can flow. When certain impurities, called dopants, are carefully added to the silicon crystal, it can be made to carry electricity. However, a semiconductor, such as silicon, and a typical electrical conductor, such as copper wire, each carry electricity in a slightly different way.
In a typical conductor, every atom has one or more negatively charged electrons loosely bound to it. When a battery is hooked up to the conductor, the electrons move away from the conductor’s negative end and toward its positive end. This movement of electrons creates an electric current. The electrons are called charge carriers, because an electric current is the flow of electric charge.
The semiconductor silicon can be made to conduct a small amount of electricity by adding a few atoms of phosphorus to the silicon crystal. Each phosphorus atom has an electron that is less tightly bound than the electrons in the silicon atoms. In the silicon crystal, these loosely bound electrons are freed from their phosphorus atoms and become charge carriers like those in a conductor. Thus an electric current can flow in the impure silicon, but not as well as in a typical conductor. This is because there are many fewer atoms with free electrons.
When an atom of the chemical element boron is added to a silicon crystal, the boron acts as if it has a space for an extra electron. This space will attract an electron to it, leaving a space with a positive charge at the original place of the electron.
An electron from a nearby silicon atom can fill the empty space in the boron atom. This leaves an empty space in the silicon atom. An electron from another silicon atom can fill that space, and so on. It is as if a positively charged “hole” is moving through the crystal.