Semiconductors
Since conductors have a single valence electron, and insulators have a full valence ring of eight electrons, it makes sense that semiconductors such as silicon have four valence electrons. This also means that there is four spots for valence electrons in a silicon atom. When atoms of silicon combine they create covalent bonds. Co as in shared, and valent, meaning valence electrons. The result is a silicon crystal, which can be thought of as a lattice of silicon atoms, all connected by their shared electrons.
Doping is the process of adding impurities to silicon (or other semiconductors) to alter the electrical characteristics of the semiconductor. If we think of a pure, or intrinsic piece of silicon, there is ideally no free electrons, and no free “holes” in the valence bands for electrons to go.
We add an atom with 3 valence electrons. Elements with 3 valence electrons are aluminum, boron, gallium, and indium. This creates a tri-valent bond and leaves on open “hole” for an electron to flow in and out of. Doping a semiconductor this way creates a P-type material. To remember, you can think of the “P” as positive. There is a deficiency of one electron, so the 3 valence atom added is known as a acceptor impurity element.
The other method of doping is to add an atom with 5 valence electrons to a piece of silicon. Elements with 5 valence electrons are arsenic, antimony and phosphorus. This creates a crystal with an extra electron that is free to move around and is known as a penta-valent bond. This is known as a N-type material and can likewise be remembered that the “N” is for negative. There is an extra electron, so the 5 valence atom is known as a donor impurity element.
As you can see, the amount of impurities added directly effects the electrical characteristics, and can be used to regulate the amount of electrons moving through the material. Simply having a P or N type material on their own might have some uses, but when the two are used together, a P-N junction is formed, and is the basis of many electronic devices used today.
When the two materials are put together, they repel each other. The free electrons spread out, and some of them diffuse across the junction. This is known as the depletion region. Each time an electron crosses over, it leaves a pentavalent ion, with a relative positive charge. This electron in turn falls into a hole in the P-type material, and causes a negatively charged trivalent ion. It has space for one electron, and when it is filled we can say that it has gained a relative negative charge. This region, with positively charged extra electron ions, and negatively charged electron deficient ions, creates a potential difference between them. This is known as barrier potential. The barrier potential is usually .7v for silicon, and varies for other types of semiconductors. The basic idea is exactly the same though.
The barrier potential must be overcome to allow electron flow in the P-N type material. Biasing a P-N junction is the process of adding a voltage source to either allow or prevent the flow of electrons.
When the N-type is negative with respect to the P-type material, the electrons easily flow from the power supply, to the junction, then from one side to the other. The N-type material constantly feeds the electrons to the P-type, and the electrons flow from the p-type back to the power supply. This is known as forward bias. The P-N junction is arranged in the circuit to allow electron flow.
Reverse biasing is done by changing the polarities of the voltage source, so the negative terminal is connected to the P-type, and the positive terminal is connected to the N_type material. This causes the depletion layer to widen, because the negative terminal attracts the free “holes” and the positive terminal attracts the free electrons. Current is not allowed to flow.
So far we have been looking at this in a perfect world, but in reality, there are a few holes in a N-type material and likewise there is a few extra electrons in a P-type material. These are known as the minority carriers, and are mostly caused by thermal energy, or heating of the P-N junction. Under normal operating temperatures, this amount is negligible. Datasheets are invaluable when seeking the maximum temperatures, voltages, and dissipation of power for any electronic device.
