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Silicon and germanium both have a valence of four, meaning that there are four electrons in the outermost layer of electron orbits. For example, silicon is element number 14, meaning it has a total of 14 electrons. The first orbit (or energy level) has two electrons, the second eight, and the third four.
In a crystal lattice, the outermost orbits of the atoms touch each other. The electrons in this orbit don't stay with one particular atom—instead, they move from orbit to orbit. It’s this sharing of electrons that holds the atoms together. This ability to move from atom to atom is also the basis of electrical conduction—in a conductor, the electrons roam widely and are easily enticed to move in an electrical field, whereas in an insulator, they stay close to home.
Doping Silicon to Create N- and P-Types
Electrically, pure silicon is a terribly uninteresting material. It’s an insulator, but not a very good one. The fun begins when we add the right impurities, or dopants.
Just to the right of silicon in the periodic table is phosphorus, element number 15. Like silicon, it has two electrons in the first orbit and eight in the second. The difference is in the third orbit, which has five electrons to silicon’s four.
Now, let's say we were able to pluck out an atom in a block of silicon and replace it with a phosphorus atom. Four of the valence electrons of this new atom would circulate with the silicon electrons, but the fifth one wouldn’t fit in. This excess electron creates a negative charge, and the silicon becomes what we now call N-type.
This introduction of excess electrons isn’t like static charge. When you brush your hair so that it stands upright, you’ve simply moved some electrons temporarily. When you "dope" silicon, the charge is permanent, fixed in the crystal lattice (and doesn’t become a battery).
Similarly, to the left of silicon and one space up in the periodic table is boron, element number 5. It has two electrons in its first level and three in its second; its valence is three. If we replace a silicon atom with a boron atom, that means a missing electron in the valence orbital. We’ve now created a positively charged, or P-type, material.
Charge Conduction in N- and P-Type Silicon
As with the excess electron in N-type silicon, we can apply an electric field and cause a current to flow. However, the net effect this time is the flow of holes, not electrons. This is what makes the Hall effect go the wrong way.
It’s important to understand this mechanism of moving holes and electrons in doped semiconductors. In N-type material, an excess phosphorus electron wanders into the path of a neighboring silicon electron and displaces it. The displaced electron then takes the orbit of another one, and so on, until the last electron ends up at the starting point, the phosphorus atom.
This endless game of musical chairs—proceeding at near the speed of light—depends greatly on the temperature. At absolute zero, there is no movement. At about –60 °C, the movement is sufficient for the semiconductor effect to start in silicon. At about 200 °C, there’s so much movement that silicon practically becomes a conductor. It’s only within a relatively narrow range, about –55 °C to 150 °C, that silicon is a useful semiconductor.
In P-type material, the movement starts with an electron in the neighborhood of the boron atom. It fills the vacancy and then is itself replaced by another electron, and so on until the first electron moves away from the boron atom again. The moving is done by electrons, but the net effect is a moving hole.
When an electric field is present, the movement takes on a direction: electrons flow toward the positive electrode and are replaced by other electrons flowing out of the negative electrode.
It’s amazing how few dopants it takes to make an N-type or P-type material. Silicon has 5 x 1022 atoms per cubic centimeter. A doping level can easily be as low as 5 x 1015 boron or phosphorus atoms per cubic centimeter—in other words, one dopant atom for every 10 million silicon atoms. No wonder it took so long to discover the true nature of the semiconductor effects—in nature, the number of miscellaneous impurities is far larger than one in 10 million.
