To understand why solid-state devices function as they do, it is necessary to examine the composition and nature of semiconductors. The two most common materials used for semiconductors are germanium and silicon. The essential characteristic of these elements is that each atom has four valence electrons to share with adjacent atoms in forming bonds. While both elements are used in semiconductor construction, silicon is preferred in most modern applications due to its ability to operate over a wider range of temperatures. The nature of a bond between two silicon atoms is such that each atom provides one electron to share with the other. The two electrons shared are in fact shared equally between the two atoms. This form of sharing is known as a covalent bond. Such bonds are very stable, and hold the two atoms together very tightly, requiring much energy to break this bond. [Figure 10-195] In this case, all of the outer electrons are used to make covalent bonds with other silicon atoms. In this condition, because all of the outer shell atoms are used, silicon takes on the characteristic of a good insulator, due to the fact that there are no open positions available for electrons to migrate through the orbits.
For the silicon crystal to conduct electricity there must be some means available to allow some electrons to move from place to place within the crystal, regardless of the covalent bonds present between the atoms. One way to accomplish this is to introduce an impurity such as arsenic or phosphorus into the crystal structure, which will either provide an extra electron or create a vacant position in the outer shell for electrons to pass though. The method used to create this condition is called doping.
Doping is the process by which small amounts of additives called impurities are added to the semiconductor material to increase their current flow by adding a few electrons or a few holes. Once the material is doped, it then falls into one of two categories: the N-type semiconductor and the P-type semiconductor.
An N-type semiconductor material is one that is doped with an N-type or a donor impurity. Elements such as phosphorus, arsenic and antimony are added as impurities and have five outer electrons to share with other atoms. This will cause the semiconductor material to have an excess electron. Due to the surplus of electrons, the electrons are then considered the majority current carriers. This electron can easily be moved with only a small applied electrical voltage. Current flow in an N-type silicon material is similar to conduction in a copper wire. That is, with voltage applied across the material, electrons will move through the crystal towards the positive terminal just like current flows in a copper wire.
A P-type semiconductor is one that is doped with a P-type or an acceptor impurity. Elements such as boron, aluminum, and gallium have only three electrons in the valence shell to share with the silicon atom. Those three electrons will form covalent bonds with adjacent silicon atoms. However, the expected fourth bond cannot be formed and a complete connection is impossible here, leaving a “hole" in the structure of the crystal. There is an empty place where an electron would naturally go, and often an electron will move into that space to fill it. However, the electron filling the hole left a covalent bond behind to fill this empty space, which leaves another hole behind as it moves. Another electron may then move into that particular hole, leaving another hole behind. As this progression continues, holes appear to move as positive charges throughout the crystal. This type of semiconductor material is designated P-type silicon material. Figure 10-196 shows the progression of a hole moving through a number of atoms. Notice that the hole illustrated at the far left of the top depiction of Figure 10-196 attracts the next valance electron into the vacancy, which then produces another vacancy called a hole in the next position to the right.
Once again this vacancy attracts the next valance electron. This exchange of holes and electrons continues to progress, and can be viewed in one of two ways: electron movement or hole movement. For electron movement, illustrated by the top depiction of Figure 10-196, the electron is shown as moving from the right to the left through a series of holes. In the second depiction in Figure 10-196, the motion of the vacated hole can be seen as migration from the left to the right, called hole movement. The valence electron in the structure will progress along a path detailed by the arrows. Holes, however, move along a path opposite that of the electrons.
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