Semiconductor
Electrical conductivity is the ability to conduct electrical current under the application of a voltage, and it is defined as the ratio of the electric current density to the electric field in a material. A semiconductor is a material whose electrical conductivity lies between that of a typical conductor, like copper, and that of a typical insulator, like rubber. Semiconducting properties are usually strongly temperature-dependent. Although most common semiconductors are of a solid, crystalline material, some semiconductors are also found in liquid and amorphous states (an amorphous state lacks a distinct crystalline structure). Semiconductors commonly produced include the chemical elements silicon, germanium, diamond (carbon), tellurium, boron, and selenium, and the chemical compounds gallium arsenide, zinc selenide, and lead telluride.
The sign of the majority of its charged carriers—i.e., its electrons—normally indicates the classification of a semiconductor. Therefore, a semiconductor with an excess number of negatively charged carriers is termed n-type, while a semiconductor with an excess number of positively charged carriers is called p-type. Many of the important semiconductor devices depend upon fabricating a sharp discontinuity between the n- and p-type materials, the discontinuity being called a p-n junction.
In a pure semiconductor such as silicon or germanium, the outer electrons (commonly called valence electrons) of an atom are shared with adjacent atoms, producing the covalent bonds that hold the crystal together. As a result, these outer electrons poorly conduct a current (and so in this state are called "non-conduction" electrons). With the introduction of higher temperatures or radiation (such as visible light), or with the addition of impurities, semiconductors increase their electrical conductivity by several orders of magnitude; this is unlike other low-conductivity materials such as metals, which decrease their electrical conductivity in such circumstances. The increase in conductivity with temperature, radiation, or impurities is brought about by an increase in the number of conduction electrons that are the carriers of the electrical current.
In order to produce conduction electrons, temperature or radiation (but not impurities) may be used as a catalyst to excite the outer electrons out of their covalent bonds (from their occupied levels to unoccupied levels), allowing them to freely conduct electrical current. These excited electrons, along with the empty states that are left (often called "holes"), may move under the influence of an electric field, providing a means for conduction of electricity. (These holes act like electrons with positive charges.) These electron-hole pairs are the physical origin of the increase in the electrical conductivity of semiconductors with the use of temperature and light.
Another method to produce conduction electrons (the free carriers of electricity) is to add impurities to the semiconductor. The difference in the number of outer electrons between the added impurity (called the doping material, or dopant), and the host (the pure semiconducting material) provides the impetus for the negative (n-type) or positive (p-type) carriers of electricity. (As mentioned earlier, the n- and p-type carriers characterize the semiconducting material.) Adding impurities is the way that semiconductors are normally manufactured. Impurities, such as antimony, arsenic, bismuth, and phosphorous, are introduced to the semiconducting material by a chemical process called "doping" that increases the conductivity of the semiconductor. The type and level of doping determines whether the semiconductor is n-type (the electrical current is conducted by an excess number of free electrons, called "donors" for extra negative charge carriers) or p-type (the electrical current is conducted by a vacant number of free electrons, called "acceptors" for extra positive charge carriers). Each method produces a material that, when joined to the opposite type, produces a device that conducts electricity better in one direction than the other. Specifically, when n-type and p-type semiconductor regions are placed next to each other, the combination forms a semiconductor diode, and the contact region is called a p-n junction. Said another way, a diode is an electronic two-terminal device that highly restricts electrical current in one direction, but allows a minimal amount of electrical flow in the other direction. The conducting ability of each p-n junction is determined by the particular direction of the voltage. As a result, this determination can be exploited in order to regulate the characteristics of electrical devices. A series of these p-n junctions are used to make transistors and other semiconductor devices. In fact, a transistor's three-layer structure contains an n-type semiconductor layer sandwiched between two p-type layers (a p-n-p configuration), or contains a p-type layer sandwiched between two n-type layers (an n-p-n configuration).
In 1926 Dr. Julius Edgar Lilienfield from New York filed the first patent on what is now recognized as an n-p-n junction transistor. The first true transistor, a point-contact germanium device, was invented in December 1947 by John Bardeen (1908-1991), William Bradford Shockley (1910-1989), and Walter H. Brattain (1902-1987), scientists from Bell Telephone Laboratories and co-recipients of the 1956 Nobel Prize for this invention. Beginning in the 1950s transistors began to replace vacuum tubes. Eventually they helped to start the modern computer industry, being the critical ingredient for all digital circuits within computers.
Because their conductivities can be readily and reliably manipulated in a variety of ways, semiconductors can be effectively used in the fabrication of popular electronic devices, such as the crystal diode, transistor, integrated circuit, microprocessor, photo-detector, light switch, and others. The most common method of semiconductor manufacture is chemical vapor deposition. This method is usually used in the fabrication of semiconductors because of its ability to provide for various types and concentrations of introduced impurities (or dopants, as described previously). Because of increasing consumer demands for computer and general electronic equipment, a tremendous amount of attention has been devoted to the research, development, and manufacture of high-quality, high-purity semiconductors. There is ongoing research on other materials (such as gallium arsenide) that might yield semiconductors with useful properties for computing devices, such as faster switching speeds.
This is the complete article, containing 960 words
(approx. 3 pages at 300 words per page).
