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1、Semiconductors1 Semiconductors Fundamentals1.1 Semiconductors MaterialsSolid-state materials can be grouped into three classesinsulators, semiconductors and conductors. Insulators such as fused quartz and glass have very low conductivities. Semiconductors have conductivities, typically between those
2、 of insulators and those of conductors. The conductivity of a semiconductor is generally sensitive to temperature, illumination, magnetic field, and minute amount of impurity atoms. This sensitivity in conductivity makes the semiconductor one of most important materials for electronic application.Th
3、e study of semiconductor materials began in the early nineteenth century. Over the years many semiconductors have been investigated. The element semiconductors, those composed of single species of atoms, such as silicon (Si) and germanium (Ge)can be found in Column 4.however, numerous compound semic
4、onductors are composed of two or more elements. For example, gallium arsenide (GaAs) is an 3-5 compound that is a combination of gallium (Ga) from Column 3 and arsenic (As) from Column 5.Prior to the invention of the bipolar in 1947, semiconductors were used only as two-terminal devices, such as rec
5、tifiers and photodiodes. In the early 1950s, germanium was the major semiconductor material. However, germanium proved unsuitable in many applications because germanium devices exhibited high leakage currents at only moderately elevated temperatures. In addition, germanium oxide is water soluble and
6、 unsuited for device fabrication. Since the early 1960s silicon has become a practical substitute and has now virtually supplanted germanium as a material for semiconductor fabrication. The main reasons we now use silicon are that silicon devices exhibit much lower leakage currents, and high-quality
7、 silicon dioxide can be grown thermally. There is also an economic consideration. Silicon in the form of silica and silicates comprises 25% of the Earths crust, and silicon is second only to oxygen in abundance. At present, silicon is one of the most studied elements in the periodic table; and silic
8、on technology is by far the most advanced among all semiconductor technologies.Many of the compound semiconductors have electrical and optical properties that are absent in silicon. There semiconductors especially gallium arsenide (GaAs) are used mainly for microwave and photonic applications. Altho
9、ugh we do not know as much about the technology of compound semiconductors as we do about that of silicon, compound semiconductor technology has partly because of the advances in silicon technology.1.2 Crystal StructureSolids may be classified by structural organization into crystalline, polycrystal
10、line, and, amorphous types. An amorphous solid does not have a well-define structure; in fact, it is distinguished by its formlessness. In the past decade, amorphous silicon has received a great deal of attention, primarily due to its application in lowcost solar cells. Recently, amorphous silicon s
11、olar cells with an efficiency of greater than 10 percent have been realized, and lower-efficiency devices are being used in consumer electronics, e. g., hand-held calculators and cameras. Exploratory work is being performed to build amorphous field-effect transistors (FETs) for large-area displays a
12、nd image sensors. Nevertheless, amorphous semiconductors are not expected to play an important role in microelectronics in the foreseeable future. It shoud be pointed out that silicon dioxide, an extremely important material in semiconductor technology is also an amorphous solid. However, it is used
13、 as an insulator, so its electrical conduction property is not of importance to us.In a polycrystalline solid, there are many small regions, each having a well-organized structure but differing from its neighboring regions. Such a material can be produced inexpensively and is used extensively in mic
14、roelectronics, e.g., polycrystalline silicon (Poly-Si),which is used as a conductor , contact, or gate in transistors. Both amorphous and polycrystalline materials are structurally more complex, resulting in less well-defined device physics.1.3 Valence Bonds Model of SolidIn a crystal lattice, a pos
15、itively charged nucleus is surrounded by negatively charged orbiting electrons in each constituent atom. If the atoms are closely packed, the orbits of the outer-shell electrons will overlap to produce strong interatomic forces. The outer electrons, i.e., valence electrons, are of primary importance
16、 in determining the electrical properties of the solid. In a metallic conductor such as aluminum or gold, the valence electrons are shared by all the atoms in the solid. These electrons are not bound to individual atoms and are free to contribute to the conduction of current upon the application of
17、an electric field. The free-electron density of a metallic conductor is on the order of 10 23 cm-3, and the resulting resistivity is smaller than 10-5 cm. In an insulator such as quartz (SiO2), almost all the valence electrons remain tightly bound to the constituent atoms and are not available for c
18、urrent conduction. As a result, the resistivity of silicon is greater than 10 16 cm. As discussed in Section 1.1, each atom in a diamond lattice is surrounded by four nearest neighbors. Each atom has four electrons in the outer orbit, and each atom shares there valance electrons with its four neighb
19、ors. This sharing of electrons is known as covalent bonding; each electron pair constitutes a covalent bond. Covalent bonding occurs between atoms of the have similar outer-shell electron configurations. Each electron spends an equal amount of time with each nucleus. However, both electrons spend mo
20、st of their time between the two nuclei. The force of attraction for the electrons by both nuclei holds the two atoms together. For a zincblende lattice such as gallium arsenide, the major bonding force is from the covalent bonds. However, gallium arsenide has a slight ionic bonding force that is an
21、 electrostatic attractive force between each Ga- ion and its four neighboring As+ ion and its four neighboring Ga- ions.At low temperature, the electrons are bound in their respective tetrahedron lattice; consequently, they are not available for conduction. At high temperatures, the thermal energy e
22、nables some electrons to break the bond, and the liberated electrons are then free to contribute to current conduction. Thus, a semiconductor behaves like an insulator at low temperatures and a conductor at high temperatures. At room temperature, the resistivity of pure silicon is 210 5 cm, which is
23、 considerably higher than that of a good conductor.Whenever a valence electron is liberated in a semiconductor, a vacancy is left behind in the covalent bond. This deficiency may be filled by one of the neighboring electrons, which results in a shift of the deficiency location, as from location A to
24、 location B. We may therefore consider this deficiency as a particle similar to an electron. This fictitious particle is called a hole. It carries a positive charge and moves, under the influence of an applied electric field, in the direction opposite to that of an electron. The concept of a hole is
25、 analogous to that of a bubble in a liquid. Although it is actually the liquid that moves, it is much easier to talk about the motion of the bubble in the opposite direction.1.4 Donors and AcceptorsWhen a semiconductor is doped with impurities, the semiconductor becomes extrinsic and impurity energy
26、 levels are introduced. A silicon atom is replaced (or substituted) by an arsenic atom with five valence electrons. The arsenic atom forms covalent bond with its four neighboring silicon atoms. The fifth electron becomes a conduction electron that is “donated” to the conduction band. The silicon bec
27、omes n-type because of the addition of the negative charge carrier, and the arsenic atom is called a donor. Similar, when a boron atom with three valence electrons substitute for a silicon atom, additional electron is accepted to form four covalent bonds around the boron, and a positively charged “h
28、ole” is created in the valence band. This is a p-type semiconductor, and the boron is an acceptor. For shallow donors in silicon and gallium arsenide, there usually is enough thermal energy to supply the energy ED to ionize all donor impurities at room temperature and thus provides an equal number o
29、f electrons in the conduction band. This condition is called complete ionization. Under a complete ionization condition, we can write the electron density as N=ND (1.1)We obtain the Fermi level in term of the effective density of states Nc and the donor concentration ND EC-EF=ktln (NC/ND) (1.2)It is
30、 clear that the higher the donor concentration, the smaller the energy difference (EC-EF), that is, the Fermi level will move closer to the bottom of the conduction band. Similarly, for higher acceptor concentration, the Fermi level will move closer to the top of the valence band.2 Characteristics o
31、f Diodes2.1 IntroductionMost electronic devices depend on the electrical characteristics of junctions between different materials. Such junctions are two-terminal devices and are referred to as diodes. When n-type and p-type silicon crystals are joined together, a pn junction is formed. In practice,
32、 a pn junction is formed by adding accepter impurities to an n-type wafer or donors to a p-type wafer. There are a variety of methods for junction formation.The techniques of alloying, epitaxy, diffusion, and ion implantation have reported in some references to produce the pn junction. In alloying t
33、echnique, a thin aluminum film is evaporated onto a clean n-type silicon wafer, called a substrate, inside a vacuum chamber. The silicon is then placed in a furnace set at about 600 for 30 min, Al and Si constitute a eutectic system. When the semiconductor is cooled down, the silicon from the liquid
34、 alloy will form a recrystallized layer which contains a significant amount of aluminum atom atoms. Since aluminum is a p-type impurity in silicon, the recrystallized region is p region. Consequently, a pn junction is produced. The alloying process is simple and inexpensive in making a single pn jun
35、ction, but it does not produce a uniform and smooth junction. And the control of aluminum concentration is also difficult. For these reasons, the alloyed junction is seldom used in practical devices.Epitaxy describes the growth technique of arranging atoms in single-crystal fashion upon a crystallin
36、e substrate so that the lattice structure of the newly grown film duplicates that of the substrate. An important reason for using this growth technique is the flexibility of impurity control in the epitaxial film. The dopant in the film may be Nor p type and independent of the substrate doping. Ther
37、efore, epitaxial growth can be used to form a lightly doped layer on a heavily doped substrate, or a pn junction between the epitaxial film and the substrate. Three different methods are available to produce epitaxial films: vapor-phase epitaxy (VPE), liquid-phase epitaxy (LPE), and molecular-beam e
38、pitaxy (MBE).The most wisely used technique in forming a pn junction is sold-state impurity diffusion. The diffusion of impurity is, in principle, the same as carrier diffusion system, a wafer is placed ina gaseous atmosphere containing impurity atoms inside a furnace. There impurity atoms may come
39、from slices of the boron nitride inserted between silicon wafers. The temperature of the furnace usually ranges between 900 and 1200. The number of impurity atoms taken in by the solid is limited by the solid solubility, which is the maxmum impurity concentration that the solid can accommodate at a
40、given temperature.The values of solid solubility for B, P, AND As in silicon in the nomal range of diffusion temperatures are approximately 6(10 2010 21), and 210 21 cm-3, respectively.Ion implantation is a primary method for introducing dopants into a semiconductor and making pn junction. The ion-i
41、mplantation process is attractive because it can be performed at low temperature with negligible impurity diffusion. In addition, the impurity concentration introduced is better controlled than with standard diffusion techniques. A beam of dopant ions is accelerated through a desired energy potentia
42、l ranging between 30 and 500 keV. The ion beam is aimed at the semiconductor target so that the high-energy ions penetrate the semiconductor surface. The energetic ions will lose their energy through collisions with the target nuclei and electrons so that the ions will finally come to rest. The dist
43、ance traveled by the ions, i.e. the penetration depth, is called the rage. The range is a function of the kinetic energy of the ions and the semiconductors structural properties, e.g., lattice spacing spacing and mass of atoms.A typical impurity-range distribution in an amorphous target is collides
44、with lattice atoms and the host atoms are dislodged. As a result, the crystalline region is turned into a disordered or amorphous layer.for this reason,it is necessary to anneal the semiconductor after implantation to reestablish the crystalline structure. In the annealing step, the semiconductor is
45、 placed in a high-temperature oven set at a temperature between 200 and 800 . The annealing temperatures are typicality well below those used in solid-state diffusion. The doping in the p type substrate (Na) is assumed constant throughout the crystal. Whenever there are more donors thanacceptors(NdN
46、a), the semiconductor is n type in that region. It is p type wherever NaNd. The metallurgical junction is at x0, the point at which Nd=Na, or the net doping is zero. It is the net doping profile that determines he energy band diagram.To solve for the electrostatic properties of the junction, the Nd
47、profile must be known, but it is typically not a simple mathematical function. Thus, an approximation to the Nd-Na profile is often used.The most common of these is the step approximation, in which the net doping is assumed to be a step function.A pn homojunction (often called simply a pn junction)
48、consists of a single crystal of a given semiconductorin which the doping level charges from p type to n type at some boundary. The term homojunction implies that the junction is between two regions of the same material (e.g., silicon), as opposed to the term heterojunction in which the junction is b
49、etween teo different semiconductors (for example, Ge and Si). To develop some physical understanding of diodes, we will analyze this simplified model (step junction) of a semiconductor homojunction. We refer to this as the prototype homojunction. The purpose is to illustrate the basic principles of operation of semiconductor diodes. In practice, examples of the prototype junction are seldom encountered, but the
