1. What is Conventional flow and Electron flow? • Conventional - TopicsExpress



          

1. What is Conventional flow and Electron flow? • Conventional current flow refers to a flow of positive charges. It is a kind of fictitious current. If - as is often the case -the real current is an electron flow (negative charges), then the conventional flow is a current in the opposite direction as the electron movements, since this would have the same effect (for example on the magnetic field, or on conservation of charge). • Electron flow is the movement of electrons from a negative terminal, through a circuit into a positive terminal of the source. Electric current is generally caused by electron flow. 2. What is covalent bonding and stable orbit? • Covalent bonding occurs when pairs of electrons are shared by atoms. Atoms will covalently bond with other atoms in order to gain more stability, which is gained by forming a full electron shell. By sharing their outer most (valence) electrons, atoms can fill up their outer electron shell and gain stability. Nonmetals will readily form covalent bonds with other nonmetals in order to obtain stability, and can form anywhere between one to three covalent bond with other nonmetals depending on how many valence electrons they posses. Although it is said that atoms share electrons when they form covalent bonds, they do not usually share the electrons equally. 3. Why is copper considered as a good conductor? • Copper is a good conductor of electricity because it has the ability to conduct the electron current or flow of electrons fairly easily. It allows heat to pass through it quickly and has the symbol Cu and an atomic number of 29. 4. What is Recombination and lifetime? • Recombination of electrons and holes is a process by which both carriers annihilate each other: electrons occupy - through one or multiple steps - the empty state associated with a hole. Both carriers eventually disappear in the process. The energy difference between the initial and final state of the electron is released in the process. This leads to one possible classification of the recombination processes. In the case of irradiative recombination, this energy is emitted in the form of a photon. In the case of non-irradiative recombination, it is passed on to one or more phonons and in the case of Auger recombination it is given off in the form of kinetic energy to another electron. Another classification scheme considers the individual energy levels and particles involved 5. Different types of flow in a semiconductor. 6. What do you mean by doping? • Doping is the process of add impurities to intrinsic semiconductors to alter their properties. Normally Trivalent and Pentavalent elements are used to dope Silicon and Germanium. When a intrinsic semiconductor is doped with trivalent impurity it becomes a P-Type semiconductors. The P stands for Positive, which means the semiconductor is rich in holes or Positive charged ions. When we dope intrinsic material with pentavalent impurities we get N-Type semiconductor, where N stands for Negative. N-type semiconductors have Negative charged ions or in other words have excess electrons. 7. Differentiate Extrinsic and Intrinsic Semiconductor. • An intrinsic semiconductor material is chemically very pure and possesses poor conductivity. It has equal numbers of negative carriers (electrons) and positive carriers (holes). A silicon crystal is different from an insulator because at any temperature above absolute zero temperature, there is a finite probability that an electron in the lattice will be knocked loose from its position, leaving behind an electron deficiency called a hole”. Where as an extrinsic semiconductor is an improved intrinsic semiconductor with a small amount of impurities added by a process, known as doping, which alters the electrical properties of the semiconductor and improves its conductivity. Introducing impurities into the semiconductor materials (doping process) can control their conductivity. 8. Types of Impurities. • N-Type Semiconductor The N-type impurity loses its extra valence electron easily when added to a semiconductor material, and in so doing, increases the conductivity of the material by contributing a free electron. This type of impurity has 5 valence electrons and is called a PENTAVALENT impurity. Arsenic, antimony, bismuth, and phosphorous are pentavalent impurities. Because these materials give or donate one electron to the doped material, they are also called DONOR impurities. When a pentavalent (donor) impurity, like arsenic, is added to germanium, it will form covalent bonds with the germanium atoms. Figure 1-10 illustrates this by showing an arsenic atom (AS) in a germanium (GE) lattice structure. Notice the arsenic atom in the center of the lattice. It has 5 valence electrons in its outer shell but uses only 4 of them to form covalent bonds with the germanium atoms, leaving 1 electron relatively free in the crystal structure. Pure germanium may be converted into an N-type semiconductor by doping it with any donor impurity having 5 valence electrons in its outer shell. Since this type of semiconductor (N-type) has a surplus of electrons, the electrons are considered MAJORITY carriers, while the holes, being few in number, are the MINORITY carriers. Figure 10. - Germanium crystal doped with arsenic. P-Type Semiconductor The second type of impurity, when added to a semiconductor material, tends to compensate for its deficiency of 1 valence electron by acquiring an electron from its neighbor. Impurities of this type have only 3 valence electrons and are called TRIVALENT impurities. Aluminum, indium, gallium, and boron are trivalent impurities. Because these materials accept 1 electron from the doped material, they are also called ACCEPTOR impurities. A trivalent (acceptor) impurity element can also be used to dope germanium. In this case, the impurity is 1 electron short of the required amount of electrons needed to establish covalent bonds with 4 neighboring atoms. Thus, in a single covalent bond, there will be only 1 electron instead of 2. This arrangement leaves a hole in that covalent bond. Figure 1-11 illustrates this theory by showing what happens when germanium is doped with an indium (In) atom. Notice, the indium atom in the figure is 1 electron short of the required amount of electrons needed to form covalent bonds with 4 neighboring atoms and, therefore, creates a hole in the structure. Gallium and boron, which are also trivalent impurities, exhibit these same characteristics when added to germanium. The holes can only be present in this type semiconductor when a trivalent impurity is used. Note that a hole carrier is not created by the removal of an electron from a neutral atom, but is created when a trivalent impurity enters into covalent bonds with a tetravalent (4 valence electrons) crystal structure. The holes in this type of semiconductor (P-type) are considered the MAJORITY carriers since they are present in the material in the greatest quantity. The electrons, on the other hand, are the MINORITY carriers. Figure 11. - Germanium crystal doped with indium. Doping is the process by which engineers change an insulating material into a semiconductor. The basic process inserts a small population of a foreign element into the crystal lattice of the insulator. For example, we might insert some boron atoms into a lump of silicon. It is conventional to call the main material the bulk and the small number of foreign atoms the dopant. The most important step in the fabrication of semiconductor devices is the controlled implantation of impurity ions into the semiconductor crystal. This is referred to as doping. There are two types of doping in semiconductors; n-doping and p-doping. In n-doping, donor atoms are added which have more valence electrons than are needed to complete the bonds with neighboring atoms of the crystal. This results in additional carriers being available for conduction. 9. How do you identify that the materials are semiconductor?
Posted on: Wed, 20 Nov 2013 01:09:38 +0000

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