Semiconductor impurities

NAA is well suited for Si semiconductor impurities analysis. The sensitivity and the bulk mode of analysis make this an important tool for controlling trace impurities during crystal growth or fer monitoring cleanliness of various processing operations for device manufacturing. It is expected that research reactors will ser e as the central analytical facilities for NAA in the industry. Since reactors are already set up to handle radioactive materials and waste, this makes an attractive choice over installing individual facilities in industries. 

Extrinsic Semiconductors. Impurity levels can be either donor levels near the empty zone (normal or n-type), or acceptor levels near the filled band (abnormal or p-type). Conductivity in n-type conductors will be due to electrons in the empty band donated by the impurity levels, and in p-type conductors, to positive holes in the previously filled band, arising from the transition of electrons to the impurity acceptor levels. 

Analysis of the growth process by LPE usually stipulates an equilibrium boundary condition at the solid-liquid interface. The solid-liquid phase diagrams of interest to LPE are those for the pure semiconductor and the semiconductor-impurity systems. Most solid alloys exhibit complete mis-... 

Probably the most apparent result of the low electron density is the decisive role played by impurities in the semiconductor. Impurities in a semiconductor electrode, even at levels as low as 1 part in 10 , can contribute significantly to the space charge, can act as catalysts for the recombination of holes and electrons, or can furnish additional charge-transfer paths for electrochemical reaction. It is essential, therefore, to have control and knowledge of the impurity distribution before any meaningful kinetic measurements can be made on a semiconductor electrode. 

Defects or impurities in the semiconductor crystal structure create electronic states in the gap region. In the case of impurities, the valence character of the impurity determines whether the level acts as an electron donor or electron acceptor state. In doping semiconductors, impurities are deliberately used to generate either donor or... 

Host Semiconductor Impurity Species Mode Frequency (temperature, K) cm" Reference... 

Photodiode array detectors are an offshoot of semiconductor technology. In semiconductors, impurities have been added to pure silicon to create two classes of materials. The addition of arsenic, bismuth, phosphorous, or antimony creates a pentavalent material (n-type) that is able to function as a donor of electrons. The addition of trivalent elements such as aluminium, boron, gallium, indium, etc., to silicon gives rise to the p-lypc material, in which the trivalent material is able... 

We normally define the energy level of electrons in a solid in terms of the Fermi level, eF, which is essentially equivalent to the electrochemical potential of electrons in the solid. In the case of metals, the Fermi level is equal to the highest occupied level of electrons in the partially filled frontier band. In the case of semiconductors of covalent and ionic solids, by contrast, the Fermi level is situated within the band gap where no electron levels are available except for localized ones. A semiconductor is either n-type or p-type, depending on its impurities and lattice defects. For n-type semiconductors, the Fermi level is located close to the conduction band edge, while it is located close to the valence band edge for p-type semiconductors. For examples, a zinc oxide containing indium as donor impurities is an n-type semiconductor, and a nickel oxide containing nickel ion vacancies, which accept electrons, makes a p-type semiconductor. In semiconductors, impurities and lattice defects that donate electrons introduce freely mobile electrons in the conduction band, and those that accept electrons leave mobile holes (electron vacancies) in the valence band. Both the conduction band electrons and the valence band holes contribute to electronic conduction in semiconductors. 

These lifetimes are, however, considerably longer than the sub-picosecond lifetimes typical of quantum well intersubband transitions of similar energy separation. This will aid the build up of a population inversion on the excited impurity states. In addition, the temperature stability of the intra-impurity lifetime (as measured up to 60 K), suggests that the use of the quantum dot properties of semiconductor impurities might provide a route to obtaining temperature stable far-infrared lasers. 

In n-type semiconductor, impurities like P, As are added and current flow is due to the movement of electrons from donor state to conduction band. 

In the extrinsic or doped semiconductor, impurities are purposely added to modify the electronic characteristics. In the case of silicon, every silicon atom shares its four valence electrons with each of its four nearest neighbors in covalent bonds. If an impurity or dopant atom with a valency of five, such as phosphorus, is substituted for silicon, four of the five valence electrons of the dopant atom will be held in covalent bonds. The extra, or fifth electron will not be in a covalent bond, and is loosely held. At room temperature, almost aU of these extra electrons will have broken loose from their parent atoms, and become free electrons. These pentavalent dopants thus donate free electrons to the semiconductor and are called donors. These donated electrons upset the balance between the electron and hole populations, so there are now more electrons than holes. This is now called an N-type semiconductor, in which the electrons are the majority carriers, and holes are the minority carriers. In an N-type semiconductor the free electron concentration is generally many orders of magnitude larger than the hole concentration. 

Some semiconductors have a fixed (and small) energy gap AH between valence and conduction bands. These are called intrinsic semiconductors. In others it is possible to influence the energy gap between the bands. In these types, the extrinsic semiconductors, impurities are added by doping. If silicon is doped with phosphorus, the P atom is called a donor atom. It uses four of its five valence electrons to bind sihcon, and the fifth electron can easily move to the conduction band. Such sihcon is thus called an n-type semiconductor ( n for negative). On the other hand. If silicon is doped with boron (with three valence electrons), the B atoms may accept electrons from the valence band itself and positive holes are created. The doping agent in this case is called an acceptor atom. As the conductivity now is based on the migration of positive holes, this semiconductor is called a p-type conductor. 

When a small amount of a semiconductor impurity is added to the solvent, semiconducting cBN powders (p- and n-type powders) are obtained (4). By growing an n-type cBN crystal on a p-type cBN seed, a functional pn-junction crystal (< 1 mm in size) was made using this high-pressure film method [T. Okubo, private Communication the diodes were described in his patent (254)]. 

Similar as inorganic semiconductors, impurities are an important factor that can change the energy level and the charge transport properties in organic semiconductors including photoconducting DLCs. The effect of the impurity, i.e. the other component of the system, depends on the nature of the impurity. 

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