Electronic dopants, doped semiconductor

Malleable materials can be beaten into sheets. Ductile materials can be pulled into wires. Lustrous materials have a shine. Oxides, hydrides, and halides are compounds with 0, H, and halogens respectively. Measures of intermolecular attractions other than melting point are also higher for metal oxides, hydrides, and halides than for the nonmetal compounds. A dopant is a small quantity of an intentionally added impurity. The controlled movement of electrons in doped silicon semiconductors carries digital information in computer circuitry. 

This always holds when the semiconductor is clean, without any added impurities. Such semiconductors are called intrinsic. The balance (4.126) can be changed by adding impurities that can selectively ionize to release electrons into the conduction band or holes into the valence band. Consider, for example, an arsenic impurity (with five valence electrons) in gennanium (four valence electrons). The arsenic impurity acts as an electron donor and tends to release an electron into the system conduction band. Similarly, a gallium impurity (three valence electrons) acts as an acceptor, and tends to take an electron out of the valence band. The overall system remains neutral, however now n p and the difference is balanced by the immobile ionized impurity centers that are randomly distributed in the system. We refer to the resulting systems as doped or extrinsic semiconductors and to the added impurities as dopants. Extrinsic semiconductors with excess electrons are called n-type. In these systems the negatively charged electrons constitute the majority carrier. Semiconductors in which holes are the majority carriers are calledp-type. 

In Fig. 4 (a) and (b) we compare, and contrast, the metal-insulator transition at r = 0 K in our divided metal with that of a macroscopic sample of a doped semiconductor. Si P. For the latter the average dopant separation, d, (and thus the electron carrier density) provides the key experimental control parameter separating the metallic [d < dc) and non-metallic d > dc) regimes. The metal-insulator tran-... 

The electronic structure of a doped semiconductor is shown here, (a) Which band, A or B, is the valence band (b) Which band is the conduction band (c) Which band consists of bonding molecular orbitals (d) Is this an example of an n-type or p-type doped semiconductor (e) If the semiconductor is germanium, which of the following elements could be the dopant Ga, Si, or P [Section 12.7]... 

The second common category of doped semiconductors is called the p-type. The idea here is to provide a way to decrease the number of electrons in the valence band, so that conductivity can be achieved without needing to promote electrons across the band gap. The strategy is similar to what we described for the n-type, except that now we want to use a dopant with fewer than four valence electrons. This introduces an acceptor level that is slightly higher in energy than the top of the valence band. The most common choice for the dopant in this case... 

In doped semiconductors, the concentration of charge carriers is given by the concentration of the dopant, which may be an electron acceptor or an electron donor. Cations that have a valence lower than that of the base oxide are electron acceptors. In a divalent oxide MO, monovalent cations such as Li or K act as electron... 

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. 

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