Doped Extrinsic Semiconductors

The n-type semiconductors are doped intrinsic semiconductors in which the dopant is a pentavalent element, for instance chemical elements of group VA(15) of the periodic chart such as arsenic (As), antimony (Sb), or phosphorus (P). The substitutional impurities will give a supplementary electron owing to their ns np electronic configuration containing five rather than four outer-shell electrons. Therefore, the density of holes in the valence band is exceeded by the density of electrons in the conduction band. A hole is a mobile electron vacancy in a semiconductor that acts like a positive electron charge with a positive mass. Then, the n-type behavior is induced by doping with the addition of pentavalent element 

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The highest energy occupied allowed band of a metal, or conduction band, is only partially filled with electrons, up to the so-called Fermi level. Hence, electrons located close to this Fermi energy are easily excited to the unoccupied level of the band, where they behave as free electrons. In a semiconductor (like in an insulator), the highest occupied allowed band is totally filled, and called valence band (VB), whereas the conduction band (CB) corresponds to the lowest unoccupied allowed band, which is completely empty. The injection of electrons in the CB occurs either thermally (in an intrinsic semiconductor) or through doping (extrinsic semiconductor). Electrons in the conduction band of metals or semiconductors move in delocalized states, and their wave function can be approximated to that of a free electron, that is, a progressive plane wave... 

The carrier concentrations in doped or extrinsic semiconductors to which donor or acceptor atoms have been added can be deterrnined by considering the chemical kinetics or mass action of reactions between electrons and donor ions or between holes and acceptor ions. The condition for electrical neutraHty is given by equation 6. When the predominant dopants are donors, the semiconductor is... 

Extrinsic Semiconductors are materials that contain donor or acceptor species (called doping substances) that provide electrons to the conduction band or holes to the valence band. If donor impurities (donating electrons) are present in minerals, the conduction is mainly by way of electrons, and the material is called an n-type semiconductor. If acceptors are the major impurities present, conduction is mainly by way of holes and the material is called a p-type semiconductor. For instance in a silicon semiconductor elements from a vertical row to the right of Si of... 

In doped silicon (an extrinsic semiconductor) the doping element has either three or five valence electrons (one electron less or one electron more than the four valence electrons of silicon). Substituting an arsenic or phosphorus atom (five valence electrons) for a silicon atom in a silicon crystal provides an extra loosely-bound electron that is more easily excited into the CB than in the case of the pure silicon. In such an n-type semiconductor, most of the electrical conductivity is attributed... 

Figure 11.2 Energy diagram for various semiconductors. Shown are examples for pure (intrinsic) and doped (extrinsic) n- and p-types. In each case the Fermi level is shown as a dotted line...

Figure 6.16 Temperature dependence of charge carrier density and conductivity of extrinsic semiconductor Ge doped with 2 ppb As. From K. M. Ralls, T. H. Courtney, and J. Wulff, Introduction to Materials Science and Engineering. Copyright 1976 by John Wiley Sons, Inc. This material is used by permission John Wiley Sons, Inc.

The properties of semiconductors are extremely sensitive to the presence of impurities at concentrations as low as 1 part in 10 °. For this reason, silicon manufactured for transistors and other devices must be very pure. The deliberate introduction of a very low concentration of certain impurities into the very pure semiconductor, however, alters the properties in a way that has proved invaluable in constructing semiconductor devices. Such semiconductors are known as doped or extrinsic semiconductors. Consider a crystal of silicon containing boron as an impurity. Boron has one fewer valence electron than silicon. Therefore, for every silicon replaced by boron, there is an electron missing from the valence band (Figure 4.10) (i.e., positive holes occur in the valence band and these enable electrons near the top of the band to conduct electricity). Therefore, the doped solid will be a better conductor than pure silicon. A semiconductor like this doped with an element with fewer valence electrons than the bulk of the material is called a p type semiconductor because its conductivity is related to the number of positive holes (or empty electronic energy levels) produced by the impurity. 

An extrinsic semiconductor is a material into which impurities have been incorporated, like raisins in a cake. This process is known as doping. The impurities can possess more or fewer valence electrons than the atoms of the cake or mother material and the classification into two types of semiconductors is based on this fact ... 

The above definition is actually that for intrinsic semiconductors. What make semiconductors so useful in electronics, however, is that their electronic properties can be altered in a controllable manner by adding tiny amounts of an advisedly chosen impurity. This is the well-known process of doping, which is related to the notion of extrinsic semiconductors. 

The bulk electronic properties of extrinsic semiconductors are largely determined by the level of doping that is used to make the materials n-type or p-type. For non-degenerate semiconductors, the electron concentration in the conduction band and the hole concentration in the valence band are related to the Fermi energy EF and to the effective densities of states in the conduction and valence bands (Nc and Ny respectively) by... 

Extrinsic semiconductors ate those in which the carrier concentration, either holes or electrons, are controlled by intentionally added impurities called dopants. The dopants are termed shallow impurities because their energy levels lie within the band gap close to one or other of the bands. Because of thermal excitation, -type dopants (donors) are able to donate electrons to the conduction band and p-type dopants (acceptors) can accept electrons from the valence band, the result of which is equivalent to the introduction of holes in the valence band. Band gap widening/narrowingmay occur if the doping changes the band dispersion. At low temperamres, a special type of electrical transport known as impurity conduction proceeds. This topic is discussed in Section 7.3. 

The most popular semiconductor material is silicon (hence Silicon Valley). Fig. 12.9a is a schematic representation of a pure Si crystal. Each Si atom has rout-valence electrons and bonds to four other atoms to form Lewis octets. The crysttil can become a conductor if some of the valence electrons are shaken loose. This produces both negative and positive charge carriers—electrons and l-uilcs. Much more important are extrinsic semiconductors in which the Si crystal is doped with impurity atoms, usually at concentrations of several parts per million (ppm). For example, Si can be doped with P (or As or Sb) atoms, which has five valence electrons. As shown in Fig. 12.9b, a P atom can replace a Si atom in the lattice. The fifth electron on the P is not needed for bonding and becomes available as a current carrier. Thus, Si doped with P is a n-type semiconductor. The Si can instead be doped with B (or Ga or Al). which has only three valence eleetrons. As shown in Fig. 12.9c, a B atom replacing a Si atom leaves an electron vacancy in one of its four bonds. Such positive holes can likewise become current carriers, making Si doped with B a p-type semi con duetor. 

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. 

Defects that introduce extra electrons, or that give missing electrons or holes , have a large influence on electronic conduction in nonmetallic solids. Most semiconductor devices use doped or extrinsic semiconductors rather than the intrinsic semiconduction of the pure material. Doping Si with P replaces some tetrahedrally bonded Si atoms in the diamond lattice (see Topic D2) with P. Each replacement provides one extra valence electron, which requires only a small... 

Extrinsic semiconductors are materials containing foreign atoms (FAs) or atomic impurity centres that can release electrons in the CB or trap an electron from the VB with energies smaller than Eg (from neutrality conservation, trapping an electron from the VB is equivalent to the release of a positive hole in the otherwise filled band). These centres can be inadvertently present in the material or introduced deliberately by doping, and, as intrinsic, the term extrinsic refers to the electrical conductivity of such materials. The electron-releasing entities are called donors and the electron-accepting ones acceptors. When a majority of the impurities or dopants in a material is of... 

To quantify the conductivity of an extrinsic semiconductor, consider an -type semiconductor doped with a concentration Nj) of dopant atoms. The ionization reaction of the donor can be written as... 

In the preceding subsection, the number of electronic defects was fixed by the doping level, especially at lower temperatures, and the concepts of donor and acceptor localized levels were discussed. The band picture for nonstoichiometric electronic semiconductors is very similar to that of extrinsic semiconductors, except that the electronic defects form not as a result of doping, but rather by varying the stoichiometry of the crystal. 

Stacks of conducting metallomacrocycles (e.g. porphyrins and phthalo-cyanines) are ideal organic conductors, especially as p-type extrinsic semiconductors (i.e. the positive "holes" have to be added by doping, via... 

When as semiconductor material is not doped, but is made of pure silicon, it is known as an intrinsic semiconductor. But when there is some addition of another material, such as boron or gallium, it is referred to as an extrinsic semiconductor. 

Na, metal Cdl2, layered structure octahedral site, 6-coordinate Ga-doped Si, extrinsic semiconductor Na2S, antifluorite structure perovskite, double oxide CaF2, fluorite structure GaAs, intrinsic semiconductor wurtzite and zinc blende, polymorphs Sn02, cassiterite. 

In extrinsic semiconductors the carrier concentrations are perturbed such that n = p. Again the analogy with the addition of an acid or base to water is quite instructive here. Consider the case when donor impurities are added to a neutral semiconductor. Since the intrinsic carrier concentrations are so low (sub-parts per trillion), even additions in parts per billion levels can have a profound electrical effect. This process is known as doping of the semiconductor. In this particular case, the Fermi level shifts toward the CB edge (Fig. 4b). When the donor level is... 

An alternative way to improve the conductive properties of a semiconductor (or to convert an insulator into an extrinsic semiconductor) is to dope it with an element that contains less electrons. One example is to dope a Si wafer such that B atoms occupy some of the lattice sites of the material. Because B has only three valence electrons, the B dopant provides a set of low-lying empty MOs, as shown in Figure 11.51 (b). The band gap between the top of the VB for Si and these empty MOs is very small, so that some of the electrons in the VB can be thermally excited into the empty B MOs. When this occurs, the electron deficiency creates holes in the VB, which act as positive charge carriers. Because the dopant increases the number of positive change carriers, the material is called a p-type semiconductor. 

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