Electronic conductivity extrinsic semiconductor

In an extrinsic semiconductor, tlie conductivity is dominated by tlie e (or h ) in tlie CB (or VB) provided by shallow donors (or acceptors). If tlie dominant charge carriers are negative (electrons), tlie material is called n type. If tlie conduction is dominated by holes (positive charge carriers), tlie material is called p type. 

Degenerate semiconductors can be intrinsic or extrinsic semiconductors, but in these materials the band gap is similar to or less than the thermal energy. In such cases the number of charge carriers in each band becomes very high, as does the electronic conductivity. The compounds are said to show quasi-metallic behavior. 

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... 

Thus, it follows that the Fermi level of p-1ype semiconductors ascends from an energy level near ea toward the middle of the band gap with decreasing acceptor concentration, N. From Eqns. 2-22 and 2-24, we obtain in general that the Fermi level is located at levels higher for n-type semiconductors and lower for p-type semiconductors than the middle of the band gap. As described in the foregoing, the concentration of electrons, n, in the conduction band is different from the concentration of holes, p, in the valence band in extrinsic semiconductors... 

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. 

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. 

The presence of an impurity such as an As or a Ga atom in silicon leads to an occupied level in the band gap just below the conduction band or a vacant level just above the valence band, respectively. Such materials are described as extrinsic semiconductors. The n-type semiconductors have extra electrons provided by donor levels, and the p-type semiconductors have extra holes originating from the acceptor levels. Band structures of the different types of semiconductors are shown in Fig. 4.3.4. 

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. 

Figure 8. Sketch of the density of electron states as a function of energy for a typical n-type extrinsic semiconductor, (a) 0 K, the donor atoms correspond to localized filled states just below the conduction band edge (b) T > 0 K, each donor atom is thermally ionized, this leads to a considerable density of electrons at the bottom of the CB the electrochemical potential is not far below the CB edge.

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. 

The main variables that determine the transport and screening (see below) of both intrinsic and extrinsic semiconductors are the mobile carrier densities n and 7A, Given the energetic information, that is, the electronic band structure, and the dopant concentrations, these densities can be evaluated from equilibrium statistical mechanics. For example, the density of electrons in the conduction band is... 

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... 

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... 

Fig. 1. (a) Optical excitation in a simple metal. The excited electron-hole pair can relax rapidly (path 1) so that the quantum efficiency of the photoemission process (2) is small. In addition, the back reaction (4) involving electron injection from the solution may occur before the photoemit-ted electron relaxes in the solvent (3). (b) Optical excitation in an insulator or semiconductor. Here, geminate recombination is slower due to the forbidden gap so that the quantum efficiency of photoelectrochemical reactions can approach unity under optimum conditions. In principle, either conduction band (1) or valence band (2) reactions can occur, but in practice the reaction of the minority carrier is most important for extrinsic semiconductors. 

Figure 7.14 Temperature dependence of the electronic conductivity of an extrinsic semiconductor. Region 2 is sometimes referred to as the exhaustion region.

To appreciate the similarities between an extrinsic semiconductor and a nonstoichiometric oxide, compare Fig. 7.13a with Fig. 6.4a or b. In both cases the electron(s) is(are) loosely bound to its(their) mooring(s) and is(are) easily excited into the conduction band. The corresponding energy diagrams for the singly ionized and doubly ionized oxygen vacancies are shown in Fig. 7.12c. In essence, a nonstoichiometric semiconductor is one where the electrons and holes excited in the conduction and valence bands are a result of reduction or oxidation. For example, the reduction of an oxide entails the removal of oxygen atoms, which have to leave their electrons behind to maintain electroneutrality. These electrons, in turn, are responsible for conduction. 

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