Extrinsic semiconductor acceptors

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. 

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... 

The position of the Fermi level in an extrinsic semiconductor depends upon the dopant concentrations and the temperature. As a rough guide the Fermi level can be taken as half way between the donor levels and the bottom of the valence band for n-type materials or half way between the top of the valence band and the acceptor levels for p-type semiconductor, both referred to 0 K. As the temperature rises the Fermi level in both cases moves toward the center of the band gap. 

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... 

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 most probable donor level, ered, the most probable acceptor level, eox, and the standard Fermi level, e redox) of redox electrons are characteristic of individual redox particles but the Fermi level, e m dox), of redox electrons depends on the concentration ratio of the reductant to the oxidant, which fact is similar to the Fermi level of extrinsic semiconductors depending on the concentration ratio of the donor to the acceptor. 

Figure 6.1 Schematic band structures of solids (a) insulator (kT ,) (b) intrinsic semiconductor (kT ,) (c) and (d) extrinsic semiconductors donor and acceptor levels in n-type and p-type semiconductors respectively are shown, (e) compensated semiconductor (f) metal (g) semimetal top of the valence band lies above the bottom of the conduction band.

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. 

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 9. Sketch of the density of electron states as a function of energy for a typical p-type extrinsic semiconductor, (a) 0 K, the acceptors correspond to localized empty states just below the conduction band edge (b) T > 0 K, each acceptor atom is thermally occupied with an electron from the valence band, this leads to a considerable density of holes at the top of the valence band. The electrochemical potential is not far above the VB edge.

In intrinsic semiconductors, considerable effects are possible, provided that the concentration of charge carriers remains small in comparison with the values of 10 and 10 just cited. Depending on whether the donor or acceptor type of defect is predominant, transformation may result into n- or p-type extrinsic semiconductor. 

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 differences in properties between semiconductors and conductors explained by the interaction of the whole lattice. Intrinsic and extrinsic semiconductors and donor/acceptor bands. Simple example of band theory in action. 

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... 

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. 

Figure 13.9 (a) (i) Donor impurity in a crystal of an extrinsic semiconductor and (ii) the associated energy-band diagram donor impurities add donor energy levels below tbe conduction band, (b) (i) Acceptor impurity in a crystal of an extrinsic semiconductor and (ii) the associated energy-band diagram acceptor impurities add acceptor energy levels above the valence band... 

We shall consider next the theory of condition 2 for an extrinsic photoconductor, then the theory of G RA for condition 3, and finally the dependence of t on material parameters. We shall analyze the geometrical model of Fig. 4.8 and assume a simple energy level model of an n-type extrinsic semiconductor consisting of a photoionizable donor level and a compensating acceptor level properties of a corresponding p-type model would be analogous. This material is extrinsic as both a photoconductor and semiconductor. 

It is then possible to solve (2.1) numerically to obtain the band bending Ug. A graphical solution (valid in the absence of an external field) is sketched in Fig. 5.2-65 for an intrinsic semiconductor ( b = 0) with a single surface acceptor level and various values of Uj and N y For extrinsic semiconductors ( b 0) the procedure is similar, provided the origin is translated to Ug = — b and the appropriate function for Qsc is chosen. The same procedure can also be used in the presence of an external field by translating the origin of the coordinates. 

In the case of extrinsic semiconductors for which traces of impurities are added intentionally by doping in order to modify their electrical properties, the concentration of donors (e.g., P, As, or Sb) is denoted hyN, while the concentration of acceptors (e.g., B, Al, or Ga) is denoted by Nj. To calculate the carrier concentration in this kind of semiconductor, it is necessary to use the equation of electrical-charge neutrality ... 

Electron-hole pairs can also be introduced by substitution of acceptor and donor atoms by a process called doping. These doped semiconductors are called extrinsic semiconductors. 

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. 

Fig. 1. Photoexcitation modes iu a semiconductor having band gap energy, E, and impurity states, E. The photon energy must be sufficient to release an electron (° ) iato the conduction band (CB) or a hole (o) iato the valence band (VB) (a) an intrinsic detector (b) and (c) extrinsic donor and acceptor...

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