Semiconductors intrinsic conductivity

With regard to polymers, it is widely known that most of them are found in the category of insulators. However, there are a few conducting polymers that display behavior similar to metals and semiconductors. Intrinsically conductive polymers belong to one of five families polyacetylene, polyaniline, polypyrrole, polythiophene, or polyphenylvinylenes. The common features of these polymers are as follows ... 

In an intrinsic semiconductor, tlie conductivity is limited by tlie tlieniial excitation of electrons from a filled valence band (VB) into an empty conduction band (CB), across a forbidden energy gap of widtli E. The process... 

From the mathematical standpoint, this model can be formulated as follows. Neglecting the intrinsic conductance, the condition of electrical neutrality of the semiconductor may be... 

This polymer occurs as trans- or a less stable c/s-isomer. (The ds-isomer can be converted to trans- by heating.) Both isomers are intrinsic semiconductors with conductivities around 10 9S/cm (cis) and 10 6S/cm (trans). 

Intrinsically conducting polymers, 13 540 Intrinsic bioremediation, 3 767 defined, 3 759t Intrinsic detectors, 22 180 Intrinsic fiber-optic sensors, 11 148 Intrinsic magnetic properties, of M-type ferrites, 11 67-68 Intrinsic photoconductors, 19 138 Intrinsic rate expressions, 21 341 Intrinsic semiconductors, 22 235-236 energy gap at room temperature, 5 596t Intrinsic strength, of vitreous silica, 22 428 Intrinsic-type detectors, cooling, 19 136 Intrinsic viscosity (TV), of thermoplastics, 10 178... 

A sequence may form and eventually meet a B sequence, as shown, but in doing so, a free radical, called a soliton, is produced. The soliton is a relatively stable electron with an unpaired spin and is located in a nonbonding state in the energy gap, midway between the conduction and valence bands. It is the presence of these neutral solitons which gives frany-polyacetylene the characteristics of an intrinsic semiconductor with conductivities of 10 to 10 (f2 cm) ... 

As the temperature of the semiconductor is increased electrons are thermally removed from interatomic bonds to the conduction band, simultaneously creating a positive hole and a free electron. Under the influence of an applied potential, the hole and electron move away from each other in opposite directions giving rise to an electric current. Occasionally an electron and a hole will meet in a recombination process and the electron will fall back into the interatomic bond. Upon heating to a sufficiently high temperature any insulator is expected to show this intrinsic conduction behavior. 

If the temperature dependence of the electronic conductivity of a semiconductor is to be accounted for, it is necessary to analyse how the density of charge carriers and their mobilities each depend upon T (see Eq. (2.25)). In the first place attention will be confined to the density n of electrons in the conduction band and the density p of holes in the valence band. When the intrinsic properties of the crystal are under consideration, rather than effects arising from impurities or, in the case of compounds, from departures from stoichiometry, the corresponding conductivity is referred to as intrinsic conductivity . The approach to the calculation of n and p in this instance is as follows. 

At one extreme are compounds such as MgO, for which it is doubtful whether any stoichiometric variability is detectable although, in principle, a potential n-type semiconductor at high temperatures, it is doubtful whether observed effects exceed what is attributable to residual impurities or intrinsic conduction. For lead sulfide at about 1000° the measured existence limits are from Pb10oo4S to PbSaa0ooi-... 

D Semiconductor (pure compound) intrinsic conductivity, graphite L C axis (bismuth-tin alloy) ... 

Fig. 34. Calculated capacitance for a narrow-band semiconductor with (a) intrinsic conductivity, (b) p-type extrinsic conductivity, and (c) n-type extrinsic conductivity.

For most semiconductors, kT (0.0259 eV at 300 K) is much smaller than E. Thus, few electrons and holes are produced at room temperature in such semiconductors. Intrinsic samples are far too resistive for many applications additionally, in actual semiconductor samples, the concentration of unwanted impurities often exceeds the intrinsic carrier concentrations. Under these conditions, it is difficult to maintain quality control over the electrical properties of semiconductors. For these reasons, the conductivity of semiconductors is generally deliberately controlled through a process known as doping. 

Intrinsically conducting polymers are a broad class of (often) processable materials based upon doped -it conjugated polymers. Their conductivities vary from that of insultators through to that of semiconductors and even good metals. A wide variety of electronic phenomena are observed. Because of the broad choice of materials and properties, this class of polymer is potentially of use in a large number of technologies. 

The spectacular success of the semiconductor industry is based on the production of materials selectively designed for specialized applications in electronic and optical devices. By carefully controlled doping of semiconductors with selected impurities—electron donors or electron acceptors—the conductivity and other properties can be modulated with great precision. Fig. 12.8 shows schematically how doped semiconductors work. In an intrinsic semiconductor (a), conducting electron-hole pairs can only by produced by thermal or photoexcitation across the band gap. In (b), addition of a small concentration of an electron donor creates an impurity band just below the conduction band. Electrons can then Jump across a much-reduced gap to the conduction band and act as negatively-charged current carriers. This produces a n-type semiconductor. In (c), an electron acceptor creates an empty impurity band just above the valence band. In this case electrons can jump from the valence band to leave positive holes. These can also conduct electricity, since electrons falling into positive holes create new holes, a sequence... 

Silicon is a semiconductor with an intrinsic conductivity of 4.3 x 10" Q" cm and a band gap of I.I2eV at 300K. It has a diamond crystal structure characteristic of the elements with four covalently bonded atoms. As shown in Fig. 2.1, the lattice constant, a, is 5.43 A for the diamond lattice of silicon crystal structure. The distance between the nearest two neighbors is V3a/4, that is, 2.35 A, and the radius of the silicon atom is 1.18 A if a hard sphere model is used. Some physical parameters of silicon are listed in Table 2.1. 

It is interesting that catalytically inactive tin(IV) oxide is a broadband -type semiconductor in which the conductivity has been attributed (40,41) to donor levels which result from defects such as impurity cations and anion vacancies. Although the large forbidden energy gap (38, 42) of 3.5 eV prevents intrinsic conductivity at ordinary temperatures, the introduction of... 

Intrinsic Semiconductors Semiconductors due to thermal defects. At OK, pure Si or Ge act as insulators, however at higher temperatures some of the covalent bonds are broken and the electrons so released become free to move in the crystal and thus conduct electric current. This type of conduction is known as intrinsic conduction as it can be introduced in the crystal without adding an external substance. 

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