Intrinsic charge carriers

There are three basic types of semiconductor materials depending on their ability to conduct hole (p-type), electrons (n-type), or both (ambipo-lar) under different gate bias conditions. In semiconductor materials, reduction of the bandgap (Eg) will enhance the thermal population of the conduction band and thus increase the number of intrinsic charge carriers. The decrease of Eg can led to true organic metals showing intrinsic electrical conductivity. 

If a semiconductor is so pure that impurities contribute negligibly to - charge carrier densities in the conduction and valence bands, it is called an intrinsic semiconductor and the intrinsic charge-carrier density dependence on temperature is given by n (T) oc T exp (-2 r)> where g is the band-gap energy and ks is the - Boltzmann constant. 

For a quantitative treatment analogous to intrinsic charge carriers, we have to take the defect concentration Nd and the energetic defect-band distance Ed into account ... 

Figure 22.12 Intrinsic charge carrier mobilities of DHnT as a dependence on the channel length for sub-micrometer devices.

The history of our knowledge of the electrical conductivity of organic solids has been discussed by several authors who were involved in the work on the electrical conductivity of polycyclic aromatic hydrocarbons in the years after 1950 in a special volume of Molecular Crystals, liquid Crystals [6] edited by H. Inokuchi. At the beginning of this newer period of research, there were at least five milestones Stimulated by the measurements of Eley on phthalocyanine, Akamatu and Inokuchi in the year 1950 discovered a thermally-activated specific conductivity with an activation energy of E= 0.39 eV in violanthrone, and obtained similar values for related aromatic soUds [7]. The interpretation given at the time for this value in terms of the model of an intrinsic semiconductor with a band gap AEg = 2E is, to be sure, obsolete today (see below), but the results clearly showed that no conductivity exists at T = 0 and thus no intrinsic charge carriers are present in the crystal. 

Generation of Charge Carriers and Charge Transport Experimental Methods 239 8.4.2.1 Intrinsic Charge Carrier Separation... 

When a semiconductor, due to its large band gap (Eg feT), contains practically no intrinsic charge carriers and owing to a low appUed voltage also no injected charge carriers, and nevertheless has an appreciable dark conductivity, then it must be doped. The dopants must either be able to activate electrons thermally into the conduction band (they are then referred to as donors and are oxidised to cations if they were previously neutral) or they must be able to thermally activate electrons out of the valence band and thereby be reduced (in this case they are called aceptors and become anions if they were previously neutral, or neutral if they were previously positively charged, for example as radical cations). The two redox or charge-transfer processes yield electrons with a density n in the valence band and holes with a density p in the conduction band (Fig. 8.25). Overall, the sample must remain neutral, that is... 

Stationary dark currents in disordered thin films which contain no intrinsic charge carriers require the injection of charge carriers. If Ohmic contacts have been prepared, then the stationary dark current is not limited by the contact resistance, but instead by the mobility and the injected space charge. Since about 1985, stationary dark currents through disordered thin films have attracted great interest, especially since at that time, suitable contacting methods were developed to permit injection of both electrons from the one electrode as well as holes from the other. This made... 

Semiconductors for practical applications are often doped, mainly with the aim to improve the conductivity. In metal oxide photoelectrodes, shallow donors and acceptors are almost always necessary because of the low intrinsic charge carrier mobilities. The conductivity of the material is given by a = nefif, + so... 

Note that, under static equilibrium, the electron and hole concentrations are additionally related by the familiar mass action law, CnCp = n , where rii is the intrinsic charge carrier concentration of the material. 

An intrinsic semiconductor is characterized by a small band gap and a low density of highly mobile intrinsic charge carriers. Electrons as well as holes contribute to the conductivity which increases with temperature. Phthalo-cyanine radicals such as the sandwich type PC2LU or PcLi carry intrinsic charges. Their facile oxidation and reduction suggests that intrinsic conductivity should be possible. The electrical properties of these materials, especially as thin films incorporated in various devices, have been studied [32]. 

Here m i and mj are the effective masses of the electron and hole, respectively. For an estimate of the intrinsic charge carrier concentration, n, in a liquid insulator we may assume the effective masses of electron and hole to be equal to the free electron mass (0.91 x 10" kg). Equation 11 then becomes... 

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