Organic thin-film transistors (OTFTs) have lived to see great improvements in recent years. This review presents new insight into conduction mechanisms and performance characteristics, as well as opportunities for modeling properties of OTFTs. The shifted focus in research from novel chemical structures to fabrication technologies that optimize morphology and structural order is underscored by chapters on vacuum-deposited and solution-processed organic semiconducting films. Finally, progress in the growing field of the n-type OTFTs is discussed in ample detail. The Figure, showing a pentacene film edge on SiO2, illustrates the morphology issue.

Organic Thin Film Transistors for Large Area Electronics

2.2. Materials 929 2.3. Factors Influencing Charge Mobility 931 2.3.1. Molecular Packing 931 2.3.2. Disorder 932 2.3.3. Temperature 933 2.3.4. Electric Field 934 2.3.5. Impurities 934 2.3.6. Pressure 934 2.3.7. Charge-Carrier Density 934 2.3.8. Size/molecular Weight 935 3. The Charge-Transport Parameters 935 3.1. Electronic Coupling 936 3.1.1. The Energy-Splitting-in-Dimer Method 936 3.1.2. The Orthogonality Issue 937 3.1.3. Impact of the Site Energy 937 3.1.4. Electronic Coupling in Oligoacene Derivatives 938

Charge transport in organic semiconductors.

The experimental evidence concerning the density of states in amorphous semiconductors and the ranges of energy in which states are localized is reviewed; this includes d.c. and a.c. conductivity, drift mobility and optical absorption. There is evidence that for some chalcogenide semiconductors the model proposed by Cohen, Fritzsche and Ovshinsky (1969) should be modified by introducing a band of localized states, near the centre of the gap. The values of C, when the d.c. conductivity is expressed as C exp (- E/kT), are considered. The behaviour of the optical absorption coefficient near the absorption edge and its relation to exciton formation are discussed. Finally, an interpretation of some results on photoconductivity is offered.

Conduction in non-crystalline systems V. Conductivity, optical absorption and photoconductivity in amorphous semiconductors

A new reversible photoelectronic effect is reported for amorphous Si produced by glow discharge of SiH4. Long exposure to light decreases both the photoconductivity and the dark conductivity, the latter by nearly four orders of magnitude. Annealing above 150 °C reverses the process. A model involving optically induced changes in gap states is proposed. The results have strong implications for both the physical nature of the material and for its applications in thin‐film solar cells, as well as the reproducibility of measurements on discharge‐produced Si.

Reversible conductivity changes in discharge‐produced amorphous Si

High-performance gas sensors prepared using p-type oxide semiconductors such as NiO, CuO, Cr2O3, Co3O4, and Mn3O4 were reviewed. The ionized adsorption of oxygen on p-type oxide semiconductors leads to the formation of hole-accumulation layers (HALs), and conduction occurs mainly along the near-surface HAL. Thus, the chemoresistive variations of undoped p-type oxide semiconductors are lower than those induced at the electron-depletion layers of n-type oxide semiconductors. However, highly sensitive and selective p-type oxide-semiconductor-based gas sensors can be designed either by controlling the carrier concentration through aliovalent doping or by promoting the sensing reaction of a specific gas through doping/loading the sensor material with oxide or noble metal catalysts. The junction between p- and n-type oxide semiconductors fabricated with different contact configurations can provide new strategies for designing gas sensors. p-Type oxide semiconductors with distinctive surface reactivity and oxygen adsorption are also advantageous for enhancing gas selectivity, decreasing the humidity dependence of sensor signals to negligible levels, and improving recovery speed. Accordingly, p-type oxide semiconductors are excellent materials not only for fabricating highly sensitive and selective gas sensors but also valuable additives that provide new functionality in gas sensors, which will enable the development of high-performance gas sensors.

Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview

It is shown that substitutional doping of an amorphous semiconductor is possible and can provide control of the electronic properties over a wide range. a-Si and Ge specimens have been prepared by the decomposition of silane (or germane) in a radio-frequency (r.f.) glow discharge. Doping is achieved by adding carefully measured amounts of phosphine or diborane, between 5 × 10−6 and 10−2 parts per volume, to obtain n- or p-type specimens. The room temperature conductivity of doped a-Si specimens can be controlled reproducibly over about 10 orders of magnitude, which corresponds to a movement of the Fermi level of 1·2 eV. Ion probe analysis on phosphorus doped specimens indicates that about half the phosphine molecules in the gaseous mixture introduce a phosphorus atom into the Si random network; it is estimated that 30–40% of these will act as substitutional donors. The results also show that the number of incorporated phosphorus atoms saturates at about 3 × 1019 cm−3, roughly equal to the number ...

Electronic properties of substitutionally doped amorphous Si and Ge

Substitutional doping of amorphous silicon

Drift mobility and conductivity measurements were made between 290 and 85\ifmmode^\circ\else\textdegree\fi{}K on amorphous silicon specimens prepared by glow-discharge decomposition of silane. The results suggest that excess electrons drift in the extended states with a mobility of about 10 ${\mathrm{cm}}^{2}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ ${\mathrm{V}}^{\ensuremath{-}1}$. At lower temperatures, phonon-assisted hopping occurs through localized states occupying a range of 0.2 eV below the extended states. Conductivity results also suggest hopping transport near the Fermi energy.

Electronic Transport in Amorphous Silicon Films

The characteristics of an insulated-gate field-effect transistor made from amorphous silicon (a-Si) deposited in a glow discharge are discussed. It is suggested that the a-Si device could be applied with advantage in an addressable matrix of a liquid-crystal display panel.

Amorphous-silicon field-effect device and possible application

The electronic properties of amorphous solids are largely determined by the distribution of localized states N (ϵ) in the mobility gap. In this paper, the field effect technique is applied to the experimental study of N (ϵ) in specimens of a-Si prepared by the glow discharge method and by vacuum evaporation. The experimental approach and the analysis of the results are discussed in some detail. N (ϵ) curves, extending over an energy range of up to 0.5 eV have been obtained for a series of glow discharge specimens, deposited at substrate temperatures between 310 and 570 K. The results show structure in the gap states, a well-defined minimum almost in the centre of the mobility gap and a rapid rise in N (ϵ), 0.18 eV below ϵ c , which is identified with the onset of band tail states. The field effect data confirm that the predominant conduction mechanism at room temperature changes from hole hopping to transport in extended electron states, as the Fermi level is moved through the minimum in N (ϵ) The effects of annealing on the state distribution have been investigated, showing that N ( ϵ f ) can be reduced by one or two orders of magnitude. The nature of the gap states is discussed and the divacancy is suggested as a basic model for the electronic states involved.

P. G. Le Comber

Papers

Electronic properties of substitutionally doped amorphous Si and Ge

Substitutional doping of amorphous silicon

Electronic Transport in Amorphous Silicon Films

Amorphous-silicon field-effect device and possible application

Investigation of the density of localized states in a-Si using the field effect technique