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.

Wurtzitic ZnO is a wide-bandgap (3.437 eV at 2 K) semiconductor which has many applications, such as piezoelectric transducers, varistors, phosphors, and transparent conducting films. Most of these applications require only polycrystalline material; however, recent successes in producing large-area single crystals have opened up the possibility of producing blue and UV light emitters, and high-temperature, high-power transistors. The main advantages of ZnO as a light emitter are its large exciton binding energy (60 meV), and the existence of well-developed bulk and epitaxial growth processes; for electronic applications, its attractiveness lies in having high breakdown strength and high saturation velocity. Optical UV lasing, at both low and high temperatures, has already been demonstrated, although efficient electrical lasing must await the further development of good, p-type material. ZnO is also much more resistant to radiation damage than are other common semiconductor materials, such as Si, GaAs, CdS, and even GaN; thus, it should be useful for space applications.

Recent Advances in ZnO Materials and Devices

Electron and ambipolar transport in organic field-effect transistors.

Human skin is a remarkable organ. It consists of an integrated, stretchable network of sensors that relay information about tactile and thermal stimuli to the brain, allowing us to maneuver within our environment safely and effectively. Interest in large-area networks of electronic devices inspired by human skin is motivated by the promise of creating autonomous intelligent robots and biomimetic prosthetics, among other applications. The development of electronic networks comprised of flexible, stretchable, and robust devices that are compatible with large-area implementation and integrated with multiple functionalities is a testament to the progress in developing an electronic skin (e-skin) akin to human skin. E-skins are already capable of providing augmented performance over their organic counterpart, both in superior spatial resolution and thermal sensitivity. They could be further improved through the incorporation of additional functionalities (e.g., chemical and biological sensing) and desired properties (e.g., biodegradability and self-powering). Continued rapid progress in this area is promising for the development of a fully integrated e-skin in the near future.

25th Anniversary Article: The Evolution of Electronic Skin (E-Skin): A Brief History, Design Considerations, and Recent Progress

Acenes have long been the subject of intense study because of the unique electronic properties associated with their pi-bond topology. Recent reports of impressive semiconductor properties of larger homologues have reinvigorated research in this field, leading to new methods for their synthesis, functionalization, and purification, as well as for fabricating organic electronic components. Studies performed on high-purity acene single crystals revealed their intrinsic electronic properties and provide useful benchmarks for thin film device research. New approaches to add functionality were developed to improve the processability of these materials in solution. These new functionalization strategies have recently allowed the synthesis of acenes larger than pentacene, which have hitherto been largely unavailable and poorly studied, as well as investigation of their associated structure/property relationships.

The larger acenes: versatile organic semiconductors.

We present a model for determining the threshold voltage of field-effect transistors with nanocrystalline active channel layers. In this type of device, the multiple boundaries between neighboring crystalline grains limit the charge-carrier transport. Electrons in the channel may either populate the conduction band within a grain or be trapped at an interface between neighboring grains. The relative distribution of the electrons over these states determines the conductances of the grains and of the boundaries between them. We employ simple carrier statistics to calculate the macroscopic densities of free and trapped carriers, and these densities are then used to define site and bond occupation probabilities for a two-dimensional site-bond percolation problem. The dependence of the threshold voltage on the primary model parameters: the energy of the trap states, the total density of traps, and the temperature, is explored.

Percolation model for the threshold voltage of field-effect transistors with nanocrystalline channels

We present magnetometry and charge transport data for a GaMnN film with approximately 7% (atomic) Mn grown by molecular beam epitaxy. Measurements of magnetization vs. applied magnetic field show hysteresis consistent with the existence of ferromagnetism up to 300 K. Magnetization curves as a function of temperature indicate a phase transition near 170 K. Temperature-dependent Hall effect measurements show n-type characteristics with high carrier concentration and low mobility, which are both only weakly dependent on temperature. Piezoresistance measurements under hydrostatic pressure yield pressure coefficients that show little variation for temperatures of 77, 194, and 300 K.

Electrical and magnetic characteristics of MBE-grown GaMnN

A device model for tunnel injection and extraction of spin-polarized charge carriers between ferromagnetic contacts and organic semiconductors with disordered molecular states is presented. Transition rates for tunneling are calculated based on a transfer Hamiltonian. Transport in the bulk semiconductor is described by macroscopic device equations. Tunneling predominantly involves organic molecular levels near the metal Fermi energy, and therefore typically in the tail of the band that supports carrier transport in the semiconductor. Disorder-induced broadening of the relevant band plays a critical role for the injection and extraction of charge carriers and for the resulting magneto-resistance of an organic semiconductor spin valve.

Effects of disorder on spin injection and extraction for organic semiconductor spin-valves

We present a discussion of the complexities encountered in particle simulation models
for noncubic symmetry semiconductors, focusing on the wurtzite and 4H polytypes
of GaN and SiC. We have identified three general issues, band structure, scattering
mechanisms, and band intersections, which in our opinion, constitute the most important
modifications to conventional Monte Carlo simulators for cubic symmetry
semiconductors. It is found that the band intersection points present the greatest
modeling challenge. We discuss the effect of band intersections on the transport
dynamics and our initial attempts at treating transport near these points. Comparison to
experimental data is also made.

/pdf/monte-carlo-modeling-of-wurtzite-and-4h-phase-semiconducting-5g219m7bau.pdf

Monte Carlo modeling of wurtzite and 4H phase semiconducting materials

We have studied current versus voltage characteristics of n-GaN∕u-AlGaN∕n-GaN double heterostructure devices under hydrostatic pressure up to 500MPa. Devices were grown on c-plane sapphire substrates by organometallic vapor phase epitaxy using epitaxial layer overgrowth. The effect of AlGaN layer thickness and composition on the pressure sensitivity was investigated. For a fixed applied bias, we found that the current decreases approximately linearly in magnitude with increasing hydrostatic pressure over the range of voltages and pressures applied. The decrease in current magnitude can be attributed to piezoelectric effects and is consistent with model calculations. The polarization charge densities at the GaN∕AlGaN interfaces change with hydrostatic pressure, which in turn modifies the internal potential barrier. Changes in the AlGaN layer thickness and composition also modify the interfacial polarization, with thicker AlGaN layers and higher AlN content increasing the effect of pressure on the observed ...

/pdf/current-versus-voltage-characteristics-of-gan-algan-gan-2iua8jskfg.pdf

Current versus voltage characteristics of GaN∕AlGaN∕GaN double heterostructures with varying AlGaN thickness and composition under hydrostatic pressure

P. Paul Ruden

Papers

Percolation model for the threshold voltage of field-effect transistors with nanocrystalline channels

Electrical and magnetic characteristics of MBE-grown GaMnN

Effects of disorder on spin injection and extraction for organic semiconductor spin-valves

Monte Carlo modeling of wurtzite and 4H phase semiconducting materials

Current versus voltage characteristics of GaN∕AlGaN∕GaN double heterostructures with varying AlGaN thickness and composition under hydrostatic pressure