Water management development and agriculture in Syria

Irrigated agriculture has increased steadily in Syria over the last decades, almost doubling since 1985. This mounting pace has responded to the nation’s food security policy objectives to satisfy the food production needs of an increasing population that features one of the largest growth rates in the world, namely 3,50 percent in 1985 and still 3,39 percent in 2007. Total expenditures for irrigated agriculture accounted for almost 70 percent of all expenditures in agriculture. Sustainable irrigation water policies aimed at increasing the efficiency of water use in agriculture and at conserving water resources by reducing future consumption. The Euphrates River is 2.800 km long and its middle traverses a wide floodplain in Syria, where it is used extensively for irrigation, and the Euphrates Dam is 230 ft (70 m) high. The total estimated water use volume is about 15 billion m3. The Euphrates and Orontes basins account for about 50% and 20% of the water use respectively. About 701.634 ha has been irrigated by ground water in the year 1997. This area represents 60% of the total irrigated land in Syria It has been gradually increased from 30% during 1970 to 44% in 1980 and 49% in 1990. The Government projects extended on 349.820 hectare area, which includes large, medium and small scale farms. The small scale government project is under 2000 hectare, but large scale project over 20.000 hectare areas. The Syrian Government wants to ensure the food supply for sharply increasing population based on established governmental agricultural projects, as state-owned farms. Water balance for Syria indicates that most of the basins are in deficit. This will be exacerbated further especially in basins encompassing large urban areas and if the country’s population continues to grow at its current rate (about 3%) and water use efficiency is not increased effectively.Water utilisation, Water management, Modern irrigation technologies, Benefits of agricultural sector, Governmental supports, Total Renewable Water Resources (TRWR), Crop Production/Industries, Environmental Economics and Policy, Farm Management, Risk and Uncertainty,

Microwave and millimeter-wave power generation in silicon carbide (SiC) IMPATT devices

There are two points that should be noted. First, in the thermal resistance calculations it is assumed that the device is operating at 773 K while the results of the room temperature simulations are used. This was done because there is not enough information to correctly predict the material parameters at 773 K. Since, in general, device performance degrades with increasing temperature, the cw results are perhaps a bit optimistic. Second, the electric field in these structures gets extremely high and there might be some possibility of tunneling. This was not incorporated into the simulation. Again, this could result in different device operating conditions

Differential tunnel transparency of a rectangular heterostructural barrier for the terahertz frequency range

Electron wave tunneling through a rectangular heterostructural emitter barrier is considered in the case of a homogeneous high-frequency (hf) alternating electric field directed normal to the barrier interfaces. This hf field leads not only to the well-known increase in a stationary tunnel current through the emitter barrier, which is proportional to EB2EB2 (where EBEB is the electric-field amplitude) but also to a linear ( ∼ EB)(∼EB) increase in an alternating current (ac) through this barrier with the same frequency ωω as the electric-field frequency. The ac is a sharp function of ωω, which grows significantly with an increase in ωω (typically in the terahertz range). In a certain intermediate current and frequency region, the above-mentioned increase in the ac is the dominating effect of the alternating field. Such an effect can be used to optimize tunnel barrier emitters for ballistic transit-time terahertz-range oscillators.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87677/2/093705_1.pd

Effects of biaxial strain on the intervalence‐band absorption spectra of InGaAs/InP systems

The effects of biaxial strain on the intervalence‐band absorption spectra of p‐doped InGaAs/InP bulk layers are investigated. The study is performed by calculating and comparing the absorption coefficients corresponding to the direct transitions between the heavy and light hole bands, between the heavy hole and split‐off bands, and between the split‐off and light hole bands in both the lattice matched and the strained layers. The valence‐band structures of these layers are neither isotropic nor parabolic and hence the k⋅p approach is utilized to calculate the band structures and their corresponding wave functions. The quantities are then invoked in the calculation of the (joint) density of states, the Fermi energy, and the momentum matrix element, which are needed in the evaluation of the intervalence‐band absorption coefficients. These calculated results show that the intervalence‐band absorption coefficients depend on the strain in the layer. The dependence is determined by the bands involved in the intervalence transition, the polarization of the incident light, and the type of the strain (compressive or tensile). © 1995 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70188/2/JAPIAU-77-12-6549-1.pd

Quantum wires and dots induced in a semiconductor by charged metallic filaments separated by an isolating barrier

A very thin positively charged metallic filament separated from a surface of a semiconductor (S)(S) by a thin nontunneling potential barrier (B)(B) induces a quantum wire (QWr) in the semiconductor at the B/SB∕S interface. Single-electron quantum states of this QWr are controlled by a potential (and a charge) of the metallic filament. Two close parallel metallic filaments placed over such a B/SB∕S interface form a double-quantum wire with the ground and the first excited electron states, which appear as a result of a symmetric–antisymmetric splitting of the ground electron state in the single QWr. Two crossed metallic filaments, which are parallel to the B/SB∕S interface, form a quantum dot with completely localized electron states under the crossing point of the metallic filaments. The analogous crossing of a metallic filament by a pair of close metallic filaments forms a double-quantum dot (DQD). The latter can serve as a two-level qubit cell. Such qubits can be controlled by potentials of three independent metallic filaments inducing the above-mentioned DQD. Besides this “outside” metallic wire control, the DQDs can be connected to each other across the “inside” quantum wires, which have formed these DQDs by crossing.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87666/2/033516_1.pd

Switching speeds in double‐barrier resonant‐tunneling diode structures

Switching speeds are calculated for GaAs‐AlGaAs resonant‐tunneling diode structures with different barrier widths from the time‐dependent Schrödinger equation. The speed is determined by monitoring the device current as the bias voltage is instantaneously switched. Effective mass discontinuities at the barrier and quantum well edges are included. Comparisons with previously published results using the wave packet approach are given. It is found that the turn‐off transient is dominated by the lifetime of the quasibound state; however, care must be used in calculating the lifetime.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70991/2/JAPIAU-70-12-7638-1.pd

Split donor centers and split excitons in a semiconductor heterostructure

The first subject considered in the article is a donor center embedded in a thin heterostructural barrier separating a semiconductor medium into two halves. As a result of the small thickness of this barrier, the wave function of an electron bound by the donor center shifts almost completely into both halves of the surrounding semiconductor medium. The ground and first excited electron states of such a donor center are separated from each other by a narrow energy gap determined by the symmetric-antisymmetric tunnel splitting. Such structures can be implemented in both GaAs/AlXGa1−XAsGaAs∕AlXGa1−XAs and Si/GeXSi1−XSi∕GeXSi1−X material systems. The second considered subject is an exciton formed in analogous heterostructures when the staggered band alignment takes place between the heterobarrier and semiconductor medium. As a result of such band alignment, the hole participating in the exciton creation is located in the formed quantum well and the electron, which is the hole’s opponent, is separated into halves (on different sides of the quantum well) as before. Unlike the donor center, the exciton can be shifted and localized in arbitrary positions along the staggered “barrier-well” boundary by inhomogeneous electric fields of external controlling gates.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87366/2/073711_1.pd

Time-dependent electron tunneling through time-dependent tunnel barriers

A plane electron wave incident on a tunnel-transparent potential barrier formed by the potential V(x,t)=V0(x)+V1(x)cos ωtV(x,t)=V0(x)+V1(x)cos ωt generates, in addition to the usual stationary transmitted and reflected stationary waves, also “transmitted” and “reflected” electron waves oscillating with the same frequency ωω. The transmitted oscillating wave can serve as the basis for transit-time microwave generators oscillating in the terahertz range. (Such oscillators are ballistic analogs of the tunnel-emission transit-time diode oscillators suggested almost half a century ago.) In the special case of a rectangular potential barrier, we describe the dependence of a small transmitted oscillating wave amplitude on the frequency ωω and the value of V1(x)V1(x). We consider two forms of V1(x)V1(x): (1) homogeneous oscillation of the height of the rectangular barrier and (2) V1(x)=aδ(x−x1)V1(x)=aδ(x−x1) [where δ(x)δ(x) is the Dirac delta function and 0<x1<w0<x1<w; ww is the barrier thickness]. For sufficiently high frequencies ωω determined by the time for tunneling, a much higher emission of the transmitted oscillating wave takes place in comparison with the results of quasistatic calculations.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70729/2/JAPIAU-96-7-3831-1.pd