Volume 93, Issue 11, 01 June 2003
Index of content:
- DEVICE PHYSICS (PACS 85)
93(2003); http://dx.doi.org/10.1063/1.1569030View Description Hide Description
The dc bias range and the logic-state lifetime for multivalued single-electron tunneling phase logic are examined in computer simulations. Results for quaternary operation, which is of particular interest for compact logic circuitry, are compared with those for ternary and binary logic. Quaternary logic is shown to exhibit a rotation rule similar to binary and ternary logic and to have an optimized dc bias range of 14%, compared with 20 and 37 % for ternary and binary logic, respectively. The effects of shot noise and Nyquist noise on the logic-state lifetime are strongly influenced by the number of logic states. The ratio of tunnel resistance to series resistance required to suppress the effects of shot noise below those of Nyquist noise depends on the number of logic states. To obtain a lifetime equal to 1000 phase-locked cycles together with a maximum dc bias operating range, the ratio of Coulomb energy to thermal energy must be 400, 1650, and 2830 for two, three, and four states, respectively. This corresponds to a rapid drop in the maximum operating temperature with the number of states, which can be related to an exponential dependence of lifetime on the activation energy for a thermally activated escape process. The implications of these results on the trade-offs between the operating regimes for dc bias, temperature, and junction size are discussed.
93(2003); http://dx.doi.org/10.1063/1.1570933View Description Hide Description
An experimental and modeling study of charge trapping related threshold voltage shifts in and n-type field effect transistors is reported. The dependence of threshold voltage, subthreshold slope, and gate leakage currents on stressing time and injected charge carrier density are investigated as a function positive bias stress voltage and temperature. Based on experimental data, a model for trapping of charges in the existing traps is developed. The model is similar to charge trapping models with one exception. Unlike models, the model assumes a continuous distribution in trapping capture cross sections. The model predicts that threshold voltage would increase with a power law dependence on stressing time and injected charge carrier density in the initial stages of stressing. The model calculates threshold voltage shifts as a function of stress time and thereby provides estimates of threshold voltage shifts after 10 years lifetime. It also provides insights into the nature of traps by estimating trapping capture cross sections. The calculated results are shown to be consistent with both and data over several decades of stressing time and Using the model, a comparison between and is made. In addition, the model is compatible with charge trapping data reported by other research groups.
93(2003); http://dx.doi.org/10.1063/1.1569980View Description Hide Description
The diffusion-induced bending of both single-layer and bilayer beam structure is analyzed by using linear elastic beam theory and the Moutier theorem. A closed form solution of the radius of curvature due to diffusion is obtained. For the single-layer beam structure, the radius of curvature is inversely proportional to the bending moment created by nonuniform concentration distribution. For the bilayer beam structure, the curvature is a linear function of the mismatch strain between the two layers and the bending moment introduced by diffusion. The mismatch strain depends on the concentration and the partial molar volume of the diffusing component in both layers. Application to microelectromechanical systems hydrogen sensors with a layer of Pd is shown.
93(2003); http://dx.doi.org/10.1063/1.1574596View Description Hide Description
The role of superparamagnetism in determining the ultimate density of magnetic random access memory is investigated. Numerical calculations show that superparamagnetism can easily be stabilized even in elements of just a few nanometers in size by the shape anisotropy generated in elements with lateral elongation. The limitation to increasing density is shown to come not directly from superparamagnetism, but from the high current densities which will be required to write data to elements that have been stabilized against superparamagnetism. Experimental measurements on planar elliptical Permalloy nanoelements show the capability of shape anisotropy to generate large stabilization fields.
93(2003); http://dx.doi.org/10.1063/1.1569975View Description Hide Description
Magnetoresistive random access memory (MRAM) with ferromagnetictunnel junctions (MTJs) has been recognized as a successful candidate for next generation solid-state memory. There are, however, still significant problems to realize MRAM. In particular, reliability of an ultrathin dielectric layer composing tunnel junctions is of great importance for MRAM developments. In this article, we performed systematical studies of the dielectric breakdown of MTJs. The samples have a double tunnel barrier structure with Its width is ranging from 0.4 to 30 μm. Different from previous reports, the breakdown voltage gradually decrease with decreasing junction area. The breakdown voltage only shows weak dependence on junction area in the sub-μm region. In addition, the breakdown voltage is inversely proportional to the junction perimeter. Results of time-dependent failure measurements demonstrate that, within the limit of this experiment, the failure function of the MTJs obeys a Weibull distribution. The shape parameter of the Weibull distribution is less than unity, indicating early device to failure. We consider that process-induced defects at the sidewall of the junctions may serve a certain role in their early failure and lead the failure of the junction area scaling of the breakdown voltage.