Chien-Jang Wu

Device modeling of ferroelectric memory field-effect transistor (FeMFET)

A numerical analysis of the electrical characteristics for the ferroelectric memory field-effect transistors (FeMFETs) is presented. Two important structures such as the metal-ferroelectric-insulator-semiconductor field-effect transistor (MFISFET) and metal-ferroelectric-metal-insulator-semiconductor field-effect transistor (MFMISFET) are considered. A new analytic expression for the relation of polarization versus electric field (P-E) is proposed to describe the nonsaturated hysteresis loop of the ferroelectric material. In order to provide a more accurate simulation, we incorporate the combined effects of the nonsaturated polarization of ferroelectric layers and the nonuniform distributions of electric field and charge along the channel. We also discuss the possible nonideal effects due to the fixed charges, charge injection, and short channel. The present theoretical work provides some new design rules for improving the performance of FeMFETs.

Device modeling of ferroelectric memory field-effect transistor for the application of ferroelectric random access memory

An improved theoretical analysis on the electrical characteristics of ferroelectric memory field-effect transistor (FeMFET) is given. First, we propose a new analytical expression for the polarization versus electric field (P-E) for the ferroelectric material. It is determined by one parameter and explicitly includes both the saturated and nonsaturated hysteresis loops. Using this expression, we then examine the operational properties for two practical devices such as the metal-ferroelectric-insulator-semiconductor field-effect transistor (MFIS-FET) and metal-ferroelectric-metal-insulator-semiconductor field-effect transistor (MFMIS-FET) as well. A double integral also has been used, in order to include the possible effects due to the nonuniform field and charge distribution along the channel of the device, to calculate the drain current of FeMFET. By using the relevant material parameters close to the (Bi, La)4Ti3O12 (BLT) system, accurate analyses on the capacitors and FeMFETs at various applied biases are made. We also address the issues of depolarization field and retention time about such a device.

Microwave surface impedances of BCS superconducting thin films

The microwave propagation-dominated problem in the multilayer structure made of a BCS superconducting film and a dielectric substrate is investigated theoretically by using the modified two-fluid model and transmission line theory. The effective microwave surface impedances are studied as functions of temperature, frequency, and film thickness, as well as substrate thickness. Special attention is paid to the substrate resonance phenomenon in the resonant structure. The influence of BCS coherence effects on surface impedance and resonant behavior is clearly demonstrated. The resonant effect in the stack structure is well interpreted with the help of the transverse resonance technique in the microwave theory.

Effective microwave surface impedance of superconducting films in the mixed state

The effective microwave surface impedance of multilayer structures made of high-T/sub c/ superconducting films in the mixed state, lossy dielectrics, and normal metals are theoretically calculated. The linear response of the superconductor to a microwave field is analyzed within both transmission line theory and the framework of a self-consistent treatment of vortex dynamics reported by Coffey and Clem (1991). The microwave properties are investigated as a function of static field and film thickness for nonresonant structures. The effect of substrate thickness on the resonant phenomenon is carefully studied as well. Numerical results reveal that the substrate resonance in the Meissner state behaves like a parallel lumped-parameter resonator, while in the vortex state it behaves as a series lumped resonant circuit. The basic distinction suggests that care should be taken in microwave applications when using superconducting films in the vortex state.

AC reponse of the vortex liquid in the high-Tc superconducting cylinder

Abstract The ac response of the vortex liquid of a high- T c superconducting cylinder in the parallel field configuration is examined theoretically. The ac properties are analyzed from the associated complex ac permeability calculated on the basis of hydrodynamics theory. The permeability is primarily dominated by two frequency-dependent penetration lengths which are also shown to be independent of the geometries considered. In the simple flux-flow regime, the second peak in ac loss is observed to be enhanced while the first peak is depressed for the superconducting cylinder. In the viscous flux liquid, the ac response is found to rely on the viscosity, ratio of shear modulus to nonlocal compressional modulus, and the geometries discussed. The results indicate some basic difference in ac permeabilities between superconducting cylinder and slab.

Vortex response to the ac field in anisotropic high-Tc superconductors

Abstract The ac response of anisotropic high- T c superconductors in the mixed states is theoretically analyzed. The physical picture proposed by Geshkenbein, Vinokur and Fehrenbacher together with the theory of thermally assisted flux flow is generalized to include the material anisotropy. The results show that the transition in the real part and the peak in the imaginary part of magnetic ac permeability are due to skin effect and, additionally, related to the sample dimensions. For an isotropic superconductor, the infinite slab geometry exhibits a maximum peak absorption, while a minimum dissipation peak is obtained in the case of a square rod. The peak frequency in the infinite slab is, however, the lowest as compared with other finite prisms. In the very anisotropic superconductors, the thin edges lower the dissipation peak and raise the peak frequency as well. Dependence of the irreversibility line in the mixed state on the geometric factor is therefore suggested.

AC permeability of the flux-line liquid in the anisotropic high-T/sub c/ superconducting crystals

AC properties of the flux-line liquid in the anisotropic high-temperature superconducting crystals in a parallel field are theoretically investigated. The ac responses in the simple flux-flow regime are analyzed from associated effective ac-magnetic permeability calculated on the hydrodynamic theory basis. The responses are studied as functions of anisotropic ratio and sample dimensions. The results illustrate the influence of the platelet crystals size on permeability in the anisotropic superconductors while in the isotropic superconductors, the relationship of responses between a square rod and cylinder is found. It indicates that the permeability of a cylinder can be essentially replaced by that of a square rod and vice versa. The geometric effect on response is also elucidated in the isotropic superconductors.

Critical current anisotropy and flux pinning in YBCO [001] and YBCO [110] superconducting thin films prepared by KrF pulsed laser ablation

Direct transport critical current density (J/sub c/) measurements were performed on laser-ablated high-quality Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/ superconducting thin films with T/sub co/>or=90 K, J/sub c/ approximately=5*10/sup 6/ A/cm/sup 2/ at 77 K, and intentionally controlled film orientations to investigate the J/sub c/ anisotropy and predominant pinning mechanisms in these films. It was found that, for films with [001] oriented normal to the substrate, the field dependencies of the critical current densities J/sub c/(H) showed very different behavior for different field orientations, indicating that the active pinning mechanisms may be different in two field directions. For [110]-oriented films with field applied normal to the film surface, the pinning mechanism was found to follow the manifestation of surface-core pinning. At zero field, however, the temperature dependence of J/sub c/ revealed that, in all cases, the critical current was limited by flux creep with essentially the same creep kinetic factor expected for high-T/sub c/ cuprates. Detailed results on the scaling behaviors of the pinning force density as functions of both field and temperatures for fields up to 7 T are presented and compared for different films and field orientations.<<ETX>>