Narrow Results

The Inverse Nottingham Effect (INE) cooling involves emission of electrons above the Fermi level into the vacuum. Our scheme involves the use of a Double Barrier Resonant Tunneling (DBRT) section positioned between the surface and the vacuum for a much increased emission, and to provide energy selectivity for assuring cooling, without surface structuring such as tips and ridges leading to current crowding and additional heating. Unlike resonant tunneling from contact-to-contact, where barrier heights and thicknesses are controlled by the choice of heterojunctions, the work function at the surface dictates the barrier height for tunneling into the vacuum. The calculated field emission via resonant tunneling gives at least two orders of magnitude greater than without resonance, however, without work function lowering, the large gain happens at fairly high field. The use of resonance to enhance cooling by INE results in an important byproduct, an efficient cold-cathode field emitter for vacuum electronics.

Plasma waves are oscillations of electron density in time and space. In deep submicron field effect transistors plasma wave frequencies lie in the terahertz range and can be tuned by applied gate bias. Since the plasma wave frequency is much larger that the inverse electron transit time in the device, it is easier to reach "ballistic" regimes for plasma waves than for electrons moving with drift velocities. In the ballistic regime, no collisions of electrons with impurities or lattice vibrations occur on a time scale on the order of the plasma oscillation period, and the device channel acts as a resonant cavity for the plasma waves, making possible tunable resonant detection or even emission of the electromagnetic radiation in the terahertz range. We review the theory of plasma waves in field effect transistors; discuss instabilities of these waves in different device structures and their applications for detection and generation of the terahertz radiation.

Photon assisted transport, dynamic localization and absolute negative conductance appear in the terahertz photoconductivity in semiconductor quantum structures and are close analogs of quasi-particle transport in microwave irradiated superconducting junctions. By embedding superlattice devices in quasi-optical arrays and integrating them into terahertz cavities, the dynamical conductance of electrically biased superlattices can be measured. Models including the complications of electric field domains can account for the results in a semi quantitative manner. Uniform electrically biased superlattices appear to be potentially important as a terahertz gain medium.

Reduction in the dark current and improvement in signal to noise ratio in the quantum dots in a well infrared photodetectors using resonant tunneling barriers have been demonstrated. Ultra-low dark current levels and high detectivity of 3×10¹⁰ cm.Hz^1/2/W at 77K for f/2 optics has been obtained for longwave infrared detection. In another experiment, the ability to control the excited state in the DWELL has been demonstrated by systematically varying the quantum well thickness. These detectors demonstrate high operating temperature with high detectivity values, even for high operating temperatures.

In this paper, novel features offered by Resonant Tunneling Diode (RTD) are reviewed by simulating it under different conditions. GaAs/AlGaAs based RTD is used as the reference one to obtain the characteristics due to parametric variations. To fulfil this purpose a simple model of resonant electronic transport through a double-barrier structure is developed. I-V characteristics are studied by varying barrier parameters and well width. Different peak and valley currents are measured under these conditions. For the same set of parameters both symmetric and asymmetric cases are considered. Different materials of lower effective mass are also taken into consideration to improve Peak to Valley Ratio (PVR). The Indium (In) based materials are considered to compare the characteristics obtained from the conventional GaAs based RTD structure. All these proposed structures are simulated using Silvaco Atlas software.

The I-V curves in multi-quantum wells of different semiconductors are studied theoretically using the formalism of the transmission coefficient directly derived from Schrödinger equation. Al_0.5Ga_0.5As/GaAs double-barrier quantum well, Al_0.29Ga_0.71As/GaAs multi-quantum well, and AlSb/InAs double-barrier structure are calculated. The influences of well width, barrier width, temperature, Fermi energy on I-V characteristic curves are discussed in detail. Calculated results show that obvious negative differential resistance effects presented by our simulated I-V curves has a good agreement with previous experiments. Therefore, it can be a theoretical expectation to design experimentally high-quality semiconductor devices.

An investigation for numerical method to study quantum dynamics in dissipative environments is presented, studying the resonant tunneling phenomena of the low temperature magnetization process of Mn₁₂. It is pointed out that information of pure quantum transition can be obtained from deceptive nonadiabatic transitions due to the dissipative environments.

The effect of coupling between the electronic transverse motion and longitudinal motion is considered in the theoretical investigation of the resonant tunneling in a semiconductor multi-barriers heterostructure. A numerical calculation is carried out for rectangular and parabolic-well heterostructures consisting of ZnSe/Zn_1-xCd_xSe. The result indicates that the coupling effect results in not only a movement of the resonant peaks but also a reduction of the peak-to-valley ratio in the transmission spectrum. The effect of the electronic transverse motion on the higher-lying resonant states for the resonant tunneling is more remarkable for both the zero and non-zero bias voltages. The J-V characteristic formula of tunneling current density, which is different from Esaki's result, is given by using a two-dimensional approximation. The influence of temperature and mixed crystal effect on the J-V characteristic is also investigated.

We introduce a general analysis method, which allows us to simulate the operation of high-performance molecular nano-devices and to design the expected function of a wide range of devices in nano-scale size. The method is based on the use of a resonant tunneling phenomenon, admitting strong electron correlation in a quantum dot with degenerated states. Three examples of the application of this method are given: Coulomb repulsion, uncorrelated resonant tunneling, and electron-phonon interaction. It is shown that there is a good agreement with experimental data in all three cases.

The transverse resonant tunneling transport and electric field domain formation in GaAs/AlGaAs superlattices were investigated in a strong tilted magnetic field. The magnetic field component parallel to structure layers causes intensive tunneling transition between Landau levels with Δn≠0, resulting in the considerable "inhomogeneous" broadening of intersubband tunneling resonance as well as in the shift of the resonance toward higher electric fields. This leads to noticeable changes of the I-V characteristics of the superlattice, namely to smoothing of the periodic NDC structure on plateau-like regions caused by formation of the electric field domains and to the shift of the plateaus toward the higher applied voltage. The predicted behavior of the I-V characteristics of the structures in magnetic field was found experimentally.

This work analyzes the Abrikosov's idea that high temperature conductivity is based on resonant tunneling. A rectangular rod was considered and it was shown that high values of transparency coefficient can be produced in it. It is the consequence of the fact that electrons as well as phonons have energy gap within a rectangular rod.

Restricting ourselves to a simple rectangular approximation but using properly a two-scale regularization procedure, additional resonant tunneling properties of the one-dimensional Schrödinger operator with a delta derivative potential are established, which appear to be lost in the zero-range limit. These "intrinsic" properties are complementary to the main already proved result that different regularizations of Dirac's delta function produce different limiting self-adjoint operators. In particular, for a given regularizing sequence, a one-parameter family of connection condition matrices describing bound states is constructed. It is proposed to consider the convergence of transfer matrices when the potential strength constant is involved into the regularization process, resulting in an extension of resonance sets for the transmission across a δ′-barrier.

In close analogy with optical Fabry–Pérot (FP) interferometer, we rederive the transmission and reflection coefficients of tunneling through a rectangular double barrier (RDB). Based on the same analogy, we also get an analytic finesse formula for its filtering capability of matter waves, and with this formula, we reproduce the RDB transmission rate in exactly the same form as that of FP interferometer. Compared with the numerical results obtained from the original finesse definition, we find the formula works well. Next, we turn to the elusive time issue in tunneling, and show that the "generalized Hartman effect" can be regarded as an artifact of the opaque limit βl → ∞. In the thin barrier approximation, phase (or dwell) time does depend on the free inter-barrier distance d asymptotically. Further, the analysis of transmission rate in the neighborhood of resonance shows that, phase (or dwell) time could be a good estimate of the resonance lifetime. The numerical results from the uncertainty principle support this statement. This fact can be viewed as a support to the idea that, phase (or dwell) time is a measure of lifetime of energy stored beneath the barrier. To confirm this result, we shrink RDB to a double Dirac δ-barrier. The landscape of the phase (or dwell) time in k and d axes fits excellently well with the lifetime estimates near the resonance. As a supplementary check, we also apply phase (or dwell) time formula to the rectangular well, where no obstacle exists to the propagation of particle. However, due to the self-interference induced by the common cavity-like structure, phase (or dwell) time calculation leads to a counterintuitive "slowing down" effect, which can be explained appropriately by the lifetime assumptions.

Numerical calculations by a transfer matrix method have been performed to obtain the transmission coefficient of rectangular double barrier structures. The dependence of the well width, barrier width and the barrier height was systematically investigated. When the width ratio of the two barriers was varied on condition that a total width was fixed, the transmission coefficient at a resonance is varied while that at a valley region is not. It is concluded that the resonant tunneling is characterized by two parameters: total width and the width ratio. Our results clarify the transition of transmission spectrum from a single barrier to a double barrier structure.

Double barrier structures with a gradient have been studied by numerical calculations of transmission coefficient following a transfer matrix method. As the slope of the barriers becomes steep, the resonant energy is lowered. On the other hand, the full width at half maximum does not depend on the gradient.

The spin-dependent electron transmission phenomenon in an SiGe/Si/SiGe resonant semiconductor heterostructure is employed theoretically to investigate the output transmission current polarization at zero magnetic field. Transparency of electron transmission is calculated as a function of electron energy as well as the well width, within the one electron band approximation along with the spin-orbit interaction. Enhanced spin-polarized resonant tunneling in the heterostructure due to Dresselhaus and Rashba spin-orbit coupling induced splitting of the resonant level is observed. We predict that a spin-polarized current spontaneously emerges in this heterostructure and we estimate theoretically that the polarization can reach 100%. This effect could be employed in the fabrication of spin filters, spin injectors, and detectors based on nonmagnetic semiconductors.

The inverse Nottingham Effect (INE) cooling involves emission of electrons above the Fermi level into the vacuum. Our scheme involves the use of a Double Barrier Resonant Tunneling (DBRT) section positioned between the surface and the vacuum for a much increased emission, and to provide energy selectivity for assuring cooling, without surface structuring such as tips and ridges leading to current crowding and additional heating. Unlike resonant tunneling from contact-to-contact, where barrier heights and thicknesses are controlled by the choice of heterojunctions, the work function at the surface dictates the barrier height for tunneling into the vacuum. The calculated field emission via resonant tunneling gives at least two orders of magnitude greater than without resonance, however, without work function lowering, the large gain happens at fairly high field. The use of resonance to enhance cooling by INE results in an important byproduct, an efficient cold-cathode field emitter for vacuum electronics.

Photon assisted transport, dynamic localization and absolute negative conductance appear in the terahertz photoconductivity in semiconductor quantum structures and are close analogs of quasi-particle transport in microwave irradiated superconducting junctions. By embedding superlattice devices in quasi-optical arrays and integrating them into terahertz cavities, the dynamical conductance of electrically biased superlattices can be measured. Models including the complications of electric field domains can account for the results in a semi quantitative manner. Uniform electrically biased superlattices appear to be potentially important as a terahertz gain medium.

Plasma waves are oscillations of electron density in time and space. In deep submicron field effect transistors plasma wave frequencies lie in the terahertz range and can be tuned by applied gate bias. Since the plasma wave frequency is much larger that the inverse electron transit time in the device, it is easier to reach “ballistic” regimes for plasma waves than for electrons moving with drift velocities. In the ballistic regime, no collisions of electrons with impurities or lattice vibrations occur on a time scale on the order of the plasma oscillation period, and the device channel acts as a resonant cavity for the plasma waves, making possible tunable resonant detection or even emission of the electromagnetic radiation in the terahertz range. We review the theory of plasma waves in field effect transistors; discuss instabilities of these waves in different device structures and their applications for detection and generation of the terahertz radiation.