Narrow Results

A high mobility two-dimensional electron system exhibits large changes in the resistance, and zero-resistance states, under microwave and Terahertz excitation. We describe associated experimental results and the possibility of using this system as a radiation detector.

We have fabricated and investigated several types of GaN MOSFETs with normally-off operation. The recessed-gate GaN MOSFET is preferred for normally-off operation, because the threshold voltage (V_th) of the device can be easily controlled, but it suffers from relatively modest current drivability which must be improved by adopting appropriate device structure and/or process. Enhanced performances have been achieved in this work by combining the recessed-gate technology with additional processes, such as: the post-recess tetramethylammonium hydroxide (TMAH) treatment to remove the plasma damage, the post-deposition annealing of gate oxide to decrease the gate leakage current, the re-growth of n+ GaN layer for source/drain to improve the access resistance and V_th uniformity, the stress control technology to achieve extremely high 2-D electron-gas density (2DEG) on source/drain and decrease the series resistance, and the use of the p-GaN back-barrier to decrease the buffer leakage current. The GaN-based FinFET with very narrow fin was also investigated as a possible candidate for high performance normally-off GaN MOSFETs.

In this study, we have investigated the ground state energy level of electrons in modulation doped GaAs/Al_xGa_{1 - x}As heterojunctions. For this purpose, Schrödinger and Poisson equations are solved self consistently using quantum genetic algorithm (QGA). In this way, we have found the potential profile, the ground state subband energy and their corresponding envelope functions, Fermi level, and the amount of tunneling charge from barrier to channel region. Their dependence on various device parameters are also examined.

We have addressed the central question in two-dimensional disordered systems: whether all states are localized even at infinitesimal disorder, as has been believed for over two decades but has come in question after recent experiments. We argue that the answer could still be yes since above a certain density the electrons can be treated effectively as non-interacting. Below this density, though the interactions among electrons get weaker they become more and more important as the electron density reduces. This can cause delocalization and produce a metallic phase. Some recent experimental results have been analysed to understand the nature of the newly found metallic phase at low electron-densities, and also the insulating phase at lower densities.

We have performed microwave absorption and near-field reflection experiments on a high mobility GaAs/AlGaAs heterostructure for the same conditions for which Microwave-Induced Resistance Oscillations (MIROs) are observed. It is shown that the electrodynamic aspect of the problem is important in these experiments. In the absorption experiments a broad CR line was observed due to a large reflection from the highly conductive electron gas. There were no additional features observed related to absorption at harmonics of the cyclotron resonance.

In near-field reflection experiments a very different oscillation pattern was revealed when compared to MIROs. The oscillation pattern observed in the reflection experiments is probably due to plasma effects occurring in a finite-size sample. The whole microscopic picture of MIROs is more complicated than simply a resonant absorption at harmonics of the cyclotron resonance. Nevertheless, the experimental observations are in good agreement with the model by Durst et al. involving the photo-assisted scattering in the presence of a crossed magnetic field and dc bias. The observed damping factor of MIROs may be attributed to a change in the electron mobility as a function of temperature.

MIROs may be considered as a light-induced drift effect, a broad class of phenomena associated with a light-induced asymmetry in the velocity distribution function.

The beating pattern of Shubnikov-de Haas oscillations is reproduced in crossed magnetic field configuration and in presence of zero-field valley splitting in Si-MOSFET 2D system. The features of Quantum Hall Effect in extremely dilute 2D system are discussed.

We report results of temperature and field dependent studies of interband optical absorption and photoluminescence in a two-dimensional electron gas in a strong in-plane magnetic field (up to 45 T). We observe a field dependent shift of the Fermi energy with both the magnitude and sign of the shift varying with magnetic field. Simultaneous low temperature magneto-photoluminescence measurements reveal a monotonically increasing blue shift accompanied by a quenching of the intensity. A theoretical treatment based on a single particle picture and taking into account the diamagnetic shift as well as a shift due to an anisotropic effective mass cannot qualitatively reproduce our experimental results, indicating that many-body interactions may be important.

Modifications of photoluminescence (PL) spectra taken from modulation doped CdTe/CdMgTe quantum well (QW) structures have been studied in magnetic fields up to 45T as a function of 2D electron concentration.

Microwave (MW) absorption by a high mobility 2DEG has been investigated experimentally using sensitive Electron Paramagnetic Resonance (EPR) cavity technique. It is found that MW absorption spectra are chiefly governed by confined magnetoplasmon excitations in a 2DEG stripe. Spectra of the 2D magnetoplasmons are studied as a function of magnetic field, MW frequency and carrier density. The electron concentration is tuned by illumination and monitored using optical photoluminescence technique.

Tuned narrow-band-sensing of microwave and terahertz radiation can be realized by subjecting an irradiated two-dimensional electron system to both a static and a time varying magnetic field, and detecting at the harmonics of the modulation.

We have experimentally found that the height of quantized conductance steps of a short constriction in a two-dimensional electron gas can reduce by 3% as compared to 2e²/h. We explain this phenomenon by different shift of one-dimensional subbands in response to changing gate voltage. Both simple analytical estimate and realistic modeling of the device yield the same magnitude of the effect as that in experiment.

Time-resolved and time-integrated circularly-polarized photoluminescence of excitons and trions have been studied in external magnetic fields up to 10 T. ZnSe-based quantum well structures of n-type with carrier densities varied from 5×10⁹ to 10¹¹cm^-2 were used in this study. Absence of the chemical equilibrium in the exciton–trion system has been demonstrated at low temperatures (<10 K). The recovery of the equilibrium has been found at elevated temperatures (<15 K).

Doublet splitting of single-electron peaks has been observed in the conductance of a small high-resistance ring interferometer. Realistic modeling of the device shows that the electron system of interferometer divides into two triangular quantum dots connected by single-mode channels to each other and to reservoirs. We explain the splitting of conductance peaks by charge interaction of the dots.

We have used silicon impurities in an aluminum arsenide barrier to probe an adjacent two-dimensional electron gas (2DEG). A single impurity acts as a local spectrometer and scans the local density of states of the 2DEG. Magnetotransport experiments have been performed at low temperature with a magnetic field B applied along the direction of the current. Current–voltage characteristics strongly depend on B and reveal the formation of Landau levels (LLs).

The tunneling and the lateral conductance of 2DEG formed in the GaAs with the δ-doped layer were measured simultaneously at hydrostatic pressures at helium temperatures. The resistivity of the δ-doped layer sharply increases by > 3 orders of magnitude at about 2 GPa, and its temperature coefficient changes from ~ - 0.04 to ~ - 4 K^-1. The tunneling resistance shows only slight change of exponential behavior at about 1.5 GPa, while a drastic change of the Zero Bias Anomaly (ZBA) takes place just at the metal–insulator transition. Such a behavior is interpreted in terms of pressure dependence of the GaAs band structure and the DX-level position.

In this paper, we analyze the operation of an Otto cycle heat machine driven by a non-interacting two-dimensional electron gas on a twisted geometry. We show that due to both the energy quantization on this structure and the adiabatic transformation of the number of complete twists per unit length of a helicoid, the machine performance in terms of output work, efficiency, and operation mode can be altered. We consider the deformations as in a spring, which is either compressed or stretched from its resting position. The realization of classically inconceivable Otto machines, with an incompressible sample, can be realized as well. The energy-level spacing of the system is the quantity that is being either compressed or stretched. These features are due to the existence of an effective geometry-induced quantum potential which is of pure quantum-mechanical origin.

In this proposed work, an extensive study on the linearity performance of underlap AlInN/GaN double gate metal oxide semiconductor high electron mobility transistors (MOS-HEMT) has been analyzed using 2D Sentaurus TCAD simulation. Specifically a brief comparison is made on the linearity and intermodulation distortion characteristics of the proposed device due to variation of barrier layer thickness from 2 nm to 6 nm. Various parameters such as transconductance ($g_m)$, second-order transconductance ($g_{m2}$), third-order transconductance ($g_{m3}$), second-order voltage intercept point (VIP2), third-order voltage intercept point (VIP3), third-order input intercept point (IIP3) and third-order intermodulation distortion (IMD3) of underlap AlInN/GaN double gate metal oxide semiconductor high electron mobility transistors (MOS-HEMT) are discussed. The simulated results obtained confirms that by careful optimization of barrier layer thickness linearity characteristics of this proposed device can be improved, which can be suitable for analog and circuit applications.

The performance of AlInSb/InSb heterostructure with various parameters is considered with T-Cad simulation. As the heterojunctions are having more advantageous properties that is a real support for so many application such as solar cells, semiconductor cells and transistors. Special properties of semiconductors are discussed here with various parameters that are depending up on the performance of accurate device [Pardeshi H., Pati S. K., Raj G., Mohankumar N., Sarkar C. K., J. Semicond.33(12):124001-1–124001-7, 2012]. The maximum drain current density is achieved with improving the density of two-dimensional electron gas (2DEG) and with high velocity. High electron mobility transistor (HEMT) structure is used with the different combinations of layers which have different bandgaps. Parameters such as electron mobility, bandgap, dielectric constant, etc., are considered differently for each layer [Zhang A., Zhang L., Tang Z., IEEE Trans. Electron Devices61(3):755–761, 2014]. The high electron mobility electrons are now widely used in so many applications. The proposed work of AlInSb/InSb heterostructure implements the same process which will be a promise for future research works.

We have performed microwave absorption and near-field reflection experiments on a high mobility GaAs/AlGaAs heterostructure for the same conditions for which Microwave-Induced Resistance Oscillations (MIROs) are observed. It is shown that the electrodynamic aspect of the problem is important in these experiments. In the absorption experiments a broad CR line was observed due to a large reflection from the highly conductive electron gas. There were no additional features observed related to absorption at harmonics of the cyclotron resonance.

In near-field reflection experiments a very different oscillation pattern was revealed when compared to MIROs. The oscillation pattern observed in the reflection experiments is probably due to plasma effects occurring in a finite-size sample. The whole microscopic picture of MIROs is more complicated than simply a resonant absorption at harmonics of the cyclotron resonance. Nevertheless, the experimental observations are in good agreement with the model by Durst et al. involving the photo-assisted scattering in the presence of a crossed magnetic field and dc bias. The observed damping factor of MIROs may be attributed to a change in the electron mobility as a function of temperature.

MIROs may be considered as a light-induced drift effect, a broad class of phenomena associated with a light-induced asymmetry in the velocity distribution function.

The beating pattern of Shubnikov-de Haas oscillations is reproduced in crossed magnetic field configuration and in presence of zero-field valley splitting in Si-MOSFET 2D system. The features of Quantum Hall Effect in extremely dilute 2D system are discussed.