Real-space Transfer Research Articles

The two-dimensional metal photonic band gap structure consisting of a skew lattice in a nonorthogonal coordinate system

The metal photonic band gap structure has potentialities in the areas of high-energy accelerators,microwave vacuum electron devices, and terahertz radiation sources etc. The real space transfer matrix method in a nonorthogonal coordinate system is used to study the two-dimensional metal photonic band gap structure consisting of a skew lattice. A general formula for calculating the complete band gap structure is derived for transverse-electric and transverse magnetic modes, and the influences of the filling fraction, skew angle and cross section of the metal rod on band gap are analyzed. The results are in good agreement with results of other methods in some special cases.

Real-space electron transfer in III-nitride metal-oxide-semiconductor-heterojunction structures

The real-space transfer effect in a SiO2∕AlGaN∕GaN metal-oxide-semiconductor heterostructure (MOSH) from the two-dimensional (2D) electron gas at the heterointerface to the oxide-semiconductor interface has been demonstrated and explained. The effect occurs at high positive gate bias and manifests itself as an additional step in the capacitance-voltage (C‐V) characteristic. The real-space transfer effect limits the achievable maximum 2D electron gas density in the device channel. We show that in MOSH structures the maximum electron gas density exceeds up to two times that at the equilibrium (zero bias) condition. Correspondingly, a significant increase in the maximum channel current (up to two times compared to conventional Schottky-gate structures) can be achieved. The real-space charge transfer effect in MOSH structures also opens up a way to design novel devices such as variable capacitors, multistate switches, memory cells, etc.

Electronic transport in a Cantor stub waveguide network

We theoretically investigate the character of electronic eigenstates and transmission properties of a one-dimensional array of stubs with Cantor geometry. Within the framework of real space renormalization group (RSRG) and transfer matrix methods we analyze the resonant transmission and extended wave functions in a Cantor array of stubs, which lack translational order. Apart from resonant states with high transmittance we unravel a whole family of wave functions supported by such an array clamped between two-infinite ordered leads, which have an extended character in the RSRG scheme but, for such states the transmission coefficient across the lead-sample-lead structure decays following a power law as the system grows in size. This feature is explained from renormalization group ideas and may lead to the possibility of trapping of electronic, optical, or acoustic waves in such hierarchical geometries.

Demonstration of a GaN‐spacer high electron mobility transistor with low alloy scattering

AbstractA new kind of high electron mobility transistor (HEMT) based on a GaN‐spacer is reported. To confine the two‐dimensional electron gas in this AlGaN‐free HEMT, a GaN/ultrathin AlN/GaN pseudo‐hetero‐junction has been used. The effective conduction band discontinuity is given by the AlN polarization‐induced electric field. This new transistor shows a superior general performance than the standard HEMT due to a decrease in the alloy and real space transfer scattering, as well as lower ohmic contact resistance. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhanced peak-to-valley current ratio in InGaAs∕InAlAs trench-type quantum-wire negative differential resistance field-effect transistors

Trench-type narrow InGaAs quantum-wire field-effect transistors (QWR-FETs) have been fabricated on (311)A InP V-groove substrates by hydrogen-assisted molecular-beam epitaxy. Enhanced negative differential resistance (NDR) effects with a peak-to-valley ratio (PVR) as high as 13.3 have been observed at an onset voltage of 0.16V in the QWR-FETs at 24K. The PVR increased with reductions in the InGaAs epitaxial layer thickness, which caused an enhanced mobility difference between the QWR and side quantum wells (QWs). This forms a velocity modulation transistor based on the real-space transfer of electrons from the high mobility QWR to the low mobility side QWs. The NDR effects were observed up to 230K as the gate length was decreased to 50nm. A unique feature of the QWR-FET is that NDR effects are controllable with the gate bias in a three-terminal configuration.

Excited state properties of acceptor-substitute carotenoids: 2D and 3D real-space analysis

Excited state properties of three (neutral, weak and strong) acceptor-substitute carotenoids (β-carotene) are studied theoretically with the methods of two-dimensional (2D) and three-dimensional (3D) real-space analysis. The influences of different (neutral, weak and strong) function groups attached to the polyene chain to the excited state property of the carotenoids are discussed. By the 3D real-space analysis, the charge and energy transfer of the three acceptor-substitute carotenoids after vertical absorption are studied with the transition density and charge difference density. By the 2D real-space analysis, the electron–hole coherences and excitation delocalization of the three acceptor-substitute carotenoids are studied with the transition density matrix.

Hot-phonon-induced velocity saturation in GaN

In highly polar semiconductors with electron densities typically found in heterostructure field-effect transistors (HFETs), transport cannot be described without taking hot phonons into account. Here we describe a simple analytical model applied to the case of bulk GaN, taking the nonparabolicity of the conduction band into account, and show that the production of longitudinal optical (LO) phonons reduces the mobility and causes the drift velocity to saturate at a value around 107 cm/s, depending on the density of electrons. Transfer of electrons to higher valleys is expected to be delayed to much higher fields than commonly predicted. The effect of possible hot products of the LO decay is also considered. We relate the results for bulk material to the situation in HFETs, in which real-space transfer is inhibited, by considering the effect of spatial spreading of the channel electrons.

A closed-form expression to analyze electronic properties in delta-doped heterostructures

We develop a closed-form model to describe the electronic properties for delta modulation doped heterostructures, particularly the 2DEG sheet charge density and the electric field distribution in the direction of growth. The model includes the effects of real-space charge transfer and carrier degeneracy. The electron transfer and quasi-equilibrium condition in growth direction have been used in order to express the 2DEG sheet charge density that is only a function of material parameters and constants. An empirical constant, corresponding to quantized energy states, has been employed to further simplify this description and to arrive at a closed-form expression. Results from the analytical expressions are shown to agree well with numerical simulations based on a self-consistent solution of a modified Schrödinger and Poisson equations. In addition to their use in modeling of current conduction in modulation doped field effect transistors (MODFETs), we expect that these expressions would serve as versatile tools in the modeling of optical and spectral effects that occur in the AlGaAs/GaAs heterostructures.

Double-transconductance-plateau characteristics in InGaAs/GaAs real-space transfer high-electron-mobility transistor

Bias-tunable double-transconductance-plateau characteristics due to real-space transfer through the lowering of conduction band offset at high electric fields in a δ-doped InGaAs/GaAs high-electron-mobility transistor have been observed. Extrinsic, unpassivated peak performance values, with gate dimensions of 1×125 μm2, include the transconductance plateaus of 269 and 116 mS/mm at VDS=3 V for VGS=0 V and −1.5 V, respectively, maximum drain current density of 382 mA/mm at VDS=5.5 V, unity current gain cutoff frequency of 15.5 GHz, and maximum frequency of oscillation of 23 GHz. The present structure is promising for high-speed analog-to-digital converters or multiple-state quantizer applications.

Enhancement-mode In0.52Al0.48As/In0.6Ga0.4As tunneling real space transfer high electron mobility transistor

An enhancement-mode In0.52Al0.48As/In0.6Ga0.4As tunneling real-space transfer high electron mobility transistor (HEMT) was fabricated. The studied device operates as a conventional enhancement-mode HEMT under a low gate electrode bias. As the gate electrode bias increases, the electric field applied between the gate electrode and the InGaAs channel increases and the electrons in the channel become “hot.” If the transverse field is sufficiently large, then the electrons in the InGaAs channel layer may have enough energy to overcome the barrier and tunnel into the gate electrode, thus IDS decreases as IG increases. Therefore, pronounced N-shaped negative differential resistance phenomenon and negative transconductance are observed in the studied device.

Blue sky in SOI: new opportunities for quantum and hot-electron devices

The combination of silicon-on-insulator (SOI) substrates with ultrathin Si and insulator layers opens new opportunities for quantum effect and hot-electron devices. Unlike their III–V predecessors, these devices have the crucial advantage of potential integrability with dominant silicon technology. We discuss three examples of such SOI devices: a three-terminal real-space transfer transistor with a low-barrier dielectric; a vertical tunneling transistor with an ultrathin tunneling gate oxide; and a lateral interband tunneling transistor. None of these devices has progressed beyond a rudimentary proof-of-concept demonstration, but the strong nonlinearities of their characteristics deriving from quantum tunneling or hot-electron injection, as well as their unique scaling behavior, make them an interesting playground for innovative device research.

Electron Transport Properties in a GaAs/AlGaAs Quantum Wire Grown on V-Grooved GaAs Substrate by Metalorganic Vapor Phase Epitaxy

A series of nonlinear conductance phenomena is investigated in the GaAs V-grooved quantum wire (QWR) field effect transistors (FETs) prepared by the flow-rate modulated epitaxy (FME) technique. These are attributed to Coulomb blockade at a subthreshold gate bias, universal conductance steps near the threshold, real space transfer under a forward gate bias and a large source-drain bias condition. Weak carrier coupling between sidewall quantum wells and QWR is responsible for the small step height of the measured universal conductance (Gm≪G0=2e2/h). Shubnikov de Haas oscillation measurements revealed that sidewall quantum wells in the V-groove quantum wire act as additional current paths and are switched or mixed with QWR depending on the gate bias conditions and device geometry. The gate-bias-dependent current path switching is found to be responsible for the large current steps and negative differential resistance (NDR) in the drain current (IDS)-gate bias (VGS) and IDS-drain bias (VDS) relationships, respectively.

Bias-Tunable Multiple-Transconductance with Improved Transport Characteristics of δ-doped In0.28Ga0.72As/GaAs/In0.24Ga0.76As/GaAs High Electron Mobility Transistor Using a Graded Superlattice Spacer

A δ-doped In0.28Ga0.72As/GaAs/In0.24Ga0.76As/GaAs high electron mobility transistor (HEMT) with a thickness-graded superlattice spacer exhibiting bias-tunable multiple-transconductance performance has been successfully fabricated by molecular beam epitaxy (MBE). As compared to a conventional HEMT structure with a single undoped spacer layer, the present superlattice spacer, which accommodates the lattice-mismatch-induced strain, provides improvement of 31% and 43% on electron mobility and the two-dimensional electron gas (2DEG) concentration-mobility product, respectively. In addition, the device demonstrates bias-tunable multiple-transconductance plateaus due to modulation of the 2DEG concentration and mobility product by real-space transfer at high channel voltage. The distinguished device characteristics of the present structure are promising for a high-speed analog-to-digital converter (ADC) or multiple-state quantizer applications.

Physical mechanisms on the abnormal gate-leakage currents in pseudomorphic high electron mobility transistors

In this brief, we report a new observation on the abnormal gate-leakage current and associated physical mechanisms in packaged gallium arsenide (GaAs)-based n-channel pseudomorphic high electron mobility transistors (PHEMTs) with a gate length L=0.2 /spl mu/m. Abnormal positive and negative humps in the gate current (I/sub G/), as a function of the gate voltage, have been investigated at room temperature. Qualitative models for the positive and negative humps in the experimental I/sub G/ are proposed, combining physical mechanisms of thermionic emission, impact ionization, real-space-transfer (RST), and resonant tunneling through the alignment of quantized states in the InGaAs channel and the /spl delta/-doped AlGaAs donor layer. An experimental result is also provided for the anomalous I/sub G/ under large-drain bias and forward-gate bias.

Voltage-Controlled Small Signal Analysis of Real Space Transfer Transistor

The research of determining the small signal equivalent circuit of the real space transfer (RST) transistor is investigated in this work. We propose a voltage-controlled mode model, called parameter extraction, to describe the performance of RST transistors at high frequency range. Besides, we also employ the value-determined model to simulate the microwave characteristics of RST and the theoretical and experimental results are compared. The influence of variables of RF performance is analyzed and theoretical results show that the cut-off frequency(fT)is mainly affected by the leakage resistances and the dimension of metal contact, but the maximum available gain frequencyfmax, is dependent on contact resistances.

Negative Effective Mass Terahertz Range Oscillators on a Cleaved Edge of a Superlattice

We consider electron dispersion relations in a quantum well overgrown on a cleaved edge of a superlattice and covered by a top barrier containing a donor modulation-doping sheet. Parameters of this structure are selected so that the electron wave function, which is localized inside of the quantum well at kZ = 0 (kZ is a wave vector component along the superlattice vector), enters the superlattice deeper and deeper with increasing kZ. As a result of this penetration, electrons become heavier and heavier. Such a dynamic (quantum) real-space transfer (without any scattering) leads to the appearance of a negative effective mass section in the lowest quantization subband. The position of this section as well as the width of the subband itself and its distance from the next subbands are regulated by both the quantum well parameters and parameters of the superlattice. The latter has to be with maximally high barriers and a small spatial period. The limitation of transverse motion of electrons in the superlattice layers is accomplished by an electric field of a pn-heterojunction formed as a result of an acceptor doping of the superlattice barriers and the above-mentioned donor modulation doping of the top barrier. This field also effectively controls the dispersion relations. Diodes with negative effective mass sections in the dispersion relation promise to be efficient terahertz range oscillators.

Quantum real-space transfer in a heterostructure overgrown on the cleaved edge of a superlattice

A dispersion relation for an electron in a two-layer (and also multilayer) quantum well (QW) is formed as a result of a certain combination of initial dispersion relations for each of the forming layers. Such a combination can be used to engineer new dispersion relations with desirable properties. The same relates to a two-dimensional electron gas (2DEG) induced in a multilayer medium. In this study, we consider first such a 2DEG in a specific two-layer structure where a superlattice (SL) plays the role of the second half-infinite layer, and electrons with large wave numbers along the SL vector spread from the first ordinary QW layer to this SL. As a result of such a quantum (dynamic) real-space transfer, electrons become heavier, and the dispersion relation achieves an additional negative effective mass (NEM) section. Such NEM dispersion relations were studied for several different material systems, including the two most interesting three-material systems: (1) an isomorphic Al0.15Ga0.85As//GaAs/Al0.5Ga0.5As structure and (2) a strained In0.53Ga0.47As//InxGa1−xAs/InyAl1−y As structure (x>0.53, y<0.52) with a strain-balanced InxGa1−xAs/InyAl1−yAs SL. Most of the results were verified using a simplified 1D model, but some of them were verified by more complicated 2D-model calculations.

Enhancement of hole injection for nitride-based light-emitting devices

A novel device design is proposed for a strong enhancement of hole injection current in nitride-based light-emitting heterostructures. Preliminary calculations show orders of magnitude increase in injected hole current when using the proposed superlattice hole injector device based on the real-space transfer concept.

Requirements for low intermodulation distortion in GaN-Al/sub x/Ga/sub 1-x/N high electron mobility transistors: a model assessment

A model analysis of the large-signal characteristics of GaN-Al/sub x/Ga/sub 1-x/N high-electron mobility transistors (HEMTs) with particular emphasis on intermodulation distortion (IMD) and the third-order intercept point. Since the nonlinearity depends critically on the carrier transport behavior, a Monte Carlo (MC) based numerical simulation scheme has been employed. The focus is to identify parameters and their interdependencies with a view of setting optimal limits for enhanced microwave performance. A case is made for increased mole fraction for the barrier layer, reducing the transit length, and introducing a thin AlN interfacial layer for suppressing real space transfer for enhancing the device performance. Finally, high-temperature predictions of the nonlinear behavior and IMD have been made, by carrying out the MC simulations at 600 K. In a process a favorable case is made for the GaN system as a potential candidate for microwave and RF applications at elevated temperatures.

High-Frequency Noise Sources in Quantum Wells

Hot-electron noise is investigated for InGaAs and InAs quantum wells containing a two-dimensional electron gas channel in a pulsed electric field applied parallel to the interfaces. Noise sources resulting from hot-electron thermal motion, electron temperature fluctuations, and real-space transfer are observed. The experimental results on hot-electron thermal noise are used to estimate energy relaxation time in the field range where other sources do not play any important role. Measurements of noise anisotropy in the plane of electron confinement are used to discuss real-space-transfer noise. High-frequency noise technique is used to study hot-electron trapping, and trap location in InAlAs/InGaAs/InAlAs heterostructure channels is determined.