Real-space Transfer Research Articles

Quantum Channel Extreme Bandgap AlGaN HEMT.

An extreme bandgap Al0.64Ga0.36N quantum channel HEMT with Al0.87Ga0.13N top and back barriers, grown by MOCVD on a bulk AlN substrate, demonstrated a critical breakdown field of 11.37 MV/cm-higher than the 9.8 MV/cm expected for the channel's Al0.64Ga0.36N material. We show that the fraction of this increase is due to the quantization of the 2D electron gas. The polarization field maintains electron quantization in the quantum channel even at low sheet densities, in contrast to conventional HEMT designs. An additional increase in the breakdown field is due to quantum-enabled real space transfer of energetic electrons into high-Al barrier layers in high electric fields. These results show the advantages of the quantum channel design for achieving record-high breakdown voltages and allowing for superior power HEMT devices.

Comprehensively analysis of hot electron transport in as-grown and thermally annealed n-type modulation-doped Al0.15Ga0.85As/GaAs0.96Bi0.4 quantum well structure

We report experimental and analytical results on hot electron transport in as-grown and thermally annealed n-type modulation-doped Al0.15Ga0.85As/GaAs0.96Bi0.4 quantum well (QW) structures at room temperature. The drift mobility of as-grown and thermally annealed samples is 2265 and 1414 cm2/Vs at a low electric field region, respectively. The lower electric field electron mobility for the thermally annealed sample is due to the increased 2D electron density following the annealing process. The drift velocity (vdrift) of the as-grown sample tends to saturate at 6.1 × 106 cm/s, contrary to the thermally annealed sample, for which no saturation is observed. The vdrift of the annealed sample is also lower due to the higher 2D electron density caused by annealing. Analyzing the experimental results concerning the theoretical model reveals that i) hot electron transport occurs in parallel mode due to the presence of electrons in both Al0.15Ga0.85As (barrier) and QW layers, ii) hot electrons transferred from QW to the barrier layer via so-called real space transfer (RST) mechanism is suppressed at higher electric fields even though the lower energy barrier between the barrier and QW layers, and iii) transfer of electrons from QW (Γ-valley) to the nearest L-valley, inter-valley transfer (IVT), dominates on the hot electron transfer at higher electric fields even with a higher energy barrier.

Toward Hot Carrier Extraction in Intervalley Photovoltaic Devices

InGaAs heterostructures have demonstrated the means to maintain high energy ‘“hot” carrier behavior under practical operating conditions via transfer to upper valleys of the band structure, potentially decreasing thermalization losses in solar cells if a suitable extraction mechanism is developed. A set of four InGaAs heterostructures with altered n+ top layers enable comparative analysis of the current density–voltage characterization and the relationship to the band and valley alignments. This reveals that the barriers to extraction are introduced by the challenges of real space transfer between upper valleys of different materials rather than an energy level mismatch at the top heterojunction.

Influence of Spacer Thickness on the Noise Performance in InP HEMTs for Cryogenic LNAs

InP high electron mobility transistors (InP HEMTs) with different spacer thickness 1 to 7 nm in the InAlAs-InGaAs heterostructure have been fabricated and characterized at 5 K with respect to electrical dc and rf properties. The InP HEMT noise performance was extracted from gain and noise measurements of a hybrid low-noise amplifier (LNA) at 5 K equipped with discrete transistors. When biased for optimal noise operation, the LNA using 5 nm spacer thickness InP HEMTs achieved the lowest average noise temperature of 1.4 K at 4–8 GHz. The InP HEMT channel noise was estimated from the drain noise temperature which confirmed the minimum in noise temperature for the 5 nm spacer thickness InP HEMT. It is suggested that the spacer thickness acts to control the degree of real-space transfer of electrons from the channel to the barrier responsible for the observed noise variation in the cryogenic InP HEMTs.

Theory of drain noise in high electron mobility transistors based on real-space transfer

High electron mobility transistors are widely used as microwave amplifiers owing to their low microwave noise figure. Electronic noise in these devices is typically modeled by noise sources at the gate and drain. While consensus exists regarding the origin of the gate noise, that of drain noise is a topic of debate. Here, we report a theory of drain noise as a type of partition noise arising from real-space transfer of hot electrons from the channel to the barrier. The theory accounts for the magnitude and dependencies of the drain temperature and suggests strategies to realize devices with lower noise figure.

Nonlocal electron dynamics in GaN / AlGaN transistor heterostructures

The processes of nonlocal electron heating in transistor heterostructures based on gallium nitride and gallium arsenide were compared. It has been shown that, in the case of GaAs double doped pseudomorphic heterostructures, the real space transfer of electrons significantly reduces their drift velocity overshot in the region of a strong field as compared with pure bulk GaAs, while in GaN-based heterostructures the decrease in the drift velocity overshot does not exceed 30% in the studied cases. Keywords: Real space transfer, field-effect transistor, gain factor

Нелокальная динамика электронов в AlGaN/GaN-транзисторных гетероструктурах

The nonlocal electrons heating in transistor heterostructures based on gallium nitride and arsenide is compared. It is shown that if, in comparison with a pure bulk material, in the case of GaAs double doped pseudomorphic heterostructures, the real space transfer of electrons significantly reduces their drift velocity overshot in the region of a strong field, then for GaN-based heterostructures, the decrease of the drift velocity overshot in the studied cases does not exceed 30%.

Coherent structures associated with interscale energy transfer in turbulent channel flows

The flow fields of direct numerical simulation of turbulent channel flow are decomposed into large scales and small scales based on the interscale energy transfer spectra. The former is characterized by streaks and quasi-streamwise vortices even in the outer layer, and the latter is characterized by hairpin-like vortical structures that are similar to the original flow fields. We further investigated the coherent structures associated with the real-space energy transfer. The formation of small-scale hairpin-like vortices is related to the large-scale shear layer.

Hierarchy of coherent structures and real-space energy transfer in turbulent channel flow

Abstract

Real-space-transfer mechanism of negative differential conductivity in gated graphene-phosphorene hybrid structures: Phenomenological heating model

We analyze the nonlinear carrier transport in the gated graphene-phosphorene (G-P) hybrid structures—the G-P field-effect transistors using a phenomenological model. This model assumes that due to high carrier densities in the G-P-channel, the carrier system, including the electrons and holes in both the G- and P-layers, is characterized by a single effective temperature. We demonstrate that a strong electric-field dependence of the G-P-channel conductivity and substantially non-linear current-voltage characteristics, exhibiting a negative differential conductivity, are associated with the carrier heating and the real-space carrier transfer between the G- and P-layers. The predicted features of the G-P-systems can be used in the detectors and sources of electromagnetic radiation and in the logical circuits.

Hot LO-phonon limited electron transport in ZnO/MgZnO channels

High-field electron transport in two-dimensional channels at ZnO/MgZnO heterointerfaces has been investigated experimentally. Pulsed current–voltage (I–V) and microwave noise measurements used voltage pulse widths down to 30 ns and electric fields up to 100 kV/cm. The samples investigated featured electron densities in the range of 4.2–6.5 × 1012 cm−2, and room temperature mobilities of 142–185 cm2/V s. The pulsed nature of the applied field ensured negligible, if any, change in the electron density, thereby allowing velocity extraction from current with confidence. The highest extracted electron drift velocity of ∼0.5 × 107 cm/s is somewhat smaller than that estimated for bulk ZnO; this difference is explained in the framework of longitudinal optical phonon accumulation (hot-phonon effect). The microwave noise data allowed us to rule out the effect of excess acoustic phonon temperature caused by Joule heating. Real-space transfer of hot electrons into the wider bandgap MgZnO layer was observed to be a limiting factor in samples with a high Mg content (48%), due to phase segregation and the associated local lowering of the potential barrier.

Increasing Saturated Electron-Drift Velocity in Donor–Acceptor Doped pHEMT Heterostructures

Field dependences of the electron-drift velocity in typical pseudomorphic high-electron-mobility transistor (pHEMT) heteroepitaxial structures (HESs) and in those with donor–acceptor doped (DApHEMT) heterostructures with quantum-well (QW) depth increased by 0.8–0.9 eV with the aid of acceptor layers have been studied by a pulsed technique. It is established that the saturated electron-drift velocity in DA-pHEMT-HESs is 1.2–1.3 times greater than that in the usual pHEMT-HESs. The electroluminescence (EL) spectra of DA-pHEMT-HESs do not contain emission bands related to the recombination in widebandgap layers (QW barriers). The EL intensity in these HESs is not saturated with increasing electric field. This is indicative of a suppressed real-space transfer of hot electrons from QW to barrier layers, which accounts for the observed increase in the saturated electron-drift velocity.

Увеличение насыщенной скорости дрейфа электронов в pHEMT-гетероструктурах с донорно-акцепторным легированием

AbstractField dependences of the electron-drift velocity in typical pseudomorphic high-electron-mobility transistor (pHEMT) heteroepitaxial structures (HESs) and in those with donor–acceptor doped (DApHEMT) heterostructures with quantum-well (QW) depth increased by 0.8–0.9 eV with the aid of acceptor layers have been studied by a pulsed technique. It is established that the saturated electron-drift velocity in DA-pHEMT-HESs is 1.2–1.3 times greater than that in the usual pHEMT-HESs. The electroluminescence (EL) spectra of DA-pHEMT-HESs do not contain emission bands related to the recombination in widebandgap layers (QW barriers). The EL intensity in these HESs is not saturated with increasing electric field. This is indicative of a suppressed real-space transfer of hot electrons from QW to barrier layers, which accounts for the observed increase in the saturated electron-drift velocity.

A peculiarity of quantum hot-electron real space transfer in dual-channel GaAs-based heterostructures

The model has been proposed for calculation of the electron transport via the double conducting channels formed in heterostructures with the tunnel-coupled quantum wells with the essentially different mobility of electrons. Experiments to test the dependences of the transport characteristics on the temperature and electric field were carried out on the heterostructures based on GaAs. The well with a high electron mobility was formed as an AlGaAs–GaAs–AlGaAs undoped quantum well. The tunnel-coupled well was formed either by a delta layer of impurity or a short-period superlattice (SPSL) fragment. Based on comparison of the experimental and calculated data, it is established that the electron temperatures in such coupled channels in the course of heating them by the electric field strongly differ from each other; their dependences in both wells on the electric field are found. This difference leads to a considerable redistribution of electrons between wells. For the structure with the SPSL fragment at fields more than 450 V cm−1 an inversion of the electron population between the two lower size-quantization energy subbands takes place.

The 2DEG mobility enhancement for low- and high-electric fields in a new type of AlGaAs/InGaAs heterostructures with donor-acceptor doping

The splitting of the delta-layers in the DA-pHEMT heterostructures has resulted in the increase of the spacer’s effective thickness and growth of the low-field 2DEG mobility from 4000÷5000 cm2 V-1 s-1 up to 6500 cm2 V-1 s-1 at the temperature of 300 K and 2DEG density of 4.0×1012 cm-2. The 2DEG mobility in the δ-splitted DA-pHEMT heterostructures almost coincides with the mobility in standard pHEMT heterostructures, but the 2DEG density in the DA-pHEMT heterostructures is approximately twice higher. The additional potential barrier in the DA-pHEMT heterostructures formed by the acceptors causes the reduction of the real-space transfer effect. Therefore, the drift saturation velocity in these heterostructures is higher than the drift saturation velocity in standard pHEMT heterostructures.

Electroluminescence from GaAs/AlGaAs Heterostructures in Strong in-Plane Electric Fields: Evidence for k- and Real-Space Charge Transfer

In the Gunn effect, which occurs in certain semiconductors in strong electric fields, electrons are driven out of a low-mass central valley into a heavy-mass side valley in k (momentum) space, ultimately resulting in negative differential resistance (NDR). Recently, there has been interest in the possibility of exploiting this phenomenon in heterostructured semiconductors, as a means to realize novel terahertz sources. Here, we demonstrate that a GaAs/AlGaAs heterostructure planar Gunn diode exhibits both NDR and electroluminescence (EL), the characteristics of which are consistent with the combined action of simultaneous k-space and real-space electron transfer. By making simultaneous electrical-transport and EL measurements, we reveal the following scenario as a function of the applied voltage. At sufficiently large bias, the onset of k-space transfer gives rise to NDR and results in the formation of traveling high-field domains. As each domain travels toward the anode, impact ionization taking place wi...

Interband and intraband radiation from the n-InGaAs/GaAs heterostructures with quantum wells under the conditions of injection in high lateral electric fields

The interband and intraband radiation from the n-InGaAs/GaAs heterostructures with the double and triple tunnel coupled and selectively doped quantum wells (QWs), which is appeared under the lateral electric field and in the presence of hole injection from the anode contact, has been investigated. A steep increase of the interband radiation intensity was found at the fields of E≥1.7kV/cm. This effect should be related to the big lifetime of the injected charge carriers (~10−6s) which exceeds by three orders of magnitude the lifetime in the similar bulk direct-band semiconductor. Its reason lies in spatial separation of the injected holes and electrons between coupled wells, firstly, by the built-in transverse electric field between wells and, secondly, due to the real-space transfer of carriers heated by the lateral electric field from the wide well to the narrow δ-doped one. Furthermore, an increase of the carrier concentration due to injection leads to an increase of that transition intensity and, consequently, to an intensity increase of the radiative intersubband transitions of carriers in QWs which results in a steep intensity increase of the far (50–120µm) infrared radiation.

Theory of high-field electron transport in the heterostructures AlxGa1-xAs/GaAs/AlxGa1-x with delta-doped barriers. Effect of real-space transfer

Steady-state electric characteristics of quantum heterostructures AlxGa1–xAs/GaAs/AlxGa1–xAs with  -doped barriers have been analyzed in this work. It has been shown that at high doping the additional low-conductive channels are formed in the barrier layers. Current-voltage characteristics of the structure were obtained in the wide interval of applied electric fields up to several kV/cm being based on the solution of Boltzmann transport equation. It has been found that in the electric fields higher than 1 kV/cm the effect of exchange of the carriers between the high-conductive channel of the GaAs quantum well and the channels in the AlGaAs barriers becomes essential. This effect gives rise to the appearance of the strongly nonlinear current-voltage characteristics with a portion of negative differential conductivity. The developed model of heterostructure is adequate to those recently fabricated and studied by Prof. Sarbey’s group. The obtained results explain some observation of this paper. It has been found that the effect of electron real-space transfer takes place at both low temperatures and room temperatures, which opens perspectives to design novel type nanostructured current controlled devices.

Hot-electron real-space transfer and longitudinal transport in dual AlGaN/AlN/{AlGaN/GaN} channels

Real-space transfer of hot electrons is studied in dual-channel GaN-based heterostructure operated at or near plasmon–optical phonon resonance in order to attain a high electron drift velocity at high current densities. For this study, pulsed electric field is applied in the channel plane of a nominally undoped Al0.3Ga0.7N/AlN/{Al0.15Ga0.85N/GaN} structure with a composite channel of Al0.15Ga0.85N/GaN, where the electrons with a sheet density of 1.4 × 1013 cm−2, estimated from the Hall effect measurements, are confined. The equilibrium electrons are situated predominantly in the Al0.15Ga0.85N layer as confirmed by capacitance–voltage experiment and Schrödinger–Poisson modelling. The main peak of the electron density per unit volume decreases as more electrons occupy the GaN layer at high electric fields. The associated decrease in the plasma frequency induces the plasmon-assisted decay of non-equilibrium optical phonons (hot phonons) confirmed by the decrease in the measured hot-phonon lifetime from 0.95 ps at low electric fields down below 200 fs at fields of 4 kV cm−1 as the plasmon–optical phonon resonance is approached. The onset of real-space transfer is resolved from microwave noise measurements: this source of noise dominates for 8 kV cm−1. In this range of fields, the longitudinal current exceeds the values measured for a mono channel reference Al0.3Ga0.7N/AlN/GaN structure. The results are explained in terms of the ultrafast decay of hot phonons and reduced alloy scattering caused by the real-space transfer in the composite channel.

Effects of the Real-Space Transfer of Charge Carriers in the n-AlGaAs/GaAs Heterostructures with the Delta-Layers of Impurity in the Barriers

Effects of the Real-Space Transfer of Charge Carriers in the n-AlGaAs/GaAs Heterostructures with the Delta-Layers of Impurity in the Barriers