Transport In Two-dimensional Electron Gas Research Articles

Study of electrical transport properties of GaN-based side-gate heterostructure transistors

In this study, GaN-based side-gate heterostructure transistors (SGHTs) with two electrical operating modes were fabricated. In the first operating mode, the SGHT was utilized as a common-source voltage amplifier with low power consumption and a broad input signal range. Analysis of the main scattering mechanisms affecting the electrical transport of two-dimensional electron gas (2DEG) in the channel revealed that polar optical phonon scattering and polarization Coulomb field (PCF) scattering play dominant roles under different side-gate voltages. In addition, channel current modulation of 2DEG electron mobility is primarily attributed to PCF scattering. Due to PCF scattering, the channel width also modulates the threshold voltage in this mode of operation. Moreover, in the second operating mode, the SGHT functioned as a traditional GaN high electron mobility transistor, allowing for electrically modulated threshold voltage and transconductance.

A Poisson–Schrodinger and cellular automaton coupled approach for two-dimensional electron gas transport modeling of GaN-based high mobility electron transistors

We propose the modeling of electron transport in GaN-based high electron mobility transistors (HEMTs) by combining the Poisson–Schrodinger method and the cellular automaton method. In HEMT, it is necessary to consider the influence of two-dimensional electron gas, band nonparabolicity, and upper valley on drift velocity. A wide range of electron transport modeling environments from low to high electric fields is required. By using the cellular automaton method instead of the traditional Monte Carlo carrier transport modeling method, noise-free drift velocity analysis is realized even at low electric fields. This makes it possible to stably investigate the effects of the HEMT structure and the influence of various scattering mechanisms on the drift velocity.

Impact of the hot phonon effect on electronic transport in monolayer silicene

The hot phonon effect is a non-equilibrium phenomenon that has an important role in high-field electronic transport in two-dimensional electron gases and in 2D materials like graphene. In this work, the influence of this effect on the electronic transport properties of silicene is investigated by means on an ensemble Monte Carlo simulator that accounts self-consistently for the out-of-equilibrium phonon population and the scattering events under high electric field conditions. In this way, the observed reduction in the carrier drift velocity is analyzed in terms of the microscopic scattering events, wavevector distributions, net phonon emission and phonon temperatures. A comparison with the case of graphene is also carried out, showing that despite the similarities concerning the linear bandstructure near the Dirac points, the hot phonon effect is less dominant in silicene due to the differences in the phonon branches and electron–phonon couplings. Even so, neglecting the role of an out-of-equilibrium phonon population may overestimate the high-field drift velocity in silicene up to 25% at moderate carrier concentrations.

(Invited) What's Next after GaN? How about More GaN!

Over the course of the past few years, transistors based on gallium nitride (GaN) and related III-N materials have proven to be clear contenders for solid-state power amplifier applications in the commercial and military market spaces. In microwave technology nodes of lateral GaN high-electron-mobility transistor (HEMT) technology (with gate lengths greater than 0.25 microns), the material-driven electrical performance of GaN has been demonstrated with exceptional reliability in production-released processes. While a power density of several W/mm can be easily achieved at S- and X-band with GaN HEMTs based on an AlGaN/GaN heterojunction, it generally becomes more difficult to obtain the same output power level at higher frequency nodes. This is primarily due to limitations on the operating voltage that can be used; as device dimensions scale shorter, internal electric fields increase and field management techniques, such as source-connected field plates, generally are not used because they introduce undesirable parasitic capacitance. To address these shortcomings, significant technical research and development has been carried out over the past few years focusing on extending GaN HEMT frequency performance into the millimeter wavelength (MMW) range of 30 – 300 GHz. To prevent the onset of short-channel effects in short gate length devices, vertically-scaled barrier heterostructures have been investigated. In a number of examples, materials growth development of novel III-N materials has enabled heterojunctions with desirable two-dimensional electron gas transport properties. In this talk, we will review a number of recent developments in the community related to MMW GaN HEMT technology. In addition to reported literature, we will discuss NRL’s internal materials and MMW device development using molecular beam epitaxy. Novel HEMT heterostructures based on AlN, ScxAl1-xN, and InxAl1-xN (metal- and N-polar) will be discussed.

Nonlinear transport in two-dimensional electron gas exhibiting colossal negative magnetoresistance

We report on nonlinear transport measurements in a GaAs/AlGaAs quantum well exhibiting a colossal negative magnetoresistance effect. Under applied dc bias, the magnetoresistance becomes nonmonotonic, exhibiting distinct extrema that move to higher magnetic fields with increasing current. In the range of magnetic fields corresponding to the resistivity minimum at zero bias, the resistivity increases linearly with current and the rate of this increase scales with the inverse magnetic field. The latter observation is consistent with the theory, proposed more than 35 years ago, considering classical memory effects in the presence of strong, dilute scatterers.

Anisotropic elastic scattering of stripe/line-shaped scatters to two-dimensional electron gas: Model and illustrations in a nonpolar AlGaN/GaN hetero-junction

In a semiconductor hetero-junction, the stripe/line-shaped scatters located at the hetero-interface lead to the anisotropic transport of two-dimensional electron gas (2DEG). The elastic scattering of infinitely long and uniform stripe/line-shaped scatters to 2DEG is theoretically investigated based on a general theory of anisotropic 2DEG transport [J. Schliemann and D. Loss, Phys. Rev. B 68(16), 165311 (2003)], and the resulting 2DEG mobility along the applied electrical field is modeled to be a function of the angle between the field and the scatters. The anisotropy of the scattering and the mobility originate in essence from that the stripe/line-shaped scatters act upon the injecting two-dimensional wave vector by changing only its component perpendicular to the scatters. Three related scattering mechanisms in a nonpolar AlGaN/GaN hetero-junction are discussed as illustrations, including the striated morphology caused interface roughness scattering, and the polarization induced line charge dipole scattering and the misfit dislocation scattering at the AlGaN/GaN interface. Different anisotropic behaviors of the mobility limited by these scattering mechanisms are demonstrated, but analysis shows that all of them are determined by the combined effects of the anisotropic bare scattering potential and the anisotropic dielectric response of the 2DEG.

Electron scattering in GaAs/InGaAs quantum wells subjected to an in-plane magnetic field

We study theoretically the effect of the in-plane magnetic field on two-dimensional electron gas transport in GaAs/InGaAs single quantum well structure. Our results show that, due to the scatterers (GaSb quantum dots) are one-side distributed, the in-plane magnetic field leads to an anisotropic scattering probability, which results in a higher mobility along the direction perpendicular to the magnetic field. Besides, compared with the no magnetic field case, the mobility shows a parabolic increasing trend as the in-plane magnetic field strength increases.

Anisotropic transport of two-dimensional electron gas modulated by embedded elongated GaSb/GaAs quantum dots

Geometrically anisotropic GaSb/GaAs quantum dots (QDs) that are elongated along the [110] direction are embedded in the vicinity of a modulation-doped AlGaAs/GaAs heterointerface. At 4.2 K, the electron mobilities μ∥ and μ⊥, which are parallel and perpendicular to the QD elongation axis, respectively, are systematically investigated as a function of the electron concentration NS by both experimental measurements and theoretical simulations. In the experiments, the mobility ratio μ∥/μ⊥ increased with increasing NS. The mobility anisotropy, attributed to the anisotropic scattering potential of the elongated GaSb QDs, is calculated under the theoretical model of a rectangular scattering potential of GaSb QDs.

Improved two-dimensional electron gas transport characteristics in AlGaN/GaN metal-insulator-semiconductor high electron mobility transistor with atomic layer-deposited Al2O3 as gate insulator

The effects of Al2O3 gate insulator grown by atomic layer deposition (ALD) system on the two-dimensional electron gas (2DEG) transport characteristics in AlGaN/GaN metal-insulator-semiconductor high electron mobility transistor (MISHEMT) were investigated. The shape of the electron lateral distribution in the quantum well at AlGaN/GaN interface was found to be slightly influenced by the Al2O3 thin layer. The drift mobility (μd) of the electrons in ALD-Al2O3/AlGaN/GaN MISHEMT is increased due to the surface passivation effects of the included dielectric layer. The higher dynamic channel current of the MISHEMT indicates that the electron saturation velocity (vsat) is also increased. These results show the improvement of the transport characteristics of 2DEG in Al2O3/AlGaN/GaN MISHEMT by the excellent properties of the Al2O3 grown by ALD.

Optimal block-tridiagonalization of matrices for coherent charge transport

Numerical quantum transport calculations are commonly based on a tight-binding formulation. A wide class of quantum transport algorithms require the tight-binding Hamiltonian to be in the form of a block-tridiagonal matrix. Here, we develop a matrix reordering algorithm based on graph partitioning techniques that yields the optimal block-tridiagonal form for quantum transport. The reordered Hamiltonian can lead to significant performance gains in transport calculations, and allows to apply conventional two-terminal algorithms to arbitrarily complex geometries, including multi-terminal structures. The block-tridiagonalization algorithm can thus be the foundation for a generic quantum transport code, applicable to arbitrary tight-binding systems. We demonstrate the power of this approach by applying the block-tridiagonalization algorithm together with the recursive Green’s function algorithm to various examples of mesoscopic transport in two-dimensional electron gases in semiconductors and graphene.

InAlN-barrier HFETs with GaN and InGaN channels

Advantages and drawbacks of InN-containing GaN-based heterostructure field-effect transistors (HFETs) are reviewed. The main attention is paid to fundamental processes and experimental observations associated with hot-electron transport in two-dimensional electron gas (2DEG) channels subjected to high electric fields. Complementary information is obtained from fluctuations. When interface roughness and alloy scattering are minimized, an important transport limitation originates from additional scattering of hot electrons by non-equilibrium longitudinal optical (LO) phonons (hot phonons) governed by the hot-phonon temperature and the hot-phonon decay into acoustic phonons. The ultrafast decay is observed for high supplied power where the hot-phonon lifetime of ∼60 fs is found. The optimal 2DEG density for the fastest decay of hot phonons is estimated and associated with LO-phonon–plasmon resonance. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The critical role of growth temperature on the structural and electrical properties of AlGaN/GaN high electron mobility transistor heterostructures grown on Si(111)

This work is dedicated to the study of the growth by ammonia source molecular beam epitaxy of AlxGa1−xN/GaN high electron mobility transistors on (111) oriented silicon substrates. The effect of growth conditions on the structural and electrical properties of the heterostructures was investigated. It is shown that even a slight variation in the growth temperature of the thick GaN buffer on AlN/GaN stress mitigating layers has a drastic influence on these properties via a counterintuitive effect on the dislocation density. Both in situ curvature measurements and ex situ transmission electron microscopy and x-ray diffraction experiments indicate that the relaxation rate of the lattice mismatch stress increases with the growth temperature but finally results in a higher dislocations density. Furthermore, a general trend appears between the final wafer curvature at room temperature and the threading dislocation density. Finally, the influence of the dislocation density on the GaN buffer insulating properties and the two-dimensional electron gas transport properties at the AlxGa1−xN/GaN interface is discussed.

Spin polarized transport of two-dimensional electron gas through step-magnetic barrier structure

By using the scattering matrix method, the properties of two-dimensional electron gas spin polarized transport through step-magnetic barrier structure at different bias voltage are investigated. The results show that (i) the degree of spin polarization is cyclically reducing if the incidence energy increases at zero bias voltage, and (ii) it is reduced slower and the maximum of spin polarization decreases when the number of steps increase in this structure, and (iii) it is enhanced significantly in a wider energy range under applied bias voltage, and more distinct spin-filtering properties are observed in the step-magnetic barriers.

Influence of Intersubband Scattering on the Magnetic Field Dependence of the Conductivity Tensor

Transport properties of semiconductor structures with several subbands occupied have been attracting scientific interest for a long time because of, among other things, the appearance of intersubband scattering. It is known that this extra scattering mechanism affects both classical as well as quantum transport. In this paper we show that intersubband scattering can lead to apparent inconsistency of the experimental results obtained by means of classical and quantum transport measurements and this discrepancy is entirely connected with the usage of the classical fomulae to describe the magnetic field dependence of the conductivity tensor. We have studied electron transport of two-dimensional electron gas (2DEG) in the modulation Si δ-doped, MOVPE grown pseudomorphic GaAs/InGaAs/AGaAs quantum well (QW) at temperature T = 4.2 K and in magnetic field up to 7 T. The persistent photoconductivity effect was used to control the free electron concentration. Shubnikov de Haas (SdH) oscillations seen in ρxx indicated the existence of 2DEG. When electron sheet density exceeded the value of 1.3·10 12 cm -2 , the so-called magneto-intersubband (MIS) oscillations appeared. These oscillations are related to scattering between Landau levels of two occupied subbands. This means that quantum lifetimes of both first and second subband should be sufficiently high. However, at the same time classical analysis of the conductivity tensor revealed very low mobility of electrons on the second subband which seemed to rule out the possibility of the appearance of MIS oscillations. We prove that there is no contradiction in our observations. First we show that even if intersubband scattering is present in the sample with multi-subband occupation, one still can use the classical multilayer approach to analyse the non-oscillatory part of the conductivity tensor. However, one has to be very cautious in using this approach, since all the parameters describing the carriers of the contributing conduction channels (subbands) must be redefined in that case. This redefinition applies both to density and mobility of carriers. That is why a difference between electron density values obtained from quantum oscillatory effects and classical analysis of magnetic field dependence is usually observed in such cases. The same applies to the values of mobilities extracted from the classical analysis of magnetic field dependences of conductivity tensor – these values may substantially differ from the real ones determined by intraand intersuband scattering processes. Hence we show that there is really no contradiction in our observations. Low electron mobility obtained in classical analysis for the second subband in our sample does not imply low electron quantum lifetime for this subband. Thus the observation of MIS oscillations is absolutely possible.

III-nitride transistors with capacitively coupled contacts

Al Ga N ∕ Ga N heterostructure field-effect transistor design using capacitively coupled contacts (C3HFET) is presented. Insulated-gate [C3 metal-oxide-semiconductor HFET (C3MOSHFET)] has also been realized. The capacitively coupled source, gate, and drain of C3 device do not require annealed Ohmic contacts and can be fabricated using gate alignment-free technology. For typical AlGaN∕GaN heterostructures, the equivalent contact resistance of C3 transistors is below 0.6Ωmm. In rf-control applications, the C3HFET and especially the C3MOSHFET have much higher operating rf powers as compared to HFETs. C3 design is instrumental for studying the two-dimensional electron gas transport in other wide band gap heterostructures such as AlN∕GaN, diamond, etc., where Ohmic contact fabrication is difficult.

Temperature effects on microwave-induced resistivity oscillations and zero-resistance states in two-dimensional electron systems

In this work we address theoretically a key issue concerning microwave-induced longitudinal resistivity oscillations and zero resistance states, namely, temperature. In order to explain the strong temperature dependence of the longitudinal resistivity and the thermally activated transport in two-dimensional electron gas, we have developed a microscopic model based on the damping suffered by the microwave-driven electronic orbit dynamics by interactions with the lattice ions yielding acoustic phonons. Recent experimental results show a reduction in the amplitude of the longitudinal resistivity oscillations and a breakdown of zero resistance states as the radiation intensity increases. In order to explain it we have included in our model the electron heating due to large microwave intensities and its effect on the longitudinal resistivity.

Spin-polarized transport of two-dimensional electron gas embedded in a diluted magnetic semiconductor

The spin-polarized transport property of a diluted magnetic semiconductor two-dimensional electron gas is investigated theoretically at low temperature. A large current polarization can be found in this system even at small magnetic fields and oscillates with increasing magnetic field while the carrier polarization is vanishingly small. The magnitude as well as the sign of the current polarization can be tuned by varying magnetic field, the electron density and the Mn concentration.

Growth of AlGaN/GaN based electronic device structures with semi‐insulating GaN buffer and AlN interlayer

Heterostructures of GaN/AlGaN for power applications are grown by metal organic vapor phase epitaxy in a multiwafer reactor on sapphire and SiC substrates. The insulating properties of the GaN buffer and the influence of an AlN interlayer on the two-dimensional electron gas transport properties of high electron mobility transistor structures have been studied. By optimizing the growth conditions the resistivity could be increased to 1011 Ωcm. For GaN/AlGaN heterostructures on sapphire with a 1 nm AlN interlayer a room temperature mobility of 1780 cm2/Vs and an electron sheet carrier density of 1.2 ×1013 cm–2 were achieved. Power devices were fabricated on s.i. SiC showing a power density of 4.9 W/mm at 10 GHz for a multifinger transistor with a gate width of 1 mm. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dislocation scattering effect on two-dimensional electron gas transport in GaN/AlGaN modulation-doped heterostructures

Dislocation scattering effect on two-dimensional electron gas transport in GaN/AlGaN modulation-doped heterostructures

Electron transport in Si/SiGe modulation-doped heterostructures using Monte Carlo simulation

The electron transport in two-dimensional gas formed in tensile-strained Si1−xGex/Si/Si1−xGex heterostructures is investigated using Monte Carlo simulation. First the electron mobility is studied in ungated modulation-doped structures. Calculation matches the experimental results very well over a wide range of electron densities. The mobility typically varies between 1100 cm2/V s in highly-doped structures and 2800 cm2/V s at low electron density. The mobility is shown to be significantly influenced by the thickness of the spacer layer separating the strained Si channel from the pulse-doped supply layers. Then the electron transport is investigated in a gated modulation-doped structure in which the contribution of parasitic paths is negligible. The mobility is shown to be higher than in comparable ungated structures and dependent on the gate voltage as a result of the electron density dependence of remote impurity screening.