2DEG Density Research Articles

Electron scattering analysis in 2DEG in sputtering-grown MgZnO/ZnO heterostructure

Here, we present an analytical modeling of electron mobility in two dimensional electron gas (2DEG)-yielding MgZnO/ZnO heterostructures, to ascertain dominant scattering mechanisms and physical parameters responsible for one-order lower value of electron mobility in sputtering-grown heterostructure as compared to that in molecular beam epitaxy-grown heterostructure. This work extensively probes all scattering components and their physical parameters, such as dislocation density, impurity density, mole fraction, 2DEG density, correlation length and lateral size, for their respective effects on electron mobility of sputtered heterostructure. The results suggest that dislocation density and alloy disorder scattering are the most dominant sources responsible for reduced electron mobility. This work is extremely crucial for achieving high electron mobility by optimizing the material growth parameters to attain low dislocation density, impurity density and interface roughness, for the development of low-cost ZnO-based heterostructure field effect transistors.

Anomalous DC Characteristics of AlGaN/GaN HEMTs Depending on Proton Irradiation Energies

We evaluated the effect of the proton irradiation energy on the DC characteristics of AlGaN/GaN high electron mobility transistors (HEMTs). Devices were exposed to various irradiation energies, i.e., 0.5 MeV, 5 MeV, and 60 MeV, respectively, with a fluence of 1 × 1014 p cm−2 at room temperature. The 0.5 MeV-irradiated HEMT shows the largest degradation of transfer characteristics than other HEMTs because the lower proton energy has a larger non-ionizing energy loss (NIEL). The threshold voltage of HEMTs with 0.5 and 5 MeV proton irradiation is positively shifted due to the reduction of 2-dimensional electron gas (2DEG) density by defects created during irradiation. However, the 60 MeV-irradiated HEMT showed a negative shift of threshold voltage with no degradation of drain current. Also, the gate leakage current of fabricated HEMTs decreased with an increasing irradiation energy. These anomalous DC characteristics were expected due to the creation of a proton radiation-caused interfacial oxide layer between the gate and AlGaN layer.

AlGaN/GaN High-Electron-Mobility Transistors on a Silicon Substrate Under Uniaxial Tensile Strain

This paper reports pulsed DC and RF characteristics of GaN high-electron-mobility transistors (HEMTs) on a silicon substrate under external mechanical tensile strain. Tensile strain enhanced two-dimensional electron gas (2DEG) density resulted in increasing ID, fT and fmax at CW measurement. Eliminating self-heating by pulse measurement, DC and RF characteristics between flat and bend devices were slightly increased for small-width devices, and decreased for wide-width devices. This was because maximum electron drift velocity was reduced by a hot phonon effect while 2DEG density increased by tensile strain for wide-width devices. Results indicated that the tensile strain on thinner substrates decreased the DC and RF performances of wide-width GaN-based RF power devices. This phenomenon should be considered while using GaN-based RF power devices for compact packages, especially in high-power applications.

Quasi-Normally-Off AlGaN/GaN HEMTs With SiNₓ Stress Liner and Comb Gate for Power Electronics Applications

Recess processes for the fabrication of normally-off GaN HEMTs generally compromise devices’ on-state performance. In this work, recess-free quasi-normally-off GaN HEMTs with a threshold voltage of 0.24 V is realized by local control of two-dimensional electron gas (2DEG) density. The devices feature a $0.1~{\mu }\text{m}$ gate length, SiNx stress liner, and comb gate. SiNx liner can provide significant stress to AlGaN/GaN heterostructure in the scaled gate region. The additional stress translates to the additional electric field and depletes the 2DEG in the gate region. As a result, the quasi-normally-off operation is achieved. Furthermore, the comb gate structure is introduced to suppress the short channel effects, supported by TCAD simulation. The quasi-normally-off devices’ excellent on-state performances are benchmarked against the normally-off devices reported recently and a p-GaN HEMT purchased from a commercial foundry. The results support strain engineering as a promising technique to pursue the normally-off operation of GaN HEMTs.

Two-dimensional electron gas modeling in strained InN/GaN hetero-interface under pressure and impurity effects

The two-dimensional electron gas (2DEG) density modeling in WZ strained InN/GaN hetero-interface with Ga-face arrangement is considered in this paper. We theoretically investigate the role of an externally applied hydrostatic pressure and impurity's position by analyzing their impact in the presence of built electric field due to the dielectric mismatch between the system and its surrounding matrix. To solve the coupling Schrödinger-Poisson equations, as well as the polarization induced charges, finite element method code is used. Pressure-dependence of dielectric constant, band-gap energy, lattices and internal electric field is considered. The 2DEG appears to be strongly-dependent internal and external perturbations exhibiting a non-monotonic behavior. Our results prove that: (1) the InN channel is more populated for on-center impurity compared to off-center one, (2) the electron density in the triangular shape quantum well increases with the pressure; (3) the 2DEG density is enhanced versus the positive impurity's position and dropped versus pressure.

Enhancement of 2D Electron Gas Mobility in an AlN/GaN/AlN Double‐Heterojunction High‐Electron‐Mobility Transistor by Epilayer Stress Engineering

Herein, 2D electron gas (2DEG) enhancement in an AlN/GaN/AlN double‐heterojunction high‐electron‐mobility transistor (DH‐HEMT) is achieved by epilayer stress engineering. The epistructures are grown on a SiC substrate using plasma‐assisted molecular beam epitaxy (PA‐MBE). The stress in the AlN buffer is systematically studied as a function of the III/V ratio. An optimized AlN buffer layer with the relaxation of 66% and a smooth surface morphology with a root mean square (RMS) roughness of 0.5 nm is grown using the two‐step growth method with recovery via metal consumption. It is observed that the stress in the GaN channel is influenced by the stress in the AlN buffer nonlinearly. Furthermore, the carrier mobility increases as the stress in the GaN channel is increased. However, the 2DEG density remains unchanged. An optimized AlN/GaN/AlN DH‐HEMT epistructure with very high 2DEG sheet carrier density of 4.1 × 1013 cm−2 and carrier mobility of 613 cm2 V−1 s−1 is achieved, which leads to higher device output power density.

The Effect of AlN Buffer Layer on AlGaN/GaN/AlN Double‐Heterostructure High‐Electron‐Mobility Transistor

Herein, the effect of crystal quality of AlN buffer layer on AlGaN/GaN/AlN double‐heterostructure high‐electron‐mobility transistor (DH‐HEMT) is investigated. The material quality of the GaN channel and the AlGaN barriers, such as the dislocation density and the interface roughness, deteriorates, and the 2D electron gas (2DEG) mobility decreases as the threading dislocation density (TDD) of the AlN buffer increases. It is also revealed that the thickness and the Al mole fraction of the AlGaN barrier are affected by the strain variation of the GaN channel depending on the TDD of the AlN buffer. The variation of the compressive strain of the GaN channel is responsible for the 2DEG density change by affecting the barrier condition and the piezoelectric polarization charge. Low‐temperature Hall effect measurement reveals that the interface roughness scattering is a dominant factor for the mobility of the DH‐HEMT, which is ≈2–6 × 103 cm2 (V s)−1.

Critical parameters for the presence of a 2DEG in GaN/AlxGa1−xN heterostructures

In this computational study, the influence of GaN/AlxGa1−xN layer stack parameters, such as surface potential, aluminum mole fraction, and background donor concentration, on the two-dimensional electron gas (2DEG) density in a heterostructure is verified. At a fixed Al mole fraction, the surface potential was identified to have the largest impact on the 2DEG density. The combination of a small aluminum mole fraction (x < 0.12) and large surface potential results in the absence of a 2DEG in the investigated heterostructures, while for a small surface potential value, a 2DEG will be present. For a large aluminum mole fraction (x ≥ 0.25), a 2DEG is always present, independent of the surface potential value. In the case of an intermediate aluminum mole fraction, the background donor level is one key parameter strongly influencing the 2DEG density.

Modeling the simultaneous effects of thermal and polarization in InGaN/GaN based high electron mobility transistors

In this paper, we propose the modeling of the two-dimensional electron gas (2DEG) density in InGaN/GaN hetero-interface based high electron mobility transistors (HEMT). The Schrodinger-Poisson equations, as well as the polarization-induced charges, have been utilized. The temperature effect on the effective mass, band gap energy, dielectric constant and the lattice thermal expansion is taken into account. Our numerical calculation shows that the 2DEG density decreases with temperature. This is due to the shrinkage of the conduction band offset at high temperatures. The improvement of the sheet carrier density and the carrier confinement at the InGaN/GaN hetero-interface based heterostructures is made at low temperature.

Model of HfO2/Al2O3 Dielectric ALNINNAlganmos HEMT for Power Application

The work involves to devise dielectric HfO2 /Al2O3MOS-HEMT stack to validate for the device structure through the refinement of mesh analysis. The structure of the AlN\InN\AlGaN HEMT MOS-HEMTs are designed and meshed using the script file in the Sentaurus structure editor. TheAlN\InN\AlGaN MOS-HEMTs output characteristic are examined for physics of MOS-HEMT in Hydrodynamic model TCAD. The amalgamation formation ofthe Al0.30GaN0.70 barrierinterface with the channelofhigh 2DEG density result in anincreaseddrain current density and high trans conductance. Additional the device performance of DC transfer characteristics, small signal analysis of AlN/InN/AlGaNMOS-HEMT are analyzed and simulated.

Active and buffer layers of GaN HEMT: ECV profiling and 2DEG calculation

ABSTRACT Gallium nitride transistors with high electron mobility (HEMT) have been studied by means of electrochemical capacitance–voltage (ECV) technique. The capacitance–voltage characteristics at various frequencies were measured; the free charge carrier depth distributions as well as the intensity of quantum well filling by charge carriers were obtained. The features of 2D electron gas (2DEG) appearance in GaN HEMT, related to spontaneous and piezoelectric polarisation, as well as factors affecting the density of free charge carriers in channel and the performance of GaN HEMTs were described in detail. The efficiency of ECV technique for quality control of both active and buffer layers of GaN HEMT heterostructures is shown. The calculation of 2DEG density for various technological parameters of the interface, such as mutual orientation of AlGaN/GaN layers and composition of Al x Ga1-x N, is carried out taking into account piezoelectric effects.

Hetero-integration of quasi two-dimensional PbZr0.2Ti0.8O3 on AlGaN/GaN HEMT and non-volatile modulation of two-dimensional electron gas

In theory, two-dimensional electron gas (2DEG) density in AlGaN/GaN heterostructures can be modulated via the polarization of a ferroelectric material. Therefore, the integration of ferroelectric materials with AlGaN/GaN high electron mobility transistors (HEMTs) is a very attractive method for exploring further applications of electronic devices. This paper reports on the integration of single crystal PbZr0.2Ti0.8O3 (PZT) thin films (20 nm) and AlGaN/GaN HEMT via a substrate transfer technique. Continuous and nonvolatile modulation of 2DEG density in the AlGaN/GaN heterojunction interface is achieved in accordance with the theory. By poling the PZT/AlGaN/GaN HEMT from 2 V to 7 V, the device exhibits different threshold voltages (from −3 V to 1 V). A modulation of 2DEG density up to 300% is achieved. Moreover, the retention characteristic of the device is excellent, and the variation of the threshold voltage is less than 0.25 V after 105 s. Compared to the conventional method of depositing PZT directly on AlGaN/GaN HEMT, the transfer technique can achieve better quality in a much thinner film and prevent the interdiffusion during growth. At the same time, the excellent quality of the transferred PZT film has a good ability to modulate the 2DEG. The generality of our approach paves the way for integrating ferroelectric materials on the GaN-based devices, and this is promising for using the device in more application fields.

(Invited) GaN-Based Multiple 2DEG Channel BRIDGE (Buried Dual Gate) HEMT Technology for High Power and Linearity

A unique combination of high mobility, high velocity and high sheet density of the 2DEG formed in GaN-based heterostructures has enabled GaN-based HEMTs to be used in a wide range of applications from RF power amplifiers to efficient power converters. Today’s complex communication systems require transceivers to process RF signals efficiently with large bandwidth and high fidelity. While GaN-based HEMT technology has advanced to reach higher power densities, it has not fundamentally changed the power requirements for the linearity performance. To address fundamental limitations of HEMT’s power/linearity/efficiency/frequency tradeoff, we proposed a transistor structure called BRIDGE FET (buried dual gate FET) where gate electrodes are buried into AlGaN/GaN heterostructures and contact laterally with multiple 2DEG channels [1]. A deliberate elimination of a conventional top-contact gate leads to a unique device operation principle and performance advantages for improved linearity and efficiency at large signal operations; (1) The drain-source current is controlled solely by modulating the width of the 2DEG channels by the lateral gate electric field while maintaining the 2DEG density. (2) The MESFET-like device operation enables gradual pinch-off, greatly reducing g m derivatives near pinch-off. (3) Lack of density modulation with V gs leads to a constant electron velocity at high electric field, eliminating a typical g m roll-off at high V gs. This results in a constant gain along a resistive load line. (4) The buried gates forms Schottky contacts to the GaN channels below the 2DEG layers. This enhances electron confinement and improves electrostatic isolation between the source and drain, significantly reducing g d at high V ds. (5) Elimination of the top-contact gate prevents electrons from being trapped on the surface, suppressing current collapse at high voltage operations. (6) An absence of inverse piezoelectric effect due to the reduced vertical electric field at the drain-side of the gate improves device reliability under high voltage stress. In this presentation, current status of our development of multi-2DEG channel BRIDGE FETs will be discussed. This work was sponsored by DARPA-MTO DREaM program under DARPA/CMO Contract No. FA8650-18-C-7807. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the Defense Advanced Research Projects Agency or the U.S. Government. [1] K. Shinohara et al., IEEE EDL, vol. 39, no. 3, p. 417, March 2018.

Efficient ab initio plus analytic calculation of the effect of GaN layer tensile strain in AlGaN/GaN heterostructures

We have addressed the existing ambiguity regarding the effect of tensile strain in the underlying GaN layer on AlxGa1−xN/GaN heterostructure properties. The bandgaps and band-offsets for AlxGa1−xN on strained GaN were first computed using density functional theory (DFT), in the generalized gradient approximations (GGA) and hybrid functional Gaussian–Perdew–Burke–Ernzerhof (Gau–PBE) regimes. We propose a simple model to relate the GGA and Gau–PBE bandgaps, which is used to determine the realistic bandgaps of strained AlGaN. The bandgaps and band-offsets from the DFT calculations are then used to analytically calculate the two-dimensional electron gas density in an AlxGa1−xN/GaN hetero-interface. Our bandstructure calculations show that it is not possible to induce significant change in band-offsets through strain in the GaN layer. The charge-density calculations indicate that such strain can, however, modulate the polarization charge and thereby enhance the 2DEG density at the AlGaN/GaN interface substantially.

Study of electronic transport properties in AlGaN/AlN/GaN/AlGaN double-heterojunction transistor

The electronic transport properties in AlGaN/AlN/GaN/AlGaN double heterostructures are investigated by an analytical model, considering the effect of the modified Fang-Howard wave function. The alloy disorder scattering is assumed to be eliminated by the introduction of an AlN insertion layer, and the rest five possible scattering mechanisms, acoustic deformation potential scattering, piezoelectric field scattering, polar optical phonons scattering, interface roughness scattering, and dislocation scattering, are taken into consideration in the calculation. The relations of two-dimensional electron gas (2DEG) density and mobility with alloy composition in the top AlGaN barrier layer and AlGaN buffer layer and the channel thickness are estimated and discussed. Finally, we compare the temperature dependences of the 2DEG mobility in Al0.25Ga0.75N/AlN/GaN/Al0.05Ga0.95N double heterostructures and in the conventional Al0.25Ga0.75N/AlN/GaN single heterostructures and explain them with detailed scattering processes.

Effects of carrier confinement in MgZnO/CdZnO thin-film transistors: Towards next generation display technologies

The authors propose a novel CdZnO/MgZnO single-well heterojunction thin-film transistor for achieving carrier confinement in a low potential well lying between two high potential barriers. The working mechanism of the TFT is elaborated with the help of energy band diagrams which indicate the formation of high density 2DEG at the channel/insulator interface. The effects of carrier confinement and 2DEG formation on device performance has been studied by TCAD simulation. The device exhibits an uncharacteristically sharp transition from OFF to ON states, high ON state current (10−3), low OFF state current (10−14), high mobility (8cm2/V-s) but suffers from a very narrow range of operating gate bias. In order to overcome this, a double-well heterojunction TFT is propounded as an improvement over the existing heterostructure, where two wells work in parallel to increase the range of operation substantially. The double-well TFT displays a two order increase in ON current and a two order reduction in OFF current with respect to its single-well counterpart, concomitantly achieving a four-fold increment in mobility with a peak mobility value of 32cm2/V-s.

Epi-Gd2O3/AlGaN/GaN MOS HEMT on 150 mm Si wafer: A fully epitaxial system for high power application

In this letter, we report the impact of epitaxial Gd2O3 on the electrical properties of an AlGaN/GaN high electron mobility transistor (HEMT) grown on a 150 mm diameter Si (111) substrate. Incorporation of epitaxial Gd2O3 grown by the molecular beam epitaxy technique under a metal gate (metal/Gd2O3/AlGaN/GaN) causes six orders of magnitude reduction in gate leakage current compared to metal/AlGaN/GaN HEMT. We observe that epi-Gd2O3 undergoes complete structural changes from hexagonal to monoclinic as the thickness of the layer is increased from 2.8 nm to 15 nm. Such structural transformation is found to have a strong impact on electrical properties whereby the gate leakage current reaches its minimum value when the oxide thickness is 2.8 nm. We find a similar trend in the density of interface traps (Dit) having a minimum value of 2.98 × 1012 cm−2 eV−1 for the epioxide layer of thickness 2.8 nm. Our measurements also confirm a significant increase in the two dimensional electron gas (2DEG) density (∼40%) at AlGaN/GaN interface with epioxide grown on AlGaN, thus confirming the contribution of epitaxial lattice strain on 2DEG modulation.

(Invited) GaN-Based Multiple 2DEG Channel BRIDGE (Buried Dual Gate) HEMT Technology for High Power and Linearity

We report on a GaN-based field effect transistor with laterally-gated multiple 2DEG channels, called BRIDGE (buried dual gate) HEMT. Unique operation principle of the transistor enables unprecedented device characteristics suitable for efficient and linear millimeter-wave power amplifier applications. The lateral gate simultaneously modulate multiple 2DEG channels formed in an Al(Ga)N/GaN hetero-structure. A higher electron saturation velocity measured for a lower 2DEG density suggests that the multiple 2DEG channel structure is ideal for obtaining a high current density, and simultaneously enhancing high frequency performance of the transistor. The BRIDGE HEMTs built on a 16-channel HEMT epitaxial structure with a net 2DEG density of 3.3×1013 cm-2 exhibited a record high knee current density of 3.7 A/mm. The absence of the top contact gate results in negligible current collapse by eliminating a high electric field region at the drain end of the gate – another key feature of the BRIDGE HEMT.

Gate Current Reduction and Improved DC/RF Characteristics in GaN-Based MOS-HEMTs Using Thermally Grown TiO2 as a Dielectric

This paper demonstrates a reduction in the gate leakage current and improvement in transistor characteristics in thermally grown TiO2/AlGaN/GaN heterostructure-based metal–oxide–semiconductor high-electron-mobility transistors (MOS-HEMTs). In contrast to the conventional AlGaN/GaN HEMTs, thermionic field emission through gate is not the dominant current transport mechanism for the thermally grown TiO2/AlGaN interface. The gate current is greatly affected by the properties of the oxide material and oxide–semiconductor interface in addition to the property of the barrier layer. The MOS-HEMTs with a 3.4-nm-thick TiO2 gate insulator exhibits a low gate leakage current of 10−8 Acm−2, which leads to superior device performances in terms of saturation drain current, peak transconductance, subthreshold swing, and unity gain frequency of 620 mA/mm, 158 mS/mm, 75 mV/decade, and 7 GHz, respectively, for a 400-nm gate length device. This is further augmented by an increase in ON/OFF ratio to ${5}\times {10}^{{8}}$ and a large reduction in the subthreshold leakage current by at least two orders of magnitude in comparison to that of a control HEMT. Trap-assisted tunneling (TAT) and Poole–Frenkel (PF) emission are found to be the dominant current mechanisms for gate leakage at high temperatures and moderate electric field. The activation energy of traps in PF emission is found to be 0.49 eV, and the extracted trap energy levels for the TAT are found to be in the range of 1.7–2.2 eV. The reverse bias current is found to saturate at high voltages when the field across the diode also saturates. The transistor characteristics improvement is largely ascribed to an increase in 2-D electron gas (2DEG) density.

(Invited) Flexible Multifunctional III-N Semiconductor Materials and Devices for Energy Applications

We present recent development in flexible wide-bandgap semiconductor materials and devices. Especially, we focus on mechanically bendable group-III-nitride (III-N) thin-film heterostructures and their photonic, electronic, and energy-harvesting devices in energy applications. The presentation will cover various topics including (1) multi-functionality of flexible III-N devices, (2) direct growth of high-quality single-crystal-like GaN semiconductor thin films on low-cost flexible metal tapes, and (3) piezoelectric generators and sensors for self-powered wearable systems by harvesting and sensing ambient biomechanical energy. Flexible III-N thin-film heterostructures have an implication of more than just mechanically flexible materials. Bendable devices based on III-N heterostructures can be equipped with new functionalities and even further improved performance characteristics using a new concept of active polarization engineering by controlled external strains. We propose to develop multi-functional and/or further-improved-performance devices by utilizing the interactions between electronic and optical properties and mechanical forces in the flexible III-N heterostructures. The concept will enable new mechano-electro-photonic (MEP) devices. We will show by device modeling that new device concept in flexible transistors based on AlGaInN/GaN heterostructure is possible, including modulation of 2-dimensional electron gas (2DEG) density by bending, strain-effect transistors (Figure 1: I-V characteristics of strain-effect transistors), and high-hole-mobility transistors. Furthermore, photon emitters based on flexible III-N heterostructure can result in higher internal quantum efficiency (IQE), higher wall-plug efficiency (WPE), and color modulation by optimum bending conditions (Figure 2: Concept of color-tunable white light-emitting diodes). We study and develop nearly-single-crystalline GaN thin film on flexible metal tapes by a direct deposition technique for the demonstration of a new wide-bandgap semiconductor platform, targeting high-performance yet economical, flexible, and versatile device technology. Data and analysis are presented for the single-crystal-like film, as representatively shown in Figure 3 (Flexible hybrid tape substrate consisting of GaN thin film and Cu tape with crystallinity-transformational buffer layers). Energy harvesters that scavenge biomechanical energy are promising power supply candidates for wearable and implantable electronics. Of the most widely used energy harvesters, piezoelectric generators can generate more electric charge than their triboelectric counterparts with similar device size, thus are more suitable to make compact wearable devices. We develop a flexible piezoelectric generator (F-PEG) with chemically stable and biocompatible III-N thin film. Data and analysis are presented for the flexible III-N F-PEG. We also demonstrate that the F-PEG can directly power electronics such as light-emitting diodes and electric watches, and charge commercial capacitors and batteries to operate an optical pulse sensor, as representatively shown in Figure 4 (Pulse sensor powered by F-PEG and heart rate measurement on a fingertip). Figure 1