Nitride-based High Electron Mobility Transistors Research Articles

Phonon transport across rough AlGaN/GaN interfaces with varying Al–Ga atomic ratios

Exploring interfacial thermal transport of a heterojunction interface is crucial to achieving advanced thermal management for gallium nitride-based high electron mobility transistor devices. The current research primarily focuses on material enhancements and microstructure design at the interfaces of epitaxial layers, buffer layers, and substrates, such as the GaN/SiC interface and GaN/AlN interface. Yet, the influence of different concentrations of Al/Ga atoms and interface roughness on the interfacial thermal conductance (ITC) of AlGaN/GaN interface, the closest interface to the hot spot, is still poorly understood. Herein, we focus on the rough AlGaN/GaN interface and evaluate the changes in ITC under different Al–Ga atomic concentrations and interface roughness using atomistic simulations. When the interface is completely smooth and AlGaN and GaN are arranged according to common polarization characteristic structures, the ITC gradually increases as the proportion of Al atoms decreases. When the proportion of Al atoms is reduced to 20%–30%, the impact of the interface structure on heat transfer is almost negligible. For interface models with different roughness levels, as the interface roughness increases, the ITC drops from 735.09 MW m−2 K−1 (smooth interface) to 469.47 MW m−2 K−1 by 36.13%. The decrease in ITC is attributed to phonon localization induced by rough interfaces. The phonon modes at the interface are significantly different from those in bulk materials. The degree of phonon localization is most pronounced in the frequency range that contributes significantly to heat flux. This work provides valuable physical insights into understanding the thermal transfer behaviors across the rough AlGaN/GaN interfaces.

Modulation of localized phonon thermal transport at GaN/AlxGa1-xN heterointerface: Polar surface, doping, and compressive Strain

The phonon transport processes within a few atomic layers near heterointerfaces play a crucial role in thermoelectric applications such as nitride-based high electron mobility transistors (HEMTs) and light-emitting diodes (LEDs). However, a systematic investigation of their localized transport characteristics remains unexplored. This article employs the Interfacial Phonons method based on first-principles calculations to extract interface phonon information for GaN/AlxGa1-xN heterostructures. Molecular dynamics simulations are utilized to compare the phonon behaviors between Ga-face and N-face GaN-based heterointerfaces. The research extensively investigates the dependence of interfacial thermal transport on the concentration of Al atoms under ordered substitutional doping and the strength of uniaxial compressive strain. The research revealed that both Ga-face and N-face GaN/AlxGa1-xN heterointerfaces exhibit phonon branch drift. The differences in charge distribution at the polar surfaces lead to varying degrees of Brillouin zone distortion. The vibrational modes of the atoms at the N-face GaN interface are more compact and localized than those of Ga-face GaN, resulting in a stronger degree of localization for the optical phonon branches. When varying the doping concentration of Al atoms, some phonon branches at the GaN interface appear outside the band structure of the intrinsic bulk and surface states, exhibiting strong localized characteristics. The introduction of Al atoms at the heterointerface induces additional phonon scattering, and the lattice vibration mismatch between Al and Ga atoms results in a non-uniform residual stress field at the interface, further enhancing phonon-interface scattering. Moderate compressive strain-induced lattice distortion promotes the overlapping of phonon modes at the interface, allowing some localized phonons to serve as interface transport modes. This optimization enhances phonon transport at the heterointerface. This research provides valuable insights into a comprehensive understanding of interfacial thermal transport in GaN-based semiconductors. Moreover, it offers useful theoretical guidance for thermal management through the manipulation of phonon wave properties.

Performance analysis of gallium nitride-based DH-HEMT with polarization-graded AlGaN back-barrier layer

In this paper, polarization-graded AlGaN back-barrier nanolayer has been introduced to improve the DC and RF parameters of gallium nitride-based high electron mobility transistors (HEMT). To explore the characteristics, both graded and non-graded double heterojunction high electron mobility transistor (DH-HEMT) structures are optimized using SILVACO-ATLAS physical simulator. Enhanced DC and RF parameters have been observed in the optimized graded DH-HEMT. In this paper, we have also studied the development of the quantum wells at the AlGaN/GaN interfaces due to the conduction band discontinuity in both structures.

Models for the self-heating evaluation of a gallium nitride-based high electron mobility transistor

We propose a novel model approach for temperature evaluation in the channel region of a InAlN/AlN/gallium nitride high electron mobility transistor (HEMT) due to self-heating effects. The heat transfer in a HEMT device has been investigated experimentally by the nearby temperature sensor and compared by theoretical models solved by both numerical and analytical methods. The average temperature of the channel area of almost 160 °C for dissipated power of 2 W was determined using the drain-source current variation analysis. The electrical and thermal behavioral numerical model under quasi-static conditions have been used to describe the HEMT device. In contrast, the one-dimensional thermal model for analytical evaluation has been proposed as an alternative approach. Surprisingly, the experimental results verified not only the validity of precise numerical simulation but also the simplified analytical model that makes it a reliable tool even for complex electronic devices.

Super Field Plate Technique That Can Provide Charge Balance Effect for Lateral Power Devices Without Occupying Drift Region

The super junction has been the most important concept for the design of power devices. However, there are still two problems when the conventional super-junction techniques are applied on lateral power devices: a large portion of the drift region is occupied by a p-type region, and the super junction techniques are not suitable for the gallium nitride-based high electron mobility transistor (GaN-HEMT). To solve the problems, a super field plate (SuFP) technique is proposed as a charge balance principle. Our analyses proved that the SuFP can provide a charge balance effect for a lateral double diffused MOS (LDMOS) without occupying the drift region. As a result, the LDMOS with SuFP has a better performance than the LDMOS with other charge balance realization techniques. Moreover, as a kind of field plate, the SuFP is also suitable for GaN-HEMT. Thereby, the proposed SuFP technique overcomes the two problems in conventional super-junction techniques and is significant for lateral power devices.

Hyperspectral Electroluminescence Characterization of OFF-State Device Characteristics in Proton Irradiated High Voltage AlGaN/GaN HEMTs

Gallium nitride-based high electron mobility transistors (HEMTs) represent a critical next-generation technology for RF amplifiers. In this work, we implement electroluminescence (EL) imaging to study breakdown mechanisms under high voltage stress conditions in as-fabricated and proton irradiated devices. After irradiation, an increased breakdown voltage and reduced buffer leakage was observed and attributed to the formation of a back barrier structure due to carrier type conversion in the buffer layer from the introduction of donor trap levels near the conduction band. In addition to identifying high field regions associated with device failure, the hyperspectral filter enables spectrum extraction, identifying peaks in the 700 nm–900 nm region associated with both defects and hot electrons that are contributing to device failure, but no change in signature after irradiation implying that radiation-induced defects are non-radiative. Complementary photoluminescence spectroscopy identified a reduction in free carrier density consistent with the generation of compensating trap levels, supporting the proposed model.

MOCVD of GaN-based HEMT structures on 8 inch silicon substrates

ABSTRACTMetal organic chemical vapor deposition, as well as material and basic device properties of nitride-based high electron mobility transistor structures on (111) silicon substrates varying in diameter from 4 to 8 inch were studied using in-situ and ex-situ characterization techniques. All substrates used for the growth of the nitride structures in this study were of SEMI standard thicknesses. The total thickness of the nitride structures was in the range of 1.5 – 5 µm. It is reported that nitride structures can be grown on 4, 6 and 8 inch diameter substrates with very similar post-growth wafer shape, material and device characteristics. It is also shown that their crystal quality, 2DEG transport properties and isolation blocking voltages can be improved by increasing nitride structure thickness while maintaining post-growth wafer bow and warp less than 50 µm. The maximum thickness of nitride structures that can be successfully grown on 8 inch diameter SEMI standard substrates seems to be limited to about 4.5 µm due to plastic deformation of Si. Blocking voltages of more than 700 V were achieved using 4.5 µm thick nitride-based high electron mobility transistor structures grown on 8 inch Si substrate.

Dual-purpose BGaN layers on performance of nitride-based high electron mobility transistors

A GaN/ultrathin BGaN/GaN heterojunction is used in AlGaN/GaN high electron mobility transistors (HEMTs) to provide an electrostatic barrier to electrons and to improve the confinement of the 2-dimensional electron gas. BGaN back-barrier layers limit leakage in the GaN buffer thanks to two effects: a polarization-induced band discontinuity and a resistive barrier originating from excellent insulation properties of BGaN. Compared to conventional AlGaN/GaN HEMTs, structures grown with BGaN back-barrier showed a significant improvement of static performances, transport properties, and trapping effects involving a limited current collapse in dynamic regime. A DC maximum current increase of 58.7% was observed.

GaN/AlN electro‐optical modulator prototype at telecommunication wavelengths

We report on a prototype electro-optical modulator at telecommunication wavelength based on intersubband (ISB) transitions in a short-period GaN/AlN superlattice (SL). The device has a vertical architecture resembling a nitride-based high-electron mobility transistor, whose barrier layer has been replaced by a 5 period SL. By applying an electrical field, we were able to influence the bandstructure and as a consequence to quench the ISB absorption peak originating from the SL. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Nitride-based high electron mobility transistors with a GaN spacer

A GaN/ultrathin AlN∕GaN heterojunction has been used to introduce a GaN spacer between the GaN channel and the AlGaN barrier in AlGaN∕GaN high electron mobility transistors (HEMTs). In conventional AlGaN∕GaN devices, the alloy scattering of the electrons with the AlGaN barrier degrades the electron velocity at high electric fields. This effect is significantly reduced in GaN-spacer transistors, which therefore have much better high field transport properties. While the dc performance of these transistors is similar to conventional AlGaN∕GaN HEMTs, a 20% increase in the electron velocity has been measured by two different techniques.