AlGaN Back Barrier Research Articles

Optimization of Epitaxial Structures on GaN-on-Si(111) HEMTs with Step-Graded AlGaN Buffer Layer and AlGaN Back Barrier

Recently, crack-free GaN-on-Si growth technology has become increasingly important due to the high demand for power semiconductor devices with high performances. In this paper, we have experimentally optimized the buffer structures such as the AlN nucleation layer and step-graded AlGaN layer for AlGaN/GaN HEMTs on Si (111) substrate by varying growth conditions and thickness, which is very crucial for achieving crack-free GaN-on-Si epitaxial growth. Moreover, an AlGaN back barrier was inserted to reduce the buffer trapping effects, resulting in the enhancement of carrier confinement and suppression of current dispersion. Firstly, the AlN nucleation layer was optimized with a thickness of 285 nm, providing the smoothest surface confirmed by SEM image. On the AlN nucleation layer, four step-graded AlGaN layers were sequentially grown by increasing the Al composition from undermost layer to uppermost layer, meaning that the undermost one was close to AlN, and the uppermost was close to GaN, to reduce the stress and strain in the epitaxial layer gradually. It was also verified that the thicker step-graded AlGaN buffer layer is suitable for better crystalline quality and surface morphology and lower buffer leakage current, as expected. On these optimized buffer structures, the AlGaN back barrier was introduced, and the effects of the back barrier were clearly observed in the device characteristics of the AlGaN/GaN HEMTs on Si (111) substrate such as the transfer characteristics, output characteristics and pulsed I-V characteristics.

Characterization of Trapping Effects Related to Carbon Doping Level in AlGaN Back-Barriers for AlGaN/GaN HEMTs

Characterization of Trapping Effects Related to Carbon Doping Level in AlGaN Back-Barriers for AlGaN/GaN HEMTs

Investigation of different buffer layer impact on AlN/GaN/AlGaN HEMT using silicon carbide substrate for high-speed RF applications

The AlN/GaN heterostructure on the AlGaN back barrier with different buffer layer structures using a silicon carbide (SiC) substrate was investigated in this work. This study mainly focused on selecting a cost-effective buffer layer with a thickness of more than 1 μm and fewer defects for improving DC/RF performance. The extra epitaxial growth process steps can be avoided by choosing an appropriate, high-quality buffer material called β-Ga2O3 (β-gallium oxide). It has a good lattice match with AlGaN alloys and SiC. It leads to fabricating cost-effective, reliable HEMTs. The GaN buffer has a poor lattice match with SiC; the nucleation layer must be used between GaN and SiC for lattice match to avoid stress defects and dislocations, which helps to reduce buffer traps. The growth of thick GaN crystals leads to high costs and more defects. Creating a solid conduction band barrier with the help of a back-barrier (AlGaN) leads to a boost in the confinement of electrons in both devices. Still, the traps and defects in the GaN buffer do not effectively halt the leakage current compared to the β-Ga2O3 buffer. The T-shaped gate with the impact of LGD (gate to drain length) and LGS (gate to source length) scaling in lateral dimensions was also studied and the device performance was reported. The HEMT with β-Ga2O3 buffer delivers significant results in drain current (2.3 A/mm), transconductance (603 S/mm), and cut-off frequency (486 GHz) for the scaled lateral lengths LG (Gate Length) of 60 nm,LGD of 0.9 μm, and LGS of 0.4 μm. This excellent performance helps to use this HEMT device in cost-effective future high-speed RF applications.

Physical insight of thin AlGaN back barrier for millimeter-wave high voltage AlN/GaN on SiC HEMTs

In this work, physical mechanisms underlying carbon-doped buffer combined with an AlGaN back-barrier layer are investigated in state-of-the-art millimeter-wave AlN/GaN transistors. We have fabricated devices with and without the insertion of a thin AlGaN back-barrier layer with reduced carbon concentration to analyze the improvement resulting from this buffer architecture. More specifically, the impact of the Al mole fraction into the back-barrier, carbon doping in the buffer, and channel thickness on 100 nm gate length device performance has been studied. It appears that a 150 nm undoped GaN channel followed by a highly carbon-doped GaN buffer results in good electron confinement at the expense of a high current collapse. On the other hand, an Al mole fraction of 25% in the AlGaN back barrier layer coupled with a 150 nm undoped GaN channel provides excellent electron confinement, resulting not only in a low DIBL under high electric field but also low current collapse. Calibrated on experimental devices, TCAD simulations reveal that the electric field penetration inside the GaN buffer is prevented owing to a strong polarization from the back barrier when the Al-content is high enough. That is why, the electron confinement is superior for the 25% Al mole fraction in the back barrier along with reduced current collapse. As a result, careful engineering of the carbon concentration together with the undoped GaN channel thickness is crucial to achieve robust devices, which can, thus, deliver high device performance with superior voltage operation while using short gate lengths.

Role of AlGaN back-barrier in enhancing the robustness of ultra-thin AlN/GaN HEMT for mmWave applications

In this work, the robustness of three different AlN/GaN structures targeting high frequency applications with ultra-thin 3 nm AlN barrier and devices with different gate-to-drain spacing are evaluated. Devices were step-stress tested in off-state, semi-on-state, and on-state conditions. Different failure mechanisms were identified. Devices without AlGaN back-barrier show a fast increase of gate leakage current, leading to a significant degradation of electrical parameters such as the threshold voltage Vth, extrinsic transconductance GM, drain saturation current IDSsat and on-resistance Ron. This study reveals the role of the AlGaN back-barrier in the improvement of the device robustness presenting a good electrical parameters stability. The presence of an AlGaN back-barrier on low carbon- doped buffer leads to better gate leakage current stability, low trapping-related and self-heating mechanisms. The AlGaN back-barrier shows an extension of the operational Safe Operating Area for AlN/GaN technology.

Efficacy of back barrier engineered Π-gate InAlN/GaN high electron mobility transistors for high-power applications

This work investigates thin-barrier InAlN/GaN high electron mobility transistors (HEMTs) for high-power applications through technology computer-aided design (TCAD) simulations. To begin with, the TCAD simulations were first calibrated with an in-house fabricated InAlN HEMT sample for both DC and pulsed characteristics. The thin-barrier InAlN/GaN HEMTs showed a large leakage current through the gate electrode due to high gate injection, which severely degrades the breakdown characteristics of the device and thus acts as a bottleneck for high-power applications. To improve the two-dimensional electron gas confinement, and consequently reduce the bulk leakage, a back-barrier technique was used. The resistive GaN buffer was replaced with an AlGaN back-barrier that improved the breakdown characteristics at the cost of output power density. Thus, to scale up the output power density and further optimize the breakdown characteristics a Π-shaped gate was introduced to limit the gate leakage current through the InAlN barrier by virtue of its improved hot electron reliability. Coupled with the AlGaN back-barrier, the Π-gate significantly improved the breakdown characteristics to achieve high output power densities, albeit with minor trade-offs to the device gain. To elucidate the compatibility with high-power applications, all the device architectures were dynamically characterized by pulsed I–V simulations and the trap-related dispersive effects were investigated. The Π-shaped gate coupled with an AlGaN back-barrier outperforms conventional architectures by exercising superior electrostatic control over the channel and exhibiting a high linearity for high-power millimeter-wave applications.

Fabrication of Low On‐Resistance and Normally Off AlGaN/GaN Metal Oxide Semiconductor Heterojunction Field‐Effect Transistors with AlGaN Back Barrier by the Selective Area Regrowth Technique

Herein, a recessed‐gate AlGaN/GaN metal oxide semiconductor heterojunction field‐effect transistor (MOS‐HFET) with an AlGaN back‐barrier layer fabricated by a selective area regrowth (SAG) technique is investigated. A recessed‐gate structure enables normally off operation required for power‐switching applications. A thin AlGaN/GaN channel and the AlGaN back‐barrier structures are fabricated on a Si substrate by metal–organic chemical vapor deposition. The 50 nm thick, thin GaN channel layer with a smooth surface is grown on the AlGaN back‐barrier layer. A recessed‐gate structure is successfully formed by SAG of an AlGaN/GaN layer on the thin AlGaN/GaN layer. The regrown AlGaN/GaN high electron mobility transistor structure shows lower sheet resistance owing to high concentration and high mobility of a two‐dimensional electron gas. Transfer characteristics of the thin AlGaN/GaN channel MOS‐HFETs show normally off operation as a consequence of using the AlGaN back‐barrier structure. Channel mobility becomes five times higher than that of GaN channel in the case of using the thin AlGaN/GaN channel. These results indicate that the regrown thin AlGaN/GaN channel MOS‐HFET has the potential to realize low on‐resistance and normally off operation.

An enhancement-mode AlInN/GaN HEMTs combining intrinsic GaN cap layer and AlGaN back barrier layer

An AlInN/GaN high electron mobility transistors (HEMTs) is designed to deplete the two-dimensional electron gas (2DEG) under the gate by using the polarization modulation effect of the GaN cap layer and the AlGaN back barrier layer. Through theoretical analysis, the thin AlInN barrier layer can increase the gate control capability and improve the transconductance of the device. On the other hand, the intrinsic GaN cap layer without doping can reduce the complexity of the fabrication. The threshold voltage (VTH) of the device increases with the increase of the thickness of the cap layer and the Al component of the back barrier layer. Increasing Al component in the back barrier can improve the device performance. This structure has the advantages of simplified adjustment of the threshold voltage, high current density, simply fabrication process and to provide a better design solution for HEMTs in power applications.

A comparative analysis of GaN and InGaN/GaN coupling channel HEMTs on silicon carbide substrate for high linear RF applications

In this work, we report comparative analysis of stable transconductance and high gain linearity for AlGaN and InAlN barrier-based HEMTs using an InGaN/GaN coupling channel with AlGaN back-barrier on Silicon carbide (SiC) substrate. The proposed InGaN/GaN coupling channel HEMTs showed flat transconductance and suppressed gm derivatives. LG 55 nm AlGaN (InAlN)/GaN conventional HEMTs showed 2.9 (3.8) A/mm of IDS, 0.59 (0.62) S/mm of transconductance (gm), 51.86 (27.42) V of VBR, 212 (233) GHz of fT, 242 (255) GHz of fMAX. Whereas, Lg 55 nm InGaN/GaN coupling channel-based AlGaN (InAlN) HEMT showed 3.3 (5.1) A/mm of IDS, 0.67 (0.71) S/mm of transconductance (gm), 53.12 (41.25) V of VBR, 251 (273) GHz of fT, 262 (283) GHz of fMAX. Moreover, high gate voltage swing (GVS) and larger OIP3 of proposed coupling channel HEMTs are suitable candidates for next-generation high linear RF applications.

Effects on RF Performance for AlGaN/GaN HEMT on Si Substrate with AlGaN Buffer Engineering

In this paper, we demonstrate the AlGaN/GaN high electron mobility transistors (HEMTs) on Si substrate using an AlGaN back-barrier (BB) and super-lattice (SL) buffer to achieve high breakdown and low current collapse (CC) properties for Radio-frequency (RF) applications. The HEMTs also demonstrated low initial vertical leakage current, high thermal stability, and smaller drain-lag with reduced leakage currents as compared to the devices using step-graded (SG) AlGaN buffer. Also, the device showed improved RF performances with higher f T and f max as compared to the devices with SG buffer.

Low Trapping Effects and High Electron Confinement in Short AlN/GaN-On-SiC HEMTs by Means of a Thin AlGaN Back Barrier

In this paper, we report on an enhancement of mm-wave power performances with a vertically scaled AlN/GaN heterostructure. An AlGaN back barrier is introduced underneath a non-intentionally doped GaN channel layer, enabling the prevention of punch-through effects and related drain leakage current under a high electric field while using a moderate carbon concentration into the buffer. By carefully tuning the Al concentration into the back barrier layer, the optimized heterostructure offers a unique combination of electron confinement and low trapping effects up to high drain bias for a gate length as short as 100 nm. Consequently, pulsed (CW) Load-Pull measurements at 40 GHz revealed outstanding performances with a record power-added efficiency of 70% (66%) under high output power density at VDS = 20 V. These results demonstrate the interest of this approach for future millimeter-wave applications.

Investigation of back barrier material effects on the scalability of Fe-doped recess-gated AlN/GaN HEMTs for next generation RF power electronics

GaN HEMTs (high-electron-mobility transistors) on SiC (silicon carbide) wafer with back barrier structures have gained much attention owing to the improved sheet charge density through enhanced carrier confinement, which can accomplish further improvement of RF performance. In this simulation work, the influence of different back barrier materials on RF & DC performance of Fe-doped recess gated AlN/GaN HEMT is explored. This paper also investigates the back barrier thickness effects on carrier confinement of each structure; the results show that better confinement of carriers in the GaN channel is accompanied by an optimized thickness for each back barrier material besides which, the performance is comparatively degraded. As back-barrier based structures have the capability to reduce SCEs by enhancing the carrier confinement, the investigation is done to analyse the impact of gate length scalability on the DC & RF characteristics of the proposed device. Lg = 60 nm Fe-doped recess-gated AlN/GaN HEMT with an optimized AlGaN back barrier thickness of 60 nm provided a peak drain current of 4.63 A/mm, the highest transconductance of 2333 mS/mm, and the highest cut-off frequency of 316.6 GHz, and this outstanding DC/RF performance is due to strong carrier confinement and reduced leakage current. This outstanding performance makes them a potential choice for future RF power electronics and MMIC design.

The DC Performance and RF Characteristics of GaN-Based HEMTs Improvement Using Graded AlGaN Back Barrier and Fe/C Co-Doped Buffer

In this article, the impact of the graded AlGaN back barrier and Fe\C co-doping buffer structure on the AlGaN <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$/$ </tex-math></inline-formula> GaN high electron mobility transistors (HEMTs) is proposed and systematically investigated. Due to effective suppression of Fe tail in unintentionally doped GaN (uid-GaN) layer by the insertion of the thick graded AlGaN back barrier layer, a large maximum drain current density and a transconductance peak are achieved. Meanwhile, the breakdown voltage is significantly improved by the use of Fe\C co-doping GaN buffer design. More importantly, it is revealed that the graded-AlGaN design can reduce the range and intensity of the electrical potential distribution in uid-GaN layer and then effectively suppress the acceptor-induced trapping\detrapping effect under high drain voltage. The RF small-signal performance of Fe-doped GaN (GaN:Fe)\C co-doping buffer HEMT exhibits significant improvement. In addition, load-pull measurement at 8 GHz revealed that a saturation power increases from 38.04 to 41.07 dB, a power gain increases from 9.06 to 10.58 dB, and an associate power-added-efficiency (PAE) increased from 40.27% to 50.18%. Our proposed GaN-based epitaxial structure can not only suppress the gate lag by reduce AlGaN surface electric field, but it can also suppress the drain lag by reduce the amplitude and range of potential distribution. It indicates that our proposed device has great potential for future high-voltage RF power amplifier application.

Reduction in RF Loss Based on AlGaN Back-Barrier Structure Changes.

We designed a high electron mobility transistor (HEMT) epitaxial structure based on an AlGaN/GaN heterojunction, utilizing Silvaco TCAD, and selected AlGaN with an aluminum composition of 0.1 as the back-barrier of the AlGaN/GaN heterojunction. We enhanced the confinement of the two-dimensional electron gas (2DEG) by optimizing the structural parameters of the back barrier, so that the leakage current of the buffer layer is reduced. Through these optimization methods, a lower drain leakage current and a good radio frequency performance were obtained. The device has a cut-off frequency of 48.9 GHz, a maximum oscillation frequency of 73.20 GHz, and a radio frequency loss of 0.239 dB/mm (at 6 GHz). This work provides a basis for the preparation of radio frequency devices with excellent frequency characteristics and low RF loss.

GaN based trigate HEMT with AlGaN back-barrier layer: proposal and investigation

In the present work, we have proposed a GaN based trigate high electron mobility transistor with AlGaN back-barrier (BB) layer that effectively suppress the punch-through effects as compared to the conventional device without BB layer. Furthermore, the device performance of the proposed structure has been studied by varying the distance (t ch) between AlGaN (barrier)/GaN (channel) to AlGaN (BB)/GaN (channel) interface from 100 nm to 500 nm. It is observed that the optimization of barrier/channel to BB/channel distance (t ch) is beneficial for minimizing the off-state leakage current and thereby, a high ON/OFF ratio (∼108) and nearly ideal subthreshold slope (∼64–66 mV decade−1) is achieved. Subsequently, it is shown that the optimized distance of t ch = 300 nm results in superior breakdown voltage (∼198 V) and cut-off frequency (∼81 GHz), leading to excellent Johnson figure-of-merit. The proposed device promises great potential for use in next generations high-power microwave applications.

Investigation of ON-State Breakdown Mechanism in AlGaN/GaN HEMTs with AlGaN Back Barrier

The temperature-dependent ON-state breakdown BVON loci of AlGaN/GaN high-electron-mobility transistors (HEMTs) with an AlGaN back barrier were investigated using the gate current extraction technique. The impact ionization of acceptor-like traps was revealed to be responsible for the ON-state breakdown in HEMTs as a 2D electron gas (2DEG) channel is marginally turned on. The characteristic electric field Ei of impact ionization was extracted, exhibiting a U-shaped temperature dependence from 40 to −30 ∘C, with minimum Ei occurring at −10 ∘C. The impurity scattering effect of acceptor-like traps in AlGaN/GaN heterostructures is suggested to be responsible for the negative temperature dependence of BVON and Ei below −10 ∘C.

High temperature and trap sates characterization of Al0.35Ga0.65N/GaN/Al0.25Ga0.75N double heterojunction

In this work, the high temperature and trap sates characteristics have been investigated for Al0.35Ga0.65N/GaN/Al0.25Ga0.75N double heterojunction. By using high-Al AlGaN barrier and back barrier layers, the maximum current at 200 ℃ is reduced by 35%, which is much lower than 58% reduction for conventional double heterojunction. The ION/IOFF ratio is as high as 4.6 × 107 at 200 ℃, demonstrating the excellent high-temperature characteristics of high-Al AlGaN barrier layer. For Al0.35Ga0.65N/GaN/Al0.25Ga0.75N double heterojunction, the shallow trap state density DT of 3.57 × 1011–1.37 × 1013 cm−2eV−1 is located at energy ET in the range of 0.259–0.322 eV, and the deep DT of 0.237–2.19 × 1013 cm−2eV−1 is located at energy ET in the range of 0.388–0.441 eV. The trap densities for heterojunction with high-Al AlGaN barrier and back barrier are lower than those for heterojunction with low-Al AlGaN back barrier.

Current transport mechanism of AlGaN-channel Schottky barrier diode with extremely low leakage current and high blocking voltage of 2.55 kV

This Letter presents work on lateral AlGaN-channel Schottky barrier diodes (SBDs) with impressive reverse blocking characteristics and low onset voltage (VON). A low reverse current (IR) of 28 nA/mm and low VON of 0.60 V are obtained by utilizing an AlGaN back barrier layer with high Al composition and low work function metal as the anode. The fabricated AlGaN-channel SBD with an anode–cathode distance (LAC) of 30 μm achieves a high blocking voltage of 2.55 kV and a power figure-of-merit of 363 MW/cm2. Meanwhile, the current transport mechanism of AlGaN-channel SBDs goes through thermionic emission, thermionic field emission, and trap-assisted tunneling as the reverse bias is gradually increased. The thermal activation energy (EA) is calculated to be 141.3 meV at high reverse bias. IR at 475 K is only 3 μA/mm, which shows an adequate barrier height for rectifying at high temperature even with low VON. The AlGaN-channel SBDs show great promise for next-generation power electronics with balanced forward and reverse characteristics.

Influence of back barrier layer thickness on device performance of AlGaN/GaN MOS-HEMT

In this work, we have performed the influence of back barrier layer thickness variation on AlGaN/GaN Metal Oxide Semiconductor High Electron Mobility Transistor (MOS-HEMT) device with 0.5 µm Schottky gate length. The AlGaN back barrier layer presented increases the conduction band with respect to GaN channel layer so that more no of electron confinement into the GaN channel layer and improve the high-frequency performance. The effect of the back-barrier layer thickness is performed by using 2-D TCAD Atlas Silvaco numerical simulation tool by taking Hydrodynamic mobility model. Due to a large amount of two-dimensional electron gas (2-DEG) density at the AlGaN/GaN heterointerface of the MOS-HEMT device higher drain current density is obtained. The 2-D simulation is carried out with a variation of back barrier layer thickness for various device parameter such as transfer characteristics (Id-Vg), drain current with a drain voltage (Id-Vd), transconductance (gm), drain induced barrier lowering (DIBL), conduction band energy and electron concentration into the channel. In this simulation, we have also performed the RF performance like a gate to source capacitance (Cgs) and current gain cut-off frequency of AlGaN/GaN MOS-HEMT device. The results obtained by variation of AlGaN back barrier layer thickness can be a better solution in future analog and RF device application. Copyright © 2018 VBRI Press.

Influence of AlGaN back-barrier on irradiation tolerance of AlGaN/AlN/GaN HEMTs

The introduction of AlGaN back-barrier increases the confinement of the two-dimensional electrons and further improves the high-frequency performance of AlGaN/AlN/GaN HEMTs. We investigate the effect of AlGaN back-barrier on irradiation tolerance of GaN-based HEMTs in terms of 2DEG density and electron scattering. The Schroödinger-Poisson equations are solved using a self-consistency method to calculate the conduction energy band and 2DEG density, which takes into account the Ga vacancies caused by proton irradiation. The electron scattering depending on phonons, ionized impurities, dislocations, and the interface AlN/GaN interface roughness is also studied, and 2DEG mobility is calculated for GaN-based HEMTs with and without back-barrier. The calculation results show that, after the same proton irradiation, the 2DEG density and mobility in GaN-based HEMT with back-barrier decrease more dramatically than that without back-barrier. The results may give some suggestions to those designing the radiation-resistant GaN-based HEMTs.