A microwave power double-heterojunction high electron mobility transistor

This paper reports a systematic theoretical study of Al0.23Ga0.77N/GaN/AlxGa1‐xN double‐heterojunction high electron mobility transistors (DH‐HEMTs) with a boron‐doped GaN cap layer under the gate. Boron containing GaN is a desired replacement for Schottky gate in DH‐HEMT to improve the resistivity and surface qualities of GaN cap with good structural properties, and the influence of polarization field in the GaN cap layer can be used to lift up the conduction band for normally OFF operation of the device. This study shows that the B‐doped GaN cap layer offers excellent device characteristics such as high threshold voltage VT of 1.92 V, steep subthreshold slope of ~ 72 mV/dec, high ON/OFF drain current ratio of ~ 107, extremely low subthreshold drain current Ids,LL ~ 10−9 A/mm, and low gate leakage current, Igs,LL less than 10−15 A/mm. For an Al0.07Ga0.93N back‐barrier/buffer, a high OFF‐state breakdown voltage (VBR,OFF) of 865 V is achieved at Ids = 1 mA/mm and Vgs = 0 V with substrate grounded and gate‐to‐drain distance and gate field plate length of 10 and 5 μm, respectively. The simulated high VT and VBR,OFF values are achieved due to the presence of increased back‐barrier height of the AlxGa1‐xN buffer and high Schottky barrier between the Boron‐doped GaN cap and gate metal. Finally, a DC to AC full‐bridge inverter circuit is designed to evaluate the benefit of B‐doped DH‐HEMTs based switching devices for an ultra‐low‐loss inverter circuit.

With the proliferation of semiconductor materials nowdays, high electron mobility transistor (HEMT) plays an imperious preamble in electronic industry. Latest double heterojunction HEMTs shows improved performance concerning carrier confinement but shows higher leakage current that is found here. The device specifies a 3-nm thin InGaN layer inserted into the conventional AlGaN/GaN HEMT structure. Gate leakage mechanism in AlGaN/GaN/InGaN/GaN HEMT is analyzed here for versatile applications for a long range of electric field and temperature sustainability. Comparing with measured data Poole-Frenkel (PF) emission and Fowler-Nordheim(FN) tunneling current has been considered as a dominant leakage mechanism in reverse bias condition. In contrast to conventional HEMT, Fowler-Nordheim(FN) tunneling current component is found higher in DH-HEMT here. In this research work an additional trap assisted tunneling current is also found to be presented here like the conventional HEMT. Possible reason of leakage current in double heterojunction HEMTs are explained here along with illustrations.

Undoped AlGaN-GaN power high electron mobility transistors (HEMTs) on sapphire substrate with 470-V breakdown voltage were fabricated and demonstrated as a main switching device for a high-voltage dc-dc converter. The fabricated power HEMT realized a high breakdown voltage with a field plate structure and a low on-state resistance of 3.9 m/spl Omega//spl middot/cm/sup 2/, which is 10 /spl times/ lower than that of conventional Si MOSFETs. The dc-dc converter operation of a down chopper circuit was demonstrated using the fabricated device at the input voltage of 300 V. These results show the promising possibilities of the AlGaN-GaN power HEMTs on sapphire substrate for future switching power devices.

Numerous studies have highlighted the significant potential of III-N High Electron Mobility Transistors (HEMTs) for power applications. Despite the commercialization of III-N HEMTs fabricated on silicon-based heterostructures, they have captured only a small share of the power electronics market. A major challenge is the introduction of electrically active defects, such as traps, during device fabrication and operation. These defects, which are unevenly distributed in the device heterostructure, contribute to device aging and severely impact reliability and operational lifespan. This work investigates the temperature dependence of key device characteristics in commercial III-N HEMTs. Using the generalized Unified Charge Control Model (UCCM), we extract temperature-dependent parameters, including electron mobility, output conduction, and maximum driving current, from in situ measured device characteristics. By distinguishing between two temperature-related effects, ambient operating temperature vs. self-heating, we found that at high drain-to-source and gate-to-source voltages, self-heating becomes the dominant factor. To account for this, we incorporate the self-heating phenomenon into the UCCM model and introduce a figure of merit for comparing the impact of self-heating across devices from different manufacturers. Additionally, we analyze the maximum driving current in relation to the material properties of III-N heterojunctions. The temperature profiles inside the HEMT devices (e.g., distribution of temperature along the channel) will be evaluated experimentally and by TCAD simulations combined with heat transfer finite element simulations. Future applications of our model aim to link self-heating with long-term reliability and lifetime, two critical factors that currently limit the broader adoption of III-N HEMTs in power electronics.

Highly uniform AlGaN/GaN high electron mobility transistors (HEMTs) were fabricated on a low-cost 3-inch conductive n-SiC substrate for the first time. Average values of threshold voltage (Vth) and transconductance (gm) of AlGaN/GaN HEMT were -1.55V and 194 mS/mm. Standard variations of those values were only 0.15V and 3.9mS/mm across an entire 3-inch conductive substrate. The 1-mm-gate-periphery GaN HEMT chip, which was operated at 60V, achieved high CW output power density of 7.0W/mm, with a high linear gain of 22.2dB and power added efficiency (PAE) of 70% at 2.14GHz. Standard variations of power density, linear gain and PAE at 50V were only 0.42W/mm, 0.2dB and 3.0 point, respectively. This low-cost highly uniform high-gain chip technology is sufficient for mass production of wireless base station application using GaN-HEMTs.

In this paper, study on the heat transfer of GaN-based high electron mobility transistors (HEMTs) on SiC substrate was carried out utilizing transient thermal analysis method. Gate-source voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{{\text {gs}}}$ </tex-math></inline-formula> ) is used as the temperature sensitive parameter. Linear relationship ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${K}$ </tex-math></inline-formula> factor) between <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{{\text {gs}}}$ </tex-math></inline-formula> and temperature is obtained. It is found that the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${K}$ </tex-math></inline-formula> factor is proportional with drain-source current and does not change with the variation of drain-gate voltage in the linear region of HEMT. We proposed and proved that the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${K}$ </tex-math></inline-formula> factor should be calibrated with HEMT working at linear region to insure the accuracy of subsequent thermal evaluation. Moreover, the temperature rise and thermal resistance keeps as constant when <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${K}$ </tex-math></inline-formula> factors were calibrated within the linear region. Transient thermal measurement turns out to be a non-destructive, fast, and precise method for the thermal performance analysis and quality control of the power HEMTs.

GaN-based high electron mobility transistors (HEMTs) are expected to be used in battery charger inverters in an electric vehicle because of their low on-resistance and high switching speed. Currently, AlGaN/GaN HEMTs fabricated on Si substrates are rapidly becoming more widespread in compact AC adapters. However, in order to achieve high-voltage devices for in-vehicle applications, etc., thicker epitaxial films are required, and various problems arise, such as an increase in the growth time of the epitaxial film and an increase in the warping of the wafer due to the stress of the epitaxial film, making it difficult to manufacture epitaxial wafers suitable for high-voltage AlGaN/GaN HEMTs. Therefore, we have focused on GaN substrates that can be grown by homoepitaxy and have been developing high-breakdown-voltage AlGaN/GaN HEMTs.Kuzuhara et al. fabricated AlGaN/GaN HEMTs using iron-doped GaN freestanding substrates and demonstrated a breakdown voltage of over 5000 V. They also showed that the higher the resistivity of the semi-insulating layer, the higher the breakdown voltage [1].Iso et al. used the HVPE method to dope high-resistivity freestanding GaN substrates with iron and carbon, and showed that the resistivity of the carbon-doped substrates was higher than that of the iron-doped substrates [2].In this report, we used two types of dual-layer semi-insulating GaN substrates purchased from Mitsubishi chemical corporation. Thick semi-insulating layers containing carbon of a 2E18 cm-3 and iron of a 1E18 cm-3 were grown on conductive GaN substrates. The resistivity of the carbon-doped layer and the iron-doped layer was 1E12ohm⋅cm and 1E8ohm⋅cm, respectively, according to the specification sheet. And AlGaN/GaN HEMTs were fabricated on each substrate and evaluated their three-terminal breakdown voltage. The epitaxial structure consisted of a 3-nm-thick GaN cap layer, a 17-nm-thick AlGaN barrier layer with an Al composition of 22%, a 200-nm-undoped GaN channel layer, and a 300-nm-thick GaN buffer layer doped with carbon of a 1E17 cm-3. Ion implantation was used to isolate the devices. The gate length (Lg), gate width (Wg), and gate-to-source distance (Lgs) were fixed at 2, 100 and 2 µm, respectively, while the gate-to-drain distance (Lgd) was varied between 10 µm, 15 µm and 20 µm. The gate-field-plate length and source-field-plates were also fixed at 2, 4 µm, respectively. The curve tracer (IWATSU CS3200) was used to measure the electrical characteristics of AlGaN/GaN HEMTs. Since the top surface of the device does not have a passivation film, to prevent dielectric breakdown on the device surface, the off-state breakdown voltage of the GaN HEMTs were measured while soaked in Solvay's Garten solution. In both GaN HEMTs on the different semi-insulating substrates, the off-state breakdown voltage increased linearly with the gate-to-drain distance and was successfully confirmed to exceed 1400 V at Lgd of 20 μm. The breakdown electric field in the AlGaN/GaN HEMTs are about 0.7 MV/cm, no resistivity dependence on the breakdown electric field was observed. Kuzuhara et al. reported that, in addition to the resistivity of the semi-insulating GaN layer, isolation technology is also important for achieving high breakdown voltage [1]. When the breakdown voltage of the isolation regions formed by ion-implantation was examined, the breakdown electric field of the carbon-doped and iron-doped insulating substrate was 0.75 MV/cm, 0.86 MV/cm, respectively. These breakdown electric fields are close to the off-state breakdown field of AlGaN/GaN HEMTs, which suggests that the isolation process for AlGaN/GaN HEMTs has not been yet optimized. In order to further improve the breakdown voltage of AlGaN/GaN HEMTs on semi-insulating GaN substrates, it is necessary to further evaluate their electrical characteristics and investigate the cause of decrease in breakdown field strength compared to the physical properties of GaN. This research was performed using C-TEFs (Center for Integrated Research of Future Electronics, Transformative Electronics Facilities), Nagoya university, Japan. This research was conducted with the support of the Ministry of the Environment “Carbon Neutral Technology Research and Development Program”.Reference[1] M. Kuzuhara, J. T. Asubar, and H. Tokuda, ”AlGaN/GaN high-electron-mobility transistor technology for high-voltage and low-on-resistance operation,” Jpn. J. Appl. Phys, 55, 070101, (2016).[2] K. Iso, H. Ikeda, T. Mochizuki, T. Odai, and S. Izumikawa, ”Dual-Layer Semi-insulating GaN Substrates Doped with Fe, C, and Mn,” Phys. Status. Solidi, B, 260, 2200489 (2023).

In this paper, several methods suitable for real time on-chip temperature measurements of power AlGaN/GaN based high-electron mobility transistor (HEMT) grown on SiC substrate are presented. The measurement of temperature distribution on HEMT surface using Raman spectroscopy is presented. We have deployed a temperature measurement approach utilizing electrical I-V characteristics of the neighboring Schottky diode under different dissipated power of the transistor heat source. These methods are verified by measurements with micro thermistors. The results show that these methods have a potential for HEMT analysis in thermal management. The features and limitations of the proposed methods are discussed. The thermal parameters of materials used in the device are extracted from temperature distribution in the structure with the support of 3-D device thermal simulation. The thermal analysis of the multifinger power HEMT is performed. The effects of the structure design and fabrication processes from semiconductor layers, metallization, and packaging up to cooling solutions are investigated. The analysis of thermal behavior can help during design and optimization of power HEMT.

A novel AlGaN/GaN/InGaN/GaN double heterojunction high electron mobility transistors (DH‐HEMTs) structure with an AlN interlayer on sapphire substrate has been grown by MOCVD. The structure featuered a 6‐10 nm In0.1Ga0.9N layer inserted between the GaN channel and GaN buffer. And we also inserted one ultrathin AlN interlayer into the AlGaN/GaN interface, which significantly enhanced the mobility of two‐dimensional electron gas (2DEG) existed in the GaN channel. AFM result of this structure shows a good surface morphology and a low dislocation density, with the root‐mean‐square roughness (RMS) of 0.196 nm for a scan area of 5 μm× 5 μm. Temperature dependent Hall measurement was performed on this sample, and a mobility as high as 1950 cm2/Vs at room temperature (RT) was obtained. The sheet carrier density was 9.89×1012cm–2, and average sheet resistance of 327 Ω/sq was achieved. The mobility obtained in this paper is about 50% higher than other results of similar structures which have been reported. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

We have demonstrated an Al-free Ti/Mo/Ta/Au-based n-type ohmic contact for high power AlGaN/GaN high electron mobility transistors (HEMTs). Our fabricated ohmic contact has an extremely smooth surface morphology and almost the same specific contact resistance (ρc) of 9.1 × 10–6 Ωcm2 as conventional Ti/Al-based contacts. We have found that Mo, which is generally inserted as a barrier layer, has the effect of reducing contact resistance. Moreover, we have applied this ohmic technique to fabricate an AlGaN/GaN HEMT and obtained sufficient transconductance (gm) of 210 mS/mm. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Field effect transistors: FETs and HEMTs

1-mm gate periphery AlGaN/AlN/GaN HEMTs on SiC with output power of 9.39 W at 8 GHz

We studied the performance of AlGaN/GaN double heterojunction high electron mobility transistors (DH-HEMTs) with an AlGaN buffer layer, which leads to a higher potential barrier at the backside of the two-dimensional electron gas channel and better carrier confinement. This, remarkably, reduces the drain leakage current and improves the device breakdown voltage. The breakdown voltage of AlGaN/GaN double heterojunction HEMTs (∼100 V) was significantly improved compared to that of conventional AlGaN/GaN HEMTs (∼50 V) for the device with gate dimensions of 0.5 × 100 μm and a gate—drain distance of 1 μm. The DH-HEMTs also demonstrated a maximum output power of 7.78 W/mm, a maximum power-added efficiency of 62.3% and a linear gain of 23 dB at the drain supply voltage of 35 V at 4 GHz.

γ-ray radiation effect has been studied on transport and noise properties of high electron mobility transistors (HEMTs) with gate lengths in the range from 350 to 150 nm at room temperature. Current-voltage (I-V) characteristics of the devices demonstrate higher radiation hardness to 60 Co γ-rays up to doses of 10 9 Rad at larger gate lengths. This confirms the very important role of surface passivation for channel transport of the HEMTs. The deviation of the I-V characteristics parameters saturated current, transconductance, channel conductance, and threshold voltage does not exceed 20% at highest radiation dose. The noise spectra of pre-irradiated devices and after γ-irradiation show different frequency dependences corresponding to different fluctuation processes in the HEMTs. The results are confirmed by dynamic current measurements of the channel conductivity.

With the wide band-gap of Gallium Nitride (GaN) and high mobility of 2 dimensional electron gas(2DEG), AlGaN/GaN high electron mobility transistors (HEMTs) are considered ideal candidates for high speed and power applications ranging from hybrid vehicles to space communication. Developments in the fabrication of GaN-based blue LEDs have been incorporated into the processing of AlGaN/GaN HEMT structures leading to improvements in amount of defects and dislocations resulting from heteroepitaxial growth. But evident degradation of dynamic performance of such devices due to the presence of as-grown defects still remains a subject of major concern. Using FLorida Object Oriented Device Simulator (FLOODS) TCAD, this work looks to analyze the impact of variation of defect properties as well as their spatial distribution and concentration on the steady-state and dynamic behavior of AlGaN/GaN HEMTs. FLOODS TCAD’s defect modeling capability will be applied to examine how multiple defect levels, of both donor and acceptor-like nature, can influence device performance. Particular emphasis will be on transient switching, such as gate-lag and drain-lag experiments, in consideration of the slow nature observed frequently in literature. While comprehensive investigation of the role of charged defects at the AlGaN/passivation layer interface on performance degradation has already been performed, contribution by as-grown defects, expected to be in significant quantity in the GaN buffer layer, is yet to be investigated. Building on experimental results, which suggest the presence of multiple acceptor-like defects, impact on steady-state current-voltage characteristics will be observed followed by an examination of how such defects will affect dynamic performance by isolating their contribution to transient and switching behavior. Figure: (a) Drain current against gate voltage at VDS=0.5V for HEMT A with 1 acceptor-like level (nT=1014 cm-3, Et=Ec-0.7eV) and HEMT B with 2 acceptor levels(Level 1: nT=1014 cm-3, Et=Ec-0.7eV; Level 2: nT=1015 cm-3, Et=Ec-0.5eV) (b) Optical pumping like detrapping of electrons, biasing at VDS=1V and VGS=0V. HEMT A(Level 1: nT=1013 cm-3, Ec-0.2eV, τ=1s; Level 2: nT=4x1013 cm-3, Ec-0.5eV, τ=1ms), HEMT B(Level 1: nT=1013 cm-3, Ec-0.2eV, τ=1s; Level 2: nT=1013 cm-3, Ec-0.5eV, τ=1ms) Figure 1