Gate-drain Spacing Research Articles

Comparison of performance in GaN-HEMTs on thin SiC substrate and sapphire substrates

We have investigated the electrical properties of GaN-based high electron mobility transistors (HEMT) on N+-type 4H SiC substrates and on sapphire substrate as a reference. The three-terminal off-state breakdown voltage (BV) as a function of gate-drain spacing (LGD) was measured. Both devices possess superior Baliga figure of merit parameters (BFOM= BV3/Ron). For GaN/Sapphire with LGD of 10 μm, Ron is 14.65 mΩ·cm2 and the corresponding BFOM yields a remarkably high value of 9.88 × 108V2·Ω−1·cm−2. Conversely, GaN/N+ SiC with 20 μm LGD exhibited an Ron of 43.23 mΩ·cm2 and a BFOM value of 1.55 × 107V2·Ω−1·cm−2. In addition, we perform AFM, XRD, SEM and TEM analysis to further explore the differences in the DC-current properties for these two distinct substrates. The XRD FWHM scales with surface roughness, moreover grain boundaries for GaN-based HEMTs grown on SiC are more distinct compared to Sapphire substrate. This study provide valuable insight into the structural and morphological disparities that may contribute to the observed variations in device performance.

Effect of Source–Drain Opposite Side Gate on the AlGaN/GaN High Electron Mobility Transistor Devices

To explore the influence of different gate positions on the performance of AlGaN/GaN high electron mobility transistor devices, two model structures are proposed in this paper: an inverted T‐type gate and source–drain opposite side structure (ITGS–DOSS), and an embedded ITGS–DOSS. It is shown in the simulation results that compared with the traditional T‐type gate structure, these two structures have better transfer characteristics and significantly reduce the on‐state resistance, which can effectively improve the virtual gate effect and suppress the current collapse effect. Furthermore, these two structures can also improve the frequency characteristics of the device, with a maximum cutoff frequency of about 625 and 635 GHz, respectively. The threshold voltage of the ITGS–DOSS is about −30 V, which is significantly shifted to the left compared to the traditional T‐type structure. With a gate–drain spacing of 4.4 μm, the breakdown voltage is still as high as 1661 V. As the device size and gate–drain spacing decrease, this structure has better voltage withstand characteristics, thus achieving low threshold and high breakdown device performance.

A Simulation Optimization Factor of Si(111)-Based AlGaN/GaN Epitaxy for High Frequency and Low-Voltage-Control High Electron Mobility Transistor Application.

The effects of barrier layer thickness, Al component of barrier layer, and passivation layer thickness of high-resistance Si (111)-based AlGaN/GaN heterojunction epitaxy on the knee-point voltage (Vknee), saturation current density (Id-sat), and cut-off frequency (ft) of its high electron mobility transistor (HEMT) are simulated and analyzed. A novel optimization factor OPTIM is proposed by considering the various performance parameters of the device to reduce the Vknee and improve the Id-sat on the premise of ensuring the ft. Based on this factor, the optimized AlGaN/GaN epitaxial structure was designed with a barrier layer thickness of 20 nm, an Al component in the barrier layer of 25%, and a SiN passivation layer of 6 nm. By simulation, when the gate voltage Vg is 0 V, the designed device with a gate length of 0.15 μm, gate-source spacing of 0.5 μm, and gate-drain spacing of 1 μm presents a high Id-sat of 750 mA/mm and a low Vknee of 2.0 V and presents ft and maximum frequency (fmax) as high as 110 GHz and 220 GHz, respectively. The designed device was fabricated and tested to verify the simulation results. We demonstrated the optimization factor OPTIM can provide an effective design method for follow-up high-frequency and low-voltage applications of GaN devices.

A Novel Ring-Gate AlGaN/GaN HEMT Device and Electrode Structure Optimization

In this paper, a novel ring-gate structure AlGaN/GaN HEMT device is proposed and fabricated successfully. When the gate-source spacing Lgs = 5 μm, gate-drain spacing Lgd = 7 μm, gate length Lg = 3 μm, the maximum drain current Idmax of this ring-gate AlGaN/GaN HEMT device improved by 161.8% comparing with the conventional structure device, the threshold voltage Vth increased by 66.7% from 1.65 V to 2.5 V. In order to further improve the performance of the device, a series of electrode structure optimization designs have been carried out. Firstly, the effect of source-drain electrode alloy type and etching depth under source-drain region on the transfer and output characteristics was investigated, we fabricated devices with two alloy electrodes of multi-layer Ti/Al/Ti/Al/Ti/Al/Ni/Au and single layer Ti/Al/Ni/Au, then perform groove etching under the source and drain electrodes, the etching depth is set to 10/20 nm, after analysis and calculation, it is found that among ring-gate and conventional-gate devices, the device with multi-layer electrodes and an etched depth of 10 nm performs best. Then, the influence of device size parameters on transfer and output characteristics was explored, devices with different Lg and Lgd were prepared, after testing it is found that with the increase of Lg, the Vth of the conventional-gate and ring-gate HEMT devices both showed a positive-shift trend, in conventional device Vth increased from 1.53 V to 1.7 V, and this value increased from 1.5 V to 2.5 V in ring-gate device; the saturation drain current decreases when Lg increasing, and the decrease of the ring-gate device is more obvious, from 51.28 mA at Lg = 3 μm to 24.48 mA at Lg = 6 μm; when Lds decreases, the Vth of the two structures doesn’t change significantly, but the output current increases with the reduction of Lds, among them, the Idmax of the conventional structure device at Lgd = 19 um is 79.07% lower than that at Lgd = 7 μm; the value of the ring-gate device is reduced by 113.7%. In addition, among all the above devices, the ring-gate devices all show better output characteristics and higher Vth than conventional devices.

High-Temperature Performance of AlN MESFETs With Epitaxially Grown n-Type AlN Channel Layers

We fabricated AlN metal semiconductor field effect transistors (MESFETs) with an epitaxially grown n-type AlN channel layer and characterized the temperature dependence of their device properties. As the temperature was varied from room temperature to 500°C, maximum drain current increased from 0.42 to 45 mA/mm and maximum transconductance increased from 0.05 to 4.5 mS/mm. This enhanced device performance with increasing temperature is attributed to increased carrier concentration in n-type AlN due to the large ionization energy of Si donors. Owing to a large Schottky barrier height of 2.08 eV for the Ni gate electrode/n-type AlN interface, reverse leakage current is less than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-{10}}$ </tex-math></inline-formula> A/mm at temperatures ranging from room temperature to 500°C. Off-state breakdown voltage is as high as 1720 V for gate-drain spacing of 16 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mu }\text{m}$ </tex-math></inline-formula> . These results indicate that AlN MESFETs are promising for high-voltage electronic devices operating at high temperature.

Lattice-matched AlInN/GaN multi-channel heterostructure and HEMTs with low on-resistance

In this paper, a high-performance multi-channel heterostructure based on lattice-matched AlInN/GaN has been reported. The stacking of five heterostructures yields a high two-dimensional electron gas density of 3.67 × 1013 cm−2 and a small sheet resistance (RSH) of 74.5 Ω/sq. Compared with the AlGaN/GaN sample with the same number of heterojunctions, the AlInN/GaN sample reduces the RSH by 51.2%. Since the AlInN barrier and GaN channel are lattice-matched, the strain defects caused by piezoelectric strain can be alleviated. The high-resolution x-ray diffraction results show that the total dislocation density in AlInN/GaN multi-channels is reduced by 18.9%. The calculation models of multiple-channel heterostructures are obtained to investigate the electron population and energy band diagram, and the calculated results are roughly consistent with the experimental results. With a gate–drain spacing of 11.5 μm, the on-resistance (RON) of the AlInN/GaN multi-channel HEMT was only 2.26 Ω mm, indicating that the lattice-matched multi-channel AlInN/GaN heterostructure can substantially enhance the current drive efficiency and improve the output performance of the devices.

A novel process for adjusting the gate-drain spacing in InP-HEMTs with

To enable mass production of InP-HEMTs operating in microwave and THz bands with T shaped gates as narrow as 30 nm, industry friendly processes for source/drain contacts are urgently needed. So far, although a self-aligned technique using pre-fabricated T shaped gates as masks has been reported, the limited adjustments of the gate-source/gate-drain spacings by this process restricts the device performance optimization. In this work, 30 nm T shaped gates by a novel bilayer of RE650/UV5 are successfully fabricated. Using the T shaped gates as masks, a self-aligned process with angled evaporation of contacts has been developed, which offers us a new degree of freedom to adjust the locations of the contacts in a large range for the optimization of the device structure. By numerical simulations, the influences of the gate-source/gate-drain spacings on the device performance such as the DC/RF characteristics, the spatial distributions of both electric fields and the carrier density, etc., have been studied. The property of InP-HEMTs with the contacts by the traditional alignment is also compared. Optimized spacings for maximizing the device performances are figured out. The progress achieved in this work should be an important guide in both the manufacturing process development and the construction of the device structure.

AlN/GaN Superlattice Channel HEMTs on Silicon Substrate

High-electron-mobility transistor (HEMT) with AlN/GaN superlattice (SL) channel has been demonstrated on a silicon substrate. High OFF-state breakdown voltage ${V}_{\text {BR}}$ of 670 V (gate–drain spacing ${L}_{\text {GD}} = 8\,\,\mu \text{m}$ ) along with a maximum output current of 196 mA/mm, a channel electron total mobility of 507 cm2/ $\text{V}\cdot \text{s}$ , and an ON/ OFF ratio of over 107 was achieved in this novel HEMT. For the HEMT with ${L}_{\text {GD}} = 22\,\,\mu \text{m}$ , a high ${V}_{\text {BR}}$ of 1700 V was obtained with substrate floated. The influence of ${L}_{\text {GD}}$ , the gate–source voltage ${V}_{\text {GS}}$ , the isolated pattern, and substrate grounded and floated type was discussed to analyze the breakdown characteristics of HEMT. We investigated the trap states in the AlN/GaN SL channel of HEMTs by frequency-dependent capacitance and conductance measurements. A trap state density of $7.4\times 10^{{12}}$ – $1.2\times 10^{{13}}$ cm−2eV−1 is located at ${E}_{\text {T}}$ in a range of 0.29–0.33 eV of the main channel, while the trap state density in the parasitic channel decreases from $3.9\times 10^{{11}}$ cm−2eV−1 at an energy of 0.27 eV to $1.1\times 10^{{11}}$ cm−2eV−1 at an energy of 0.38 eV. The fabricated AlN/GaN SL channel HEMTs in this work reveal a great step toward the realization of the SL channel HEMTs on cost-effective silicon substrate and provide a novel technology for AlGaN multichannel devices to obtain high output current.

Electrostatic Engineering Using Extreme Permittivity Materials for Ultra-Wide Bandgap Semiconductor Transistors

The performance of ultra-wide bandgap semiconductors like β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> is critically dependent on achieving high average electric fields within the active region of the device. In this article, we show that dielectrics like BaTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> with extremely high dielectric constant can provide an efficient field management strategy by improving the uniformity of electric field profile within the gate-drain region of lateral field-effect transistors. Using this strategy, we achieved high average breakdown field of 1.5 and 4 MV/cm at gate-drain spacing (L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">gd</sub> ) of 6 and 0.5 μm, respectively in β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , at a high channel sheet charge density of 1.6 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . The high channel charge density along with the high breakdown field enabled a record power figure of merit (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">br</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> ) of 376 MW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at a gate-drain spacing of 3 μm.

Low Static and Dynamic On‐Resistance with High Figure of Merit in AlGaN/GaN High Electron Mobility Transistors on Chemical Vapor Deposited Diamond

AlGaN/GaN high electron mobility transistors (HEMTs) on chemical vapor deposited (CVD) diamond with different gate–drain spacings (Lgd) are fabricated, and off‐state breakdown voltage (BVgd) along with a dynamic specific on‐resistance (Dyn.‐Ron,sp) is measured. Ron,sp is obtained in the range of 0.32–0.98 mΩ cm2 for HEMTs with Lgd of 2–8 μm. The HEMTs exhibit a BVgd of 415 V (Lgd = 8 μm) with a maximum lateral breakdown strength of 0.72 MV cm−1. The power device figure of merit (FOM) of ≈0.18 GW cm−2 is obtained from HEMTs with Lgd of 8 μm. The HEMTs exhibit about 60% of higher FOM when compared with the reported HEMTs on CVD diamond (≈0.11 GW cm−2). This is mainly due to the improvement of Ron,sp through the reduction of contact resistance (Rc) and sheet resistance (Rsh). The Dyn.‐Ron,sp is estimated from pulsed (pulse width/period = 200 ns/1 ms) ID–VD characteristics, which is comparable with the static Ron,sp. These improved results of the fabricated AlGaN/GaN HEMTs on CVD diamond make it a promising candidate for high‐voltage power switching device applications.

Ionizing radiation damage mechanism and biases correlation of AlGaN/GaN high electron mobility transistor devices

In this paper, the total dose effect on AlGaN/GaN high-electron-mobility transistor (HEMT) devices after &lt;sup&gt;60&lt;/sup&gt;Co γ-ray irradiation with a total dose of 1 Mrad(Si) was investigated at different biases (&lt;i&gt;V&lt;/i&gt;&lt;sub&gt;GS&lt;/sub&gt; = –3 V, &lt;i&gt;V&lt;/i&gt;&lt;sub&gt;DS&lt;/sub&gt;&lt;italic/&gt; = 0.5 V; &lt;i&gt;V&lt;/i&gt;&lt;sub&gt;GS&lt;/sub&gt; = –1.9 V, &lt;i&gt;V&lt;/i&gt;&lt;sub&gt;DS&lt;/sub&gt; = 0.5 V; &lt;i&gt;V&lt;/i&gt;&lt;sub&gt;GS&lt;/sub&gt; = 0 V, &lt;i&gt;V&lt;/i&gt;&lt;sub&gt;DS&lt;/sub&gt; = 0 V). The experimental results were analyzed using 1/&lt;i&gt;f&lt;/i&gt; low-frequency noise and direct current electrical characteristics. The electrical parameters degraded mostly under zero bias condition because of the radiation-induced defect charge of the oxide layer and the interface state. Wherein, the saturation drain current was reduced by 36.28%, and the maximum transconductance was reduced by 52.94%. The reason was that the oxide dielectric layer of AlGaN/GaN HEMT devices generated electron-hole pairs under γ-ray irradiation, and most of the electrons were quickly swept out of the oxide region corresponding to the gate-source and gate-drain spacer regions, and most of the holes remained in the oxide. Under the action of the built-in electric field, holes slowly moved towards the interface between the oxide and AlGaN, which depleted the two-dimensional electron gas of the channel.According to the McWhorter model, the low-frequency noise in the AlGaN/GaN HEMT devices results from random fluctuations of carriers, which are caused by the capture and release processes of carriers by traps and defect states in the barrier layer. The extracted defect densities in AlGaN/GaN HEMT devices increased from 4.080 × 10&lt;sup&gt;17 &lt;/sup&gt;cm&lt;sup&gt;–3&lt;/sup&gt;·eV&lt;sup&gt;–1&lt;/sup&gt; to 6.621 × 10&lt;sup&gt;17 &lt;/sup&gt;cm&lt;sup&gt;–3&lt;/sup&gt;·eV&lt;sup&gt;–1&lt;/sup&gt; under the condition of zero bias, and the result was in good agreement with test results of the direct currentelectrical characteristics. The damage mechanism was the radiation-induced defect charge in the oxide layer and the interface state, which increased the flat-band voltage noise power spectral density of the AlGaN/GaN HEMT devices. According to the charge tunneling mechanism, the spatial distribution of defect in the barrier layer was extracted, and the result also proved that the densities of radiation-induced defect charges under zero bias were more than the other biases. The experimental results showed that zero bias was the worst bias for AlGaN/GaN HEMT devices irradiation.

Breakdown voltage enhancement in GaN channel and AlGaN channel HEMTs using large gate metal height**Project supported by the National Key Science & Technology Special Project of China (Grant No. 2017ZX01001301), the National Key Research and Development Program of China (Grant No. 2016YFB0400100), and the National Natural Science Foundation of China (Grant Nos. 51777168 and 61801374).

A large gate metal height technique is proposed to enhance breakdown voltage in GaN channel and AlGaN channel high-electron-mobility-transistors (HEMTs). For GaN channel HEMTs with gate–drain spacing LGD = 2.5 μm, the breakdown voltage VBR increases from 518 V to 582 V by increasing gate metal height h from 0.2 μm to 0.4 μm. For GaN channel HEMTs with LGD = 7 μm, VBR increases from 953 V to 1310 V by increasing h from 0.8 μm to 1.6 μm. The breakdown voltage enhancement results from the increase of the gate sidewall capacitance and depletion region extension. For Al0.4Ga0.6N channel HEMT with LGD = 7 μm, VBR increases from 1535 V to 1763 V by increasing h from 0.8 μm to 1.6 μm, resulting in a high average breakdown electric field of 2.51 MV/cm. Simulation and analysis indicate that the high gate metal height is an effective method to enhance breakdown voltage in GaN-based HEMTs, and this method can be utilized in all the lateral semiconductor devices.

Electrothermal Characteristics of Delta-Doped $\beta$ -Ga2O3 Metal–Semiconductor Field-Effect Transistors

A 2-D electrothermal model of delta-doped b eta-gallium oxide ( $\beta $ -Ga2O3) metal–semiconductor field-effect transistor (MESFET) is developed by using TCAD Sentaurus to investigate its electrical and thermal characteristics. The temperature and electric field-dependent electron mobility model is incorporated to predict I – V characteristics of the FET, which are in good agreement with the measured I – V characteristics. We investigated the effect of bias voltages on an electric field, a current path, a volumetric heat generation profile, and a peak temperature at a given total heat dissipation. The peak temperature is observed to be significantly different at the same power dissipation but different bias conditions, e.g., the peak temperature is ~9 K higher when the drain-to-source voltage ( V ds) = 25 V and the gate-to-source voltage ( V gs) = −6 V compared with V ds = 8.3 V and V gs = 2 V at the total power dissipation of 1.16 W/mm. This difference is attributed to the change in the electron Joule heating profile with the applied bias voltage. The effects of the location of the delta-doping layer, the gate–drain spacing, and the source–drain spacing are investigated to guide a future device fabrication. The variation in these parameters mainly affects electrical characteristics such as ON-resistance, saturation current, threshold voltage, and so on. The thermal characteristics, such as peak temperature, temperature profile, and so on, are not significantly affected at a low-power dissipation.

Lateral 1.8 kV $\beta$ -Ga2O3 MOSFET With 155 MW/cm2 Power Figure of Merit

Lateral $\beta $ -Ga2O3 MOSFET for power switching applications with a 1.8 kV breakdown voltage and a record power figure of merit of 155 MW/cm2 are demonstrated. Sub- $\mu \text{m}$ gate length combined with gate recess was used to achieve low ON-state resistances with reasonable threshold voltages above −24 V. The combination of compensation-doped high-quality crystals, implantation-based inter-device isolation, and SiN x -passivation yielded in consistently high average breakdown field strengths of 1.8–2.2 MV/cm for gate–drain spacings between 2 and 10 $\mu \text{m}$ . These values outperform the results of more established wide-bandgap device technologies, such as SiC or GaN, and the major Ga2O3 material promise—a higher breakdown strength—is well demonstrated.

High Breakdown Voltage in RF AlN/GaN/AlN Quantum Well HEMTs

In evaluating GaN high-electron mobility transistors (HEMTs) for high-power applications, it is crucial to consider the device-level breakdown characteristics. This letter replaces the conventional AlGaN barrier and common AlGaN backbarrier with unstrained AlN, and it assesses the breakdown voltage of AlN/GaN/AlN quantum well HEMTs for gate-drain spacings in the range of 0.27– $5.1~\mu \text{m}$ . The results are highlighted by a high breakdown voltage of 78 V for a gate-drain spacing of 390 nm, among the best reported for submicron-channel devices. In addition, small-signal RF measurements showed record performance for HEMTs on the AlN platform, with $\text {f}_{\text {t}}/\text {f}_{\text {max}} = {161}/{70}$ GHz. The cut-off frequency and corresponding drain bias are benchmarked against the state-of-the-art GaN HEMTs using the Johnson figure of merit, with measured devices highlighted by a JFoM value of 2.2 THz $\cdot $ V. These results illustrate the potential for AlN/GaN/AlN quantum well HEMTs as a future platform for high-power RF transistors.

Two dimensional Green's function solution of threshold for junction less field effect transistors at low drain voltage

The threshold voltage is an important device and circuit parameter of any FET structure. In this paper, closed form analytic expressions are derived which determine the dependence of the threshold voltage of DGJLFETs on device dimensions, and the drain voltage. The depletion approximation and the two dimensional Green's function are used to determine the terminal behavior of the device. The model includes the effect of the depletion in the gate-drain spacing. The threshold condition is reached when the depletion layers from the two sides cross i.e., when the channel pinches off. The widening of the depletion layer under sub-threshold condition, and the movement of the pinch-off point with the drain voltage are calculated. Finally, the shift in threshold voltage and DIBL with the gate length is determined. We can also determine the potential distribution within the device. To verify our calculations, the analytical results are compared with 2-D numerical simulations. The agreement is especially good at low drain voltages.

Demonstration of lateral field-effect transistors using Sn-doped β-(AlGa)2O3 (010)

This paper demonstrates a modulation-doped fisseld-effect transistor (MODFET) and a metal-semiconductor field-effect transistor (MESFET) using β-(AlGa)2O3 (010). Ohmic contacts on Sn-doped (Al0.15Ga0.85)2O3 exhibit a fairly linear behavior, which has a specific contact resistivity and sheet resistance of 9 × 10−5 Ω cm2 and 75 kΩ/□, respectively. The MODFET with Sn-doped (Al0.08Ga0.92)2O3 barrier layer showed a breakdown voltage of 610 V for gate-drain spacing (Lgd) of 8 μm, while the (Al0.16Ga0.85)2O3-channel MESFET exhibited a breakdown voltage of 940 V for Lgd of 20 μm. These results show the great potential of (AlGa)2O3 transistors for high-power applications.

High Channel Conductivity, Breakdown Field Strength, and Low Current Collapse in AlGaN/GaN/Si &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$\delta$ &lt;/tex-math&gt; &lt;/inline-formula&gt;-Doped AlGaN/GaN:C HEMTs

This paper reports the AlGaN/GaN/ Si δ-doped AlGaN/GaN:C HEMT device on silicon with high channel conductivity, high breakdown field (E-field) strength, and low current collapse by using the Si-doped AlGaN back barriers. The Si δ-doped AlGaN back barrier was used to compensate for the reduction of channel conductivity as a result of a carbon-doped semiinsulating GaN buffer layer. The maximum drain current increases from 412 to 720 mA/mm, and peak extrinsic transconductance is improved from 103 to 210 mS/mm. Due to the reduction of electric field between the gate and drain along the GaN channel by inserting the Si δ-doped AlGaN back barrier layer, it can effectively suppress the capture of electrons in channel by carbon-induced accepted traps in the GaN:C buffer. Combined with the high conductivity of Si δ-doped AlGaN back barrier and high resistance of GaN:C buffer, the device showed the high breakdown E-field strength and the low specific on-resistance. Our proposed device is observed to hold a gate-drain voltage of 769 Vat 10 μA/mm (7-μm gate-drain spacing) and 0.53 mQ · cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and the gateto-drain electric field corresponds to 1.1 MV/cm.

Simulation of a high-performance enhancement-mode HFET with back-to-back graded AlGaN layers

A novel three-dimensional hole gas (3DHG)enhancement-mode (E-mode) heterostructure field-effect transistor (HFET) isproposed and investigated. It features back-to-back graded AlGaN (BGA)barrier layers consisting of a positive-graded AlGaN layer and anegative-graded AlGaN layer, which form polarization gradient andsubsequently induce the three-dimensional electron gas (3DEG) and 3DHG inthe positive- and negative-graded AlGaN layers, respectively. The source anddrain are located at the same side of the metal-insulator-semiconductor(MIS) trench gate, and the source is in contact with the HfO$_{2}$ gateinsulator. First, the on-state current is significantly improved owing tothe high-density 3DEG in the positive-graded AlGaN. Next, the verticalconductive channel between the source and 3DEG is blocked by the 3DHG,thereby realizing the E-mode. The threshold voltage ($V_{\rm~th})$ can bemodulated by a partial doping conductive channel. Subsequently, a highbreakdown voltage (BV) is obtained, because the polarization junction formedby the polarization charges assists in depleting the drift region in theoff-state. Next, the BGA-HFET is smaller than the conventional HFET(Con-HFET) owing to the special location of the source. The BV of theproposed HFET sharply increases to 919 V from 39 V of the Con-HFET with thesame gate-drain spacing, and the saturation drain current is increased by103.5%

1.8-kV circular AlGaN/GaN/AlGaN double-heterostructure high electron mobility transistor**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0400100), the National Natural Science Foundation of China (Grant Nos. 11435010, 61474086, and 61804125), and the Natural Science Basic Research Program of Shaanxi Province, China (Grant No. 2016ZDJC-02).

In this paper, we present a 1.8-kV circular AlGaN/GaN/AlGaN double-heterostructure high electron mobility transistor (DH HEMT) with a gate-drain spacing . Compared with the regular DH HEMT, our circular structure has a high average breakdown electric-field strength that increases from 0.42 MV/cm to 0.96 MV/cm. The power figure of merit for the circular HEMT is as high as . The divergence of electric field lines at the gate edge and no edge effect account for the breakdown enhancement capability of the circular structure. Experiments and analysis indicate that the circular structure is an effective method to modulate the electric field.