Double-heterojunction High Electron Mobility Transistors Research Articles

Damage Effects and Mechanisms of High-Power Microwaves on Double Heterojunction GaN HEMT

In this paper, simulation modeling was carried out using Sentaurus Technology Computer-Aided Design. Two types of high electron mobility transistors (HEMT), an AlGaN/GaN/AlGaN double heterojunction and AlGaN/GaN single heterojunction, were designed and compared. The breakdown characteristics and damage mechanisms of the two devices under the injection of high-power microwaves (HPM) were studied. The variation in current density and peak temperature inside the device was analyzed. The effect of Al components at different layers of the device on the breakdown of HEMTs is discussed. The effect and law of the power damage threshold versus pulse width when the device was subjected to HPM signals was verified. It was shown that the GaN HEMT was prone to thermal breakdown below the gate, near the carrier channels. A moderate increase in the Al component can effectively increased the breakdown voltage of the device. Compared with the single heterojunction, the double heterojunction HEMT devices were more sensitive to Al components. The high domain-limiting characteristics effectively inhibited the overflow of channel electrons into the buffer layer, which in turn regulated the current density inside the device and improved the temperature distribution. The leakage current was reduced and the device switching characteristics and breakdown voltage were improved. Moreover, the double heterojunction device had little effect on HPM power damage and high damage resistance. Therefore, a theoretical foundation is proposed in this paper, indicating that double heterojunction devices are more stable compared to single heterojunction devices and are more suitable for applications in aviation equipment operating in high-frequency and high-voltage environments. In addition, double heterojunction GaN devices have higher radiation resistance than SiC devices of the same generation.

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.

Switching Transient Analysis and Characterization of an E-Mode B-Doped GaN-Capped AlGaN DH-HEMT with a Freewheeling Schottky Barrier Diode (SBD)

This paper presents a systematic study of Al0.23Ga0.77N/GaN/AlxGa1−xN double-heterojunction high-electron-mobility transistors (DH-HEMTs) with a boron-doped P+ GaN cap layer under the gate. The boron-doped GaN cap layer shows great potential to form a high-bandgap Schottky gate in DH-HEMT devices to increase the resistivity of the GaN cap with excellent structural characteristics. Thus, the polarization-induced field in the GaN cap layer can be used to raise the conductive band of the device in the normally OFF operation. In this paper, these AlGaN/GaN power-switching devices with freewheeling Schottky barrier diodes are examined in their working states. In comparison with conventional HEMT power devices, the HEMT with a B-doped GaN cap offers the lowest switching charges, area-specific ON-state resistance, and energy losses. Therefore, this study clearly shows the advantage of GaN transistors for power electronics applications.

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.

Device characteristics of enhancement mode double heterostructure DH‐HEMT with boron‐doped GaN gate cap layer for full‐bridge inverter circuit

AbstractThis 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.

Full geometric simulation of miniaturized GaN double-heterojunction high electron mobility transistors by a multiscale approach coupling quantum and semi-classical transport

In this paper, a multiscale approach coupling semi-classical drift-diffusion (DD) and the quantum mechanical non-equilibrium Green’s function (NEGF) formalism is established to simulate the full geometry of miniaturized GaN high electron mobility transistors (HEMTs). DD current flow is corrected locally in the HEMT channel, where electron transport represented by the NEGF is governed by quantum effects. As a result, simulated drain current–drain voltage and drain current–gate voltage curves are easily fitted to experimental data without fitting mobility. The variation of electric behavior predicted by our multiscale approach is in agreement with that expected from 2D electrostatics. This work indicates that it is practical and efficient to include NEGF in a DD solver, which is still the workhorse of the technology computer-aided design industry.

Growth and characterization of AlGaN/GaN/AlGaN double-heterojunction high-electron-mobility transistors on 100-mm Si(111) using ammonia-molecular beam epitaxy

To improve the confinement of two-dimensional electron gas (2DEG) in AlGaN/GaN high electron mobility transistor (HEMT) heterostructures, AlGaN/GaN/AlGaN double heterojunction HEMT (DH-HEMT) heterostructures were grown using ammonia-MBE on 100-mm Si substrate. Prior to the growth, single heterojunction HEMT (SH-HEMT) and DH-HEMT heterostructures were simulated using Poisson-Schrödinger equations. From simulations, an AlGaN buffer with “Al” mole fraction of 10% in the DH-HEMT was identified to result in both higher 2DEG concentration (∼1013 cm−2) and improved 2DEG confinement in the channel. Hence, this composition was considered for the growth of the buffer in the DH-HEMT heterostructure. Hall measurements showed a room temperature 2DEG mobility of 1510 cm2/V.s and a sheet carrier concentration (ns) of 0.97 × 1013 cm−2 for the DH-HEMT structure, while they are 1310 cm2/V.s and 1.09 × 1013 cm−2, respectively, for the SH-HEMT. Capacitance-voltage measurements confirmed the improvement in the confinement of 2DEG in the DH-HEMT heterostructure, which helped in the enhancement of its room temperature mobility. DH-HEMT showed 3 times higher buffer break-down voltage compared to SH-HEMT, while both devices showed almost similar drain current density. Small signal RF measurements on the DH-HEMT showed a unity current-gain cut-off frequency (fT) and maximum oscillation frequency (fmax) of 22 and 25 GHz, respectively. Thus, overall, DH-HEMT heterostructure was found to be advantageous due to its higher buffer break-down voltages compared to SH-HEMT heterostructure.

Scaling of GaN HEMTs and Schottky Diodes for Submillimeter-Wave MMIC Applications

In this paper, we report state-of-the-art high frequency performance of GaN-based high electron mobility transistors (HEMTs) and Schottky diodes achieved through innovative device scaling technologies such as vertically scaled enhancement and depletion mode (E/D mode) AlN/GaN/AlGaN double-heterojunction HEMT epitaxial structures, a low-resistance <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$n^{+}$</tex></formula> -GaN/2DEG ohmic contact regrown by MBE, a manufacturable 20-nm symmetric and asymmetric self-aligned-gate process, and a lateral metal/2DEG Schottky contact. As a result of proportional scaling of intrinsic and parasitic delays, an ultrahigh <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$f_{T}$</tex></formula> exceeding 450 GHz (with a simultaneous <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$f_{\rm max}$</tex></formula> of 440 GHz) and a <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$f_{\rm max}$</tex></formula> close to 600 GHz (with a simultaneous <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$f_{T}$</tex></formula> of 310 GHz) are obtained in deeply scaled GaN HEMTs while maintaining superior Johnson figure of merit. Because of their extremely low on-resistance and high gain at low drain voltages, the devices exhibited excellent noise performance at low power. 501-stage direct-coupled field-effect transistor logic ring oscillator circuits are successfully fabricated with high yield and high uniformity, demonstrating the feasibility of GaN-based E/D-mode integrated circuits with <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${&gt;}{1000}$</tex> </formula> transistors. Furthermore, self-aligned GaN Schottky diodes with a lateral metal/2DEG Schottky contact and a 2DEG/ <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$n^{+}$</tex></formula> -GaN ohmic contact exhibited <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$RC$</tex></formula> -limited cutoff frequencies of up to 2.0 THz.

AlGaN/GaN/InGaN/GaN DH-HEMTs with GaN channel layer grown at high temperature

The AlGaN/GaN/InGaN/GaN double heterojunction high electron mobility transistors (DH-HEMTs) sample has been grown by MOCVD on (0 0 0 1) sapphire substrate. The structure features a 7 nm In 0.046 Ga 0.954 N interlayer determined by Rutherford backscattering (RBS). Since the polarization field in the InGaN interlayer is opposite to it in the AlGaN layer, an additional potential barrier is introduced between the two-dimensional electron gas (2DEG) channel and buffer, leading to enhanced carrier confinement and improved buffer isolation. The GaN layers between the AlGaN layer and InGaN interlayer are divided into two layers consisting of GaN channel layer which provides high mobility 2DEG grown at 1070 °C and GaN spacer layer grown at the same temperature as InGaN interlayer (800 °C) to prevent indium diffusion. RBS measurement confirms that the 3 nm GaN spacer layer isolates the InGaN interlayer well and free from diffusion. Hall measurement has been performed, the mobility as high as 1552 cm 2 /V s at room temperature is obtained and the sheet carrier density is 1.55 × 10 13 cm −2 . The average sheet resistance is 331 Ω/sq, respectively. The mobility obtained in this paper is about 20% higher than similar structures reported.

Dielectric and barrier thickness fluctuation scattering in Al2O3/AlGaN/GaN double heterojunction high-electron mobility transistors

The two-dimensional electron gas (2DEG) mobility limited by dielectric and barrier thickness fluctuations (TF) scattering in Al2O3/AlGaN/GaN double heterojunction high-electron mobility transistors (HEMTs) is calculated. Calculation shows that thickness fluctuation scattering is the main limitation in Al2O3/AlGaN/GaN double heterojunction HEMTs with thin Al2O3 layer thicknesses. In addition, a study of 2DEG mobility as a function of 2DEG density, ns, shows that TF scattering acts as the main limitation when ns exceeds 2×1012cm−2. The results may be used to design HEMTs to obtain higher 2DEG mobilities by modulating the dielectric layer and barrier thicknesses or 2DEG density.

Theoretical calculation of the interfacial charge-modulated two-dimensional electron gas mobility in Al2O3/AlGaN/GaN double heterojunction high-electron mobility transistors

Influences of the dielectric layer-induced interfacial charges on two-dimensional electron gases (2DEGs) are theoretical calculated in thin-barrier Al2O3/AlGaN/GaN double heterojunction high-electron mobility transistors (HEMTs). A decrease of 2DEG mobilities limited by three main scattering mechanisms, including misfit dislocation, threading dislocation and interface roughness scatterings, is observed with increasing value of the density of interfacial charges. The results in this article can be used to design structures to generate higher mobility in Al2O3/AlGaN/GaN double heterojunction HEMTs.

Analysis of Interface Scattering in AlGaN/GaN/InGaN/GaN Double-Heterojunction High-Electron-Mobility Transistors

This paper presents detailed investigations on the direct-current (DC) characteristics of an AlGaN/GaN/InGaN/GaN double-heterojunction high-electron-mobility transistor (DH-HEMT) using two-dimensional numerical analysis. In this work, the hot-electron effect is taken into account and implemented in the hydrodynamic model. The results indicate that carrier transport in this kind of device exhibits properties significantly different from that in a conventional AlGaN/GaN HEMT. Due to imperfections at the GaN/InGaN interface, scattering caused by the interface roughness, phonons, etc. inhibit the negative differential conductance in high electric field. In addition, the velocity increment of electrons around the gate edge is dominated by the overshoot effect rather than the phonon effect. The energy exchange between phonons and electrons, as presented in this paper, illustrates that the dissipated power is just a small portion of the exchanged energy. For further performance improvement, more lattice-matched material with strong polarization for the barrier layer is proposed.

Characteristics of AlGaN/GaN/AlGaN double heterojunction HEMTs with an improved breakdown voltage

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.

Investigation on the current collapse effect of AlGaN/GaN/InGaN/GaN double-heterojunction HEMTs

A series of AlGaN/GaN/InGaN/GaN double-heterojunction high-electron-mobility-transistors (DH-HEMT) is fabricated with GaN channel layer thicknesses from 6 nm to 20 nm by two-dimensional (2D) numerical simulations. A new idea for optimizating of DH-HEMT structure is proposed. The hot electron effect and self-heating effect are investigated by using hydrodynamic model. Current collapse and negative differential conductance are observed to be directly relevant to GaN channel layer thickness. DH-HEMT with thicker GaN channel layer can confine electrons better in channel, which significantly diminishes the penetration ability of hot electrons from channel layer to buffer layer under high voltage. Increasing the thickness of GaN channel layer appropriately can effectively restrict current collapse and negative differential conductance, and consequently improve device performance under high voltage condition.

Characteristics of Two-Stage Γ-Gate on AlGaAs/InGaAs/AlGaAs DH-HEMTs by Using AlGaAs/InGaP Etching-Stop Layers

Al0.22Ga0.78As/In0.18Ga0.82As/Al0.22Ga0.78As double heterojunction high electron mobility transistors (DH-HEMTs) with gate structures of traditional planar gate (PG), one-stage gamma-gate (1SΓG) and two-stage gamma-gate (2SΓG) formed by using the Al0.22Ga0.78As/In0.49Ga0.51P etch-stop layers (ESL) are simulated and presented in this work. Based on this proposed ESL structure design, the fabrication and implementation of studied DH-HEMT device with 1SΓG and 2SΓG could be expected. Both ΓG-structure devices show the better electric field property compared to PG-device. Simulated results reveal that there are no significant differences in common-source voltage-current characteristics among all studied devices. The obtained drain current density and transconductance of all studied devices are about 220 mAmm-1and 265 mSmm-1. However, the current stability of ΓG-devices with larger bias operation would be improved due to its edge-effect of ΓG extended-region. In addition, the electric field intensity under the gate-footprint is effectively reduced by both studied ΓG structures. The electric field peak value of PG-device is 498 kV cm-1, and it would be reduced down to about 210 kVcm-1and 178 kVcm-1for 1SΓG- and 2SΓG-device, respectively. On the other hand, some frequency property dropping is observed from studied device with 1SΓG or 2SΓG due to the side-edge extension of ΓG-device would create the additional parasitic capacitance. The obtained cut-off frequencies are 15 GHz, 10.5 GHz and 10 GHz for PG-, 1SΓG- and 2SΓG-device (at VGS=+5 V and VGS=-0.75 V), respectively.

The role of ultrathin AlN barrier in the reduction in the hot electron and self-heating effects for GaN-based double-heterojunction high electron mobility transistors

We propose an AlN/GaN/InGaN/GaN double-heterojunction high electron mobility transistor (DH-HEMT) structure with a 4 nm thin AlN barrier layer. The performance of the DH-HEMT device is investigated by using two-dimensional numerical simulation. The conduction band profile is obtained by using the Poisson’s equation and Fermi–Dirac statistics in combination with the polarization charges. Due to large conduction-band offset of the AlN/GaN interface and strong polarization of AlN, the minor channel at GaN/InGaN interface can be eliminated. Further, the hot electron and self-heating effects on the transport properties of this DH-HEMT are investigated by using hydrodynamic model. In comparison with the AlGaN barrier DH-HEMT and conventional HEMT, this kind of DH-HEMT can effectively reduce the hot electron effect under high voltage. The reason is that the maximum field strength is far below the critical value for the existence of the hot electron effect in the AlGaN barrier DH-HEMTs and conventional HEMTs with the same voltage 6 V. The simulation results also show that the ultrathin AlN barrier layer can significantly reduce thermal impedance, and then lower the self-heating effect. Furthermore, the passivation layer has significant role in the self-heating effect of the ultrathin barrier DH-HEMTs.

Comparative influence study of gate-formation structuring on Al0.22Ga0.78As/In0.16Ga0.84As/Al0.22Ga0.78As double heterojunction high electron mobility transistors

Pseudomorphic Al0.22Ga0.78As/In0.16Ga0.84As/Al0.22Ga0.78As double heterojunction high electron mobility transistors (DH-HEMTs) fabricated with different gate-formation structures of a single-recess gate (SRG), a double-recess gate (DRG) and a field-plate gate (FPG) were comparatively investigated. FPG devices show the best breakdown characteristics among these devices due to great reduction in the peak electric field between the drain and gate electrodes. The measured gate–drain breakdown voltages defined at a 1 mA mm−1 reverse gate–drain current density were −15.3, −19.1 and −26.0 V for SRG, DRG and FPG devices, respectively. No significant differences in their room-temperature common-source current–voltage characteristics were observed. However, FPG devices exhibit threshold voltages being the least sensitive to temperature. Threshold voltages as a function of temperature indicate a threshold-voltage variation as low as −0.97 mV K−1 for FPG devices. According to the 2.4 GHz load–pull power measurement at VDS = 3.0 V and VGS = −0.5 V, the saturated output power (POUT), power gain (GP) and maximum power-added efficiency (PAE) were 10.3 dBm/13.2 dB/36.6%, 11.2 dBm/13.1 dB/39.7% and 13.06 dBm/12.8 dB/47.3%, respectively, for SRG, DRG and FPG devices with a pi-gate in class AB operation. When the FPG device is biased at a VDS of 10 V, the saturated power density is more than 600 mW mm−1.

AlGaN/GaN/InGaN/GaN DH‐HEMTs structure with an AlN interlayer grown by MOCVD

AbstractA 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 9.89×1012 cm–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)

Influence of Sinking-Gate on Al0.24Ga0.76As/In0.22Ga0.78As Double Heterojunction High Electron Mobility Transistors

The influence of gate metal with thermal annealed process adopt to control the distance between gate and channel on pseudomorphic Al0.24Ga0.76As/ In0.22Ga0.78As double heterojunction high electron mobility transistors (DH-HEMTs) were studied. Compared to device with gate-recess process, the distance of gate-to-channel could be controlled through the thermal annealed process and therefore exhibit a lower series resistance. Measured transconductance of 150 mS/mm and an open- drain voltage gain of 136 for the DH-HEMT with an as deposited gate are enhanced to 175 mS/mm and 160 for the DH-HEMT with a 330-{degree sign}C annealed gate. Good device linearity is also obtained with a low second-harmonic to fundamental ratio of 3.55 %. In addition, good microwave performances such as unit-current gain- and maximum power gain- frequency were also obtained from devices with gate-annealed process.

DC and RF Characteristics of AlGaN/GaN/InGaN/GaN Double-Heterojunction HEMTs

We present the detailed dc and radio-frequency characteristics of an Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> N/GaN/In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.9 </sub> N/GaN double-heterojunction HEMT (DH-HEMT) structure. This structure incorporates a thin (3 nm) In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.9</sub> N notch layer inserted at a location that is 6-nm away from the AlGaN/GaN heterointerface. The In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.9</sub> N layer provides a unique piezoelectric polarization field which results in a higher potential barrier at the backside of the two-dimensional electron gas channel, effectively improving the carrier confinement and then reducing the buffer leakage. Both depletion-mode (D-mode) and enhancement-mode (E-mode) devices were fabricated on this new structure. Compared with the baseline AlGaN/GaN HEMTs, the DH-HEMT shows lower drain leakage current. The gate leakage current is also found to be reduced, owing to an improved surface morphology in InGaN-incorporated epitaxial structures. DC and small- and large-signal microwave characteristics, together with the linearity performances, have been investigated. The channel transit delay time analysis also revealed that there was a minor channel in the InGaN layer in which the electrons exhibited a mobility slightly lower than the GaN channel. The E-mode DH-HEMTs were also fabricated using our recently developed CF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> -based plasma treatment technique. The large-signal operation of the E-mode GaN-based HEMTs was reported for the first time. At 2 GHz, a 1times100 mum E-mode device demonstrated a maximum output power of 3.12 W/mm and a power-added efficiency of 49% with single-polarity biases (a gate bias of +0.5 V and a drain bias of 35 V). An output third-order interception point of 34.7 dBm was obtained in the E-mode HEMTs