GaN-based MIS-HEMTs Research Articles

Proton-Irradiation Effects and Reliability on GaN-Based MIS-HEMTs.

A comprehensive study of proton irradiation reliability on a bilayer dielectrics SiNx/Al2O3 MIS-HEMT, the common Schottky gate HEMT, and a single dielectric layer MIS-HEMT with SiNx and with Al2O3 for comparison is conducted in this paper. Combining the higher displacement threshold energy of Al2O3 with the better surface passivation of the SiNx layer, the bilayer dielectrics MIS-HEMT presents much smaller degradation of structural materials and of device electrical performance after proton irradiation. Firstly, the least of the defects caused by irradiation suggesting the smallest structural material degradation is observed in the bilayer dielectrics MIS-HEMT through simulations. Then, DC and RF electrical performance of four kinds of devices before and after proton irradiation are studied through simulation and experiments. The smallest threshold voltage degradation rate, the smallest maximum on-current degradation and Gm degradation, the largest cut-off frequency, and the lowest cut-off frequency degradation are found in the bilayer dielectrics MIS-HEMT among four kinds of devices. The degradation results of both structural materials and electrical performance reveal that the bilayer dielectrics MIS-HEMT performs best after irradiation and had better radiation resilience.

Low thermal budget V/Al/Mo/Au ohmic contacts for improved performance of AlGaN/GaN MIS-HEMTs

We report on the highly improved performance of Al2O3/AlGaN/GaN MIS-HEMTs using a V/Al/Mo/Au metal stack as ohmic electrodes. Transfer length method test structures using a V/Al/Mo/Au metal stack annealed at a temperature of 660 °C revealed highly linear current–voltage curves and smooth surface morphology. Compared with reference MIS-HEMTs using Ti/Al/Mo/Au annealed at the standard rapid thermal annealing temperature of 880 °C, V-based devices exhibited less hysteresis of transfer curves and showed higher gate controllability of the drain current, suggesting a highly improved Al2O3/AlGaN interface. Measurements and analyses of capacitance–voltage characteristics of corresponding MIS-capacitors corroborated these findings. The V-based ohmic contact could open new avenues towards enhanced GaN-based MIS-HEMTs performance.

Effect of in-situ SiNx grown with different carrier gas on structural and electrical properties of GaN-based MISHEMTs

In this work, the effect of in situ SiNx grown with different carrier gas on the structural and electrical properties of the SiNx/AlGaN/GaN MIS-HEMTs is studied. It was found that the growth rate of SiNx grown with N2 as carrier gas (N2-SiNx) is more sensitive to different growth conditions, while the growth rate of SiNx grown with H2 as carrier gas (H2-SiNx) is very stable due to the inhibiting effects of H2 carrier gas on the SiH4–NH3 forward reactions. More importantly, a continuous and smooth SiNx growth at the initial stage can be realized with H2 carrier gas due to its faster surface migration, leading to a decent surface morphology and sharp interface of H2-SiNx. As a result, the SiNx passivated device with H2 as carrier gas shows improved performance compared to that with N2 as carrier gas, featuring ultra-low interface-state density of 2.8 × 1010 cm−2 eV−1, improved on- and off-state current, reduced threshold voltage shift, and mitigated current collapse, especially after long-term electrical stress. These results not only elaborate on the growth mechanisms of in situ SiNx with different carrier gases but also highlight the advances of H2 as carrier gas for in situ SiNx growth, providing an effective strategy to tailor the passivation schemes for GaN-based devices.

Low Leakage Current and High Breakdown Field AlGaN/GaN MIS-HEMTs Using PECVD-SiNx as a Gate Dielectric.

In this paper, SiNx film deposited by plasma-enhanced chemical vapor deposition was employed as a gate dielectric of AlGaN/GaN high electron mobility transistors (HEMTs). We found that the NH3 flow during the deposition of SiNx can significantly affect the performances of metal–insulator–semiconductor (MIS) HEMTs. Compared to that without using NH3 flow, the device with the optimized NH3 flow exhibited three orders of magnitude lower gate leakage current, two orders of magnitude higher ON/OF drain current ratio, and an increased breakdown field by 69%. In addition, an in situ N2 plasma surface treatment prepared prior to SiNx deposition can further improve DC performances of MIS-HEMTs to a very low gate leakage current of 10−9 mA/mm and a high ON/OFF drain current ratio up to 109 by reducing the interface state density. These results demonstrate the great potential for using PECVD-SiNx as a gate dielectric in GaN-based MIS-HEMTs.

Normally-OFF AlGaN/GaN MIS-HEMTs With Low RON and Vth Hysteresis by Functioning In-situ SiNx in Regrowth Process

Normally-off AlGaN/GaN MIS-HEMTs with a high threshold voltage (<inline-formula> <tex-math notation="LaTeX">$\text{V}_{\text {th}}$ </tex-math></inline-formula>) more than 2.5 V and a low on- resistance of <inline-formula> <tex-math notation="LaTeX">$5.5\Omega \cdot $ </tex-math></inline-formula>mm have been achieved by an improved regrowth technique with <i>in-situ</i> SiN_<i>x</i> passivation. A thin-barrier heterojunction was used to decrease the two-dimensional electron gas (2DEG) underneath the gate, then regrown Al_0.2Ga_0.8N and <i>in-situ</i> SiN_<i>x</i> were applied to recover 2DEG at the access regions and reduce contact resistance. The O₃-based Al₂O₃ and HfO₂ were employed to cover the recessed-gate with low channel sheet resistances by atomic layer deposition. The other hybrid MIS-HEMT with <i>in-situ</i> SiN_<i>x</i> gate interlayer also enabled a normally-off operation with a <inline-formula> <tex-math notation="LaTeX">$\text{V}_{\text {th}}$ </tex-math></inline-formula> hysteresis lower than 30 mV. The damage-free recessed-gate structures with in- situ SiN_<i>x</i> as passivation and gate dielectric contribute to reducing surface scattering and interface states, resulting in a high <inline-formula> <tex-math notation="LaTeX">$\text{V}_{\text {th}}$ </tex-math></inline-formula> uniformity and channel mobility, low on- resistance and_th hysteresis in normally-off GaN-based MIS-HEMTs.

Atomic-resolved structural and electric field analysis of the passivation interface of MIS-HEMTs

The passivation interfaces of GaN-based MIS-HEMTs with Si3N4 and ZrO2/Si3N4 bilayers were investigated through atomic resolution scanning transmission electron microscope–energy dispersive spectroscopy–differential phase contrast microscopy methods. It is found that the Si3N4/GaN interface exhibits atomic disorder fluctuation, and the GaN surface is discontinuous at the depth of 1–2 atomic layers. An oxide layer of ∼2 nm is formed at the ZrO2/GaN interface, and the GaN surface is atomically flat. Furthermore, the local minimum of the potential is located at the Si3N4/GaN interface, while it is distributed in the GaN side at the ZrO2/GaN interface. The electric field or potential distribution is affected by the crystal orientation of the polycrystalline ZrO2 layer. Finally, the difference in passivation mechanism is discussed.

Improving Gate Reliability of 6-In E-Mode GaN-Based MIS-HEMTs by Employing Mixed Oxygen and Fluorine Plasma Treatment

Robust and reliable gate scheme is crucial in developing the enhancement-mode (E-mode) AlGaN/GaN metal–insulator–semiconductor (MIS)-high electron mobility transistors (HEMTs). This work reports on a method of mixed oxygen and fluorine plasma pretreatment combined with selected bi-layer gate dielectrics to improve the performances of partially recessed-gate MIS-HEMTs. The fabricated E-mode HEMTs exhibit significantly boosted threshold voltage of 2.5 V and gate breakdown voltage of 26 V, as well as improved device reliability on 6-in wafer under both bias and temperature stresses. Remarkably reduced <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {th}}$ </tex-math></inline-formula> shift is found even at 250 °C high temperature. Furthermore, measurements on gate breakdown lifetime show a ten-year lifetime of the fabricated devices when the gate bias is kept below 13.1 V which demonstrates a promising scheme in fabricating the E-mode HEMT products.

Impact of Passivation System on Device Performance and Proton Radiation Hardness in GaN-Based MIS-HEMTs

The impact of the passivation system on the device performance has been studied in GaN-based MIS-HEMTs using SiN/Al2O3 bi-layered passivation. The deposition of SiN and Al2O3 passivation layer induced the compressive and tensile stress on GaN channel layer, respectively. Through the Al2O3 deposition on top of SiN layer, the mechanical stress and device characteristics were modulated. The device properties such as carrier mobility and concentration at the hetero-interface were ameliorated when the slight tensile stress was applied on the GaN channel compared to the compressive stress. The proton radiation hardness corresponding to the passivation system was also researched. The SiN/Al2O3 passivation system exhibited stronger immunity to the proton radiation than that of SiN passivation due to the superior dielectric quality of Al2O3. These results highlight that the SiN/Al2O3 bi-layered passivation is promising technique for the optimization of the device performance and improvement of proton radiation hardness in GaN-based MIS-HEMTs.

Experimental evaluation of interface states during time-dependent dielectric breakdown of GaN-based MIS-HEMTs with LPCVD-SiNx gate dielectric**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFB0402800), the Key Research and Development Program of Guangdong Province, China (Grant Nos. 2019B010128002 and 2020B010173001), the National Natural Science Foundation of China (Grant Nos. U1601210 and 61904207), the Natural Science

We experimentally evaluated the interface state density of GaN MIS-HEMTs during time-dependent dielectric breakdown (TDDB). Under a high forward gate bias stress, newly increased traps generate both at the SiNx/AlGaN interface and the SiNx bulk, resulting in the voltage shift and the increase of the voltage hysteresis. When prolonging the stress duration, the defects density generated in the SiNx dielectric becomes dominating, which drastically increases the gate leakage current and causes the catastrophic failure. After recovery by UV light illumination, the negative shift in threshold voltage (compared with the fresh one) confirms the accumulation of positive charge at the SiNx/AlGaN interface and/or in SiNx bulk, which is possibly ascribed to the broken bonds after long-term stress. These results experimentally confirm the role of defects in the TDDB of GaN-based MIS-HEMTs.

Negative bias-induced threshold voltage instability and zener/interface trapping mechanism in GaN-based MIS-HEMTs**Project supported by the National Key Research and Development Program of China (Grant No. 2018YFB1802100), the Science Challenge Project, China (Grant No. TZ2018004), and the National Natural Science Foundation of China (Grant Nos. 61534007 and 11690042).

We investigate the instability of threshold voltage in D-mode MIS-HEMT with in-situ SiN as gate dielectric under different negative gate stresses. The complex non-monotonic evolution of threshold voltage under the negative stress and during the recovery process is induced by the combination effect of two mechanisms. The effect of trapping behavior of interface state at SiN/AlGaN interface and the effect of zener traps in AlGaN barrier layer on the threshold voltage instability are opposite to each other. The threshold voltage shifts negatively under the negative stress due to the detrapping of the electrons at SiN/AlGaN interface, and shifts positively due to zener trapping in AlGaN barrier layer. As the stress is removed, the threshold voltage shifts positively for the retrapping of interface states and negatively for the thermal detrapping in AlGaN. However, it is the trapping behavior in the AlGaN rather than the interface state that results in the change of transconductance in the D-mode MIS-HEMT.

Effect of High-k Passivation Layer on High Voltage Properties of GaN Metal-Insulator-Semiconductor Devices

In this paper, the GaN-based MIS-HEMTs with Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> single-layer passivation, Al <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> /SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> bilayer passivation, and ZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> bilayer passivation are demonstrated. High-k dielectrics are adopted as the passivation layer on MIS-HEMTs to suppress the shallow traps on the GaN surface. Besides, high permittivity dielectrics passivated MIS-HEMTs also show an improved breakdown voltage characteristic, and that is explained by 2-D simulation analysis. The fabricated devices with high-k dielectrics/SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> bilayer passivation exhibit higher power properties than the devices with plasma enhanced chemical vapor deposition-SiNx single layer passivation, including smaller current collapse and higher breakdown voltage. The Al <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> /SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> passivated MIS-HEMTs exhibit a breakdown voltage of 1092 V, and the dynamic R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</sub> is only 1.14 times the static R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</sub> after off-state VDS stress of 150 V. On the other hand, the ZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> passivated MIS-HEMTs exhibit a higher breakdown voltage of 1207 V, and the dynamic Ron is 1.25 times the static R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</sub> after off-state VDS stress of 150 V.

Improving Performances of Enhancement-Mode AlGaN/GaN MIS-HEMTs on 6-inch Si Substrate Utilizing SiON/Al2O3 Stack Dielectrics

Enhancement-mode (E-mode) GaN-based MIS-HEMTs still suffer from undeniable gate leakage or low gate breakdown voltage due to the low quality of gate dielectrics, resulting in a notorious tailing effect of the off-state current. In this letter, a gate scheme featuring SiON/Al2O3 stack dielectrics and partially recessed gate barrier has been employed in the AlGaN/GaN MIS-HEMTs. A high on/off current ratio over $10^{{9}}$ and a small threshold voltage ( ${V} _{\text {th}}$ ) hysteresis less than 20 mV are achieved in the fabricated E-mode devices with a ${V} _{\text {th}}$ around 2.5 V, mainly owing to the reduction of the net positive fixed charge density in the SiON/Al2O3 gate stack confirmed by the ${C}$ - ${V}$ measurements. Meanwhile, a good performance uniformity on 6-inch wafer is achieved which demonstrates the promising scheme for fabricating GaN-based E-mode MIS-HEMT products.

A large-signal Pspice modeling of GaN-based MIS-HEMTs

Abstract In this work we present a physics-based semi-empirical large-signal model for GaN MIS-HEMTs, which introduces the non-segmented, smooth continuous equations to describe the static and dynamic characteristics of GaN MIS-HEMTs in different working regions. The unique physical effect of threshold voltage drift was considered for MIS-HEMTs in the current model, in addition to effects of the channel length modulation and the temperature drift. In addition, a current-controlled current source was used in the dynamic model to characterize the nonlinear capacitance including the gate-drain capacitance CGD, gate-source capacitance CGS and drain-source capacitance CDS at different operating voltages. The model is in excellent agreement with the experimental data for both drain current and capacitances over a typical range of applied voltages and temperatures.

Improvement of Proton Radiation Hardness Using ALD-Deposited Al2O3 Gate Insulator in GaN-Based MIS-HEMTs

The proton radiation hardness has been investigated in GaN-based MIS-HEMTs with various gate insulating systems. Through the pulsed mode measurements and carrier mobility extraction, we have revealed that the Coulomb scattering generated by the trapped charges inside of the gate insulating layer is a key device performance degradation factor. We also have found out that SiN/Al2O3 bi-layer gate insulating system exhibits stronger immunity to the proton radiation compared to the SiN single-layer gate insulating system since the dielectric layer quality of ALD deposited Al2O3 is better than that of PECVD deposited SiN layer. Our systematic research emphasizes that to employ an excellent quality dielectric layer such as Al2O3 is essential factor for the improvement of the proton radiation hardness in GaN-based MIS-HEMTs.

Observation of Hot Electron and Impact Ionization in N-Polar GaN MIS-HEMTs

This letter reports on the observation of hot-electron and impact-ionization mechanisms in N-polar GaN-based MIS-HEMTs designed for high frequency (RF) operation. Thanks to the extremely low gate leakage of such devices, we were able to demonstrate—for the first time—a correlation between electroluminescence (EL) and gate current in the semi-on state. In the semi-on state, the devices show a non-monotonic, bell-shaped behavior of the EL-versus- ${\text {V}}_{\text {GS}}$ and of the ${I}_{\text {G}}$ -versus- ${V}_{\text {GS}}$ characteristics, and the intensity of the EL signal is proportional to the ${I}_{G}\times {I}_{D}$ product. The results are experimental evidence for impact ionization: the related mechanisms are described in detail in the letter.

Positive and negative threshold voltage instabilities in GaN-based transistors

This paper reviews the main mechanisms responsible for bias-temperature instability (BTI) in GaN-based high electron mobility transistors. In the first part of the article, we focus on the threshold voltage instabilities of GaN-based MIS-HEMTs submitted to positive and negative gate bias. We demonstrate that the shift in threshold voltage originates from the trapping/de-trapping of defects located at the insulator/semiconductor interface and/or in the bulk dielectric. In the second part of the paper, we describe the threshold voltage instabilities of GaN-transistors with p-GaN gate, designed specifically for normally-off operation. We present original data indicating that under positive gate bias these devices with p-GaN gate may show a negative threshold voltage shift, which is fully recoverable. This effect is ascribed to the injection of holes under the gate contact, and to the corresponding accumulation of positive charge, possibly at the p-GaN/AlGaN interface.The results described within this paper provide an up-to-date description of the most relevant trapping processes that impact on the stability of threshold voltage and on-resistance in GaN-based transistors.

Positive Shift in Threshold Voltage Induced by CuO and NiO&lt;italic&gt;x&lt;/italic&gt; Gate in AlGaN/GaN HEMTs

AlGaN/GaN HEMTs with two kinds of p-type metal-oxide (CuO and NiO x ) gates have been fabricated, in which the threshold voltage can be modulated effectively. Especially, the threshold voltage of the device shows a 0.7 V positive shift with a high-quality CuO gate layer. Meanwhile, a high on/off current ratio of $\sim 10^{{\mathsf {9}}}$ , a large saturated drain current of ~1030 mA/mm, and an improved transconductance of 200 mS/mm are also achieved. The pulsed transfer characteristics and output characteristics exhibit small hysteresis and ignorable current collapse, signifying an excellent CuO/AlGaN interface. Therefore, CuO can be used as a potential p-type gate for high-performance GaN-based MIS-HEMTs.

Characterization of Interface Defects With Distributed Activation Energies in GaN-Based MIS-HEMTs

Charge trapping is one of the main reliability issues for GaN-based MIS-high-electron-mobility-transistor technologies. In this paper, we focus on the defects located at or close to the interface with the dielectric, which are responsible for the threshold voltage instability at positive gate bias conditions. We present a methodology to analyze the experimental data based on the nonradiative multiphonon model for charge trapping. In particular, we show how to extract the density of interface traps as a function of their activation energy from stress and recovery experiments performed at various temperatures. Our approach is applied to two GaN/AlGaN/SiN samples with different trapping properties, at temperatures ranging from −190 °C to 200 °C. We evaluate their response to forward bias stress and finally, we extract the activation energy distribution for electron capture and emission over a continuous energy range.

Evidence of Hot-Electron Degradation in GaN-Based MIS-HEMTs Submitted to High Temperature Constant Source Current Stress

This letter demonstrates that GaN-based MIS-HEMTs submitted to stress with high-temperature, high drain bias, and constant source current (HTSC stress) may show degradation modes that are not detectable during standard high temperature reverse bias (HTRB) stress. Based on a number of stress/recovery experiments, we demonstrate the following novel results: 1) the combined presence of high drain bias and constant drain-source current (HTSC stress) can lead to a significant increase in ON-resistance (RON) that is not detected under conventional HTRB stress; 2) RON increases without changes in the threshold voltage, indicating that charge trapping takes place in the access regions, and not under the gate; 3) the RON increase has a monotonic dependence on the source current flowing during stress; and 4) for the same stress current level, the RON increase has a negative dependence on temperature. The strong correlation between RON increase and source current and the negative temperature coefficient strongly support the hypothesis that trapping originates from the injection of hot electrons toward the gate-drain access region.

Analysis of off-state leakage mechanisms in GaN-based MIS-HEMTs: Experimental data and numerical simulation

This paper presents an extensive analysis of the off-state conduction mechanisms in AlGaN/GaN Meta–Insulator–Semiconductor (MIS) transistors. Based on combined bi-dimensional numerical simulation and experimental measurements, we demonstrate the following relevant results: (i) under off-state bias conditions, the drain current can show a significant increase when the drain bias is swept up to 600V; (ii) several mechanisms can be responsible for off-state current conduction, including band-to-band tunneling and impact ionization; (iii) two-dimensional numerical simulations indicate that band-to-band tunneling plays a major role, while impact ionization does not significantly contribute to the overall leakage. Temperature-dependent I–V measurements were also carried out to identify the origin of the vertical drain-bulk leakage.