Normally-off High Electron Mobility Transistors Research Articles

Recent Developments and Prospects of Fully Recessed MIS Gate Structures for GaN on Si Power Transistors

For high electron mobility transistors (HEMTs) power transistors based on AlGaN/GaN heterojunction, p-GaN gate has been the gate topology commonly used to deplete the two dimensional electron gas (2-DEG) and achieve a normally-OFF behavior. But fully recessed MIS gate GaN power transistors or MOSc-HEMTs have gained interest as normally-OFF HEMTs thanks to the wider voltage swing and reduced gate leakage current when compared to p-GaN gate HEMTs. However the mandatory AlGaN barrier etching to deplete the 2-DEG combined with the nature of the dielectric/GaN interface generates etching-related defects, traps, and roughness. As a consequence, the threshold voltage (VTH) can be unstable, and the electron mobility is reduced, which presents a challenge for the integration of a fully recessed MIS gate. Recent developments have been studied to solve this challenge. In this paper, we discuss developments in gate recess with low impact etching and atomic layer etching (ALE) alongside surface treatments such as wet cleaning, thermal or plasma treatment, all in the scope of having a surface close to pristine. Finally, different interfacial layers, such as AlN, and alternative dielectrics investigated to optimize the dielectric/GaN interface are presented.

Simulation study on electrical properties of p-GaN gate normally-off HEMT devices affected by Al mole fraction in AlGaN barrier layer

The reason that p-GaN gate normally-off high electron mobility transistor (HEMT) device is able to draw prevail interests is not only for its outstanding electric properties of low on-resistance and high thermal conductivity, but also for its applications under high frequency and high voltage. In HEMT devices, the setting of Al component in the AlGaN barrier layer is a key parameter to affect the formation of two-dimensional electron gas (2DEG) at the AlGaN/GaN heterojunction, and therefore directly determines the performance of the devices. This paper uses the Atlas module of the Silvaco-TCAD software to design and simulate p-GaN gate normally-off HEMT devices to investigate the effect of Al mole fraction content in the AlGaN barrier layer on the electrical performance, as well as the causes in terms of the conduction band energy structure diagram. The results show that the threshold voltage of the HEMT device decreases, the maximum saturation drain current increases, and the peak transductance of the HEMT device increases with increasing the Al content. In the off-state (zero gate voltage), the AlGaN/GaN potential well depth becomes deeper as the Al mole fraction increases, resulting in a lower driven force to rebuild the minimized 2DEG concentration in the GaN layer for switching from off-state to on-state, and this leads to a lower threshold voltage. While, in the on-state (5 V gate voltage), the deeper AlGaN/GaN potential well depth as the Al mole fraction increases, the higher of the 2DEG concentration of the GaN layer, leading to a higher maximum saturation output drain current. After optimization, the integrated electrical performance of the HEMT devices has been found best when the Al mole fraction content is about 0.25. The results obtained from the simulations are helpful for the experimental design of p-GaN gate normally-off HEMT devices

Influence of Mg doping level at the initial growth stage on the gate reliability of p-GaN gate HEMTs

This letter has studied the gate reliability of p-GaN high electron mobility transistors (HEMTs) influenced by the Mg doping level at the initial growth stage of p-GaN layer. Normally-off HEMTs with p-GaN gate fabricated with relatively low and high Mg doping concentration have been analyzed and compared based on their performances at various statuses of as-fabricated, forward step-stressed, reverse step-stressed and long-term reverse stressed. It reveals that Mg over-doping has a detrimental effect on the p-GaN gate reliability through degrading the PIN diode formed by p-GaN/AlGaN/GaN structure. Severe drain current reduction, threshold voltage instability and gate leakage increase have been observed after being negatively stressed due to the high-density defects acting as apparent acceptor-like traps induced by Mg over-doping. The detailed studies on the p-GaN gate performance with various Mg concentrations give a greater depth of understanding to realize high gate reliability of p-GaN gate HEMTs.

Heteroepitaxy of Hf0.5Zr0.5O2 ferroelectric gate layer on AlGaN/GaN towards normally-off HEMTs

Ferroelectric gate High Electron Mobility Transistors (HEMTs) stands out as a promising route for normally-off HEMTs. HEMTs based on perovskite ferroelectrics, however, suffer from low threshold voltage, electrical instability, and CMOS process incompatibility. In this work, fluorite Hf0.5Zr0.5O2 (HZO) was designed to serve as ferroelectric gate layer for normally-off HEMTs, and HZO/MgO/AlGaN/GaN/Si ferroelectric-semiconductor heterostructure was constructed by pulsed laser deposition. The epitaxial relationship was revealed to be [112¯](111) HZO//[112¯](111) MgO//[101¯0](0002) GaN, and corresponding domain matching mechanism was determined as 6aMgO=5aHZO. Strong coupling effect between ferroelectric polarization of HZO and two-dimensional electron gas (2DEG) at AlGaN/GaN interface was confirmed by continuous modulation of threshold voltage from −3.8 V to +3.2 V through regulating polarization state of 15 nm HZO. Moreover, relatively weak depolarization effect of HZO on AlGaN/GaN was observed, which was crucial for device electrical stability. In addition, ultrahigh threshold voltage of +5.5 V was detected in designed heterostructure with 50 nm HZO. Our work demonstrates that epitaxial HZO ferroelectric gate layer on AlGaN/GaN possesses high threshold voltage, good device electrical stability and great semiconductor process compatibility, emerging as great potential for developing and miniaturizing high-performance normally-off HEMTs.

Accurate statistical extraction of AlGaN/GaN HEMT device parameters using the Y-function

A new protocol based on Y-function is used for accurate statistical extraction of electrical parameters of High Electron Mobility Transistor (HEMT) devices for GaN technology. This protocol presented here is used for extraction of relevant electrical parameters such as oxide capacitance, threshold voltage, effective mobilities and access resistance. This study has been verified over a large range of channel lengths for two normally-off HEMT GaN wafers having different levels of access resistances.

Ridge-channel AlGaN/GaN normally-off high-electron mobility transistor based on epitaxial lateral overgrowth

A ridge-channel AlGaN/GaN high-electron mobility transistor (HEMT) utilizing selective-area growth and epitaxial lateral overgrowth (ELO) technique is proposed in this work to achieve high-performance normally-off devices. It has a c-plane platform for the source and the drain contacts, and sidewalls of lattice plane for the gate contact. The sidewalls have characteristics of weak polarization and thin barrier, which are advantageous for realizing normally-off operation. Two ridge HEMTs with triangular and trapezoid channel are designed. Theoretical simulation demonstrates a threshold voltage of 0.03 V for the sidewall channel with reduced polarization and barrier thickness, and a threshold voltage of 1.1–1.3 V for the ridge HEMTs assuming no polarization charge in sidewall channel. The ridge-channel device also exhibits high saturation drain current. The ELO-based ridge-channel opens a new way to achieve normally-off AlGaN/GaN HEMT.

High-frequency enhancement-mode millimeterwave AlGaN/GaN HEMT with an f T/f max over 100 GHz/200 GHz* *Project supported by the National Key Research and Development Program of China (Grant No. 2020YFB1804902), the National Natural Science Foundation of China (Grant No. 61904135), the China Postdoctoral Science Foundation (Grant Nos. 2018M640957 and BX20200262), and the Natural Science Foundation of Shaanxi Province, China (Grant No.

Ultra-thin barrier (UTB) 4-nm-AlGaN/GaN normally-off high electron mobility transistors (HEMTs) having a high current gain cut-off frequency (f T) are demonstrated by the stress-engineered compressive SiN trench technology. The compressive in-situ SiN guarantees the UTB-AlGaN/GaN heterostructure can operate a high electron density of 1.27 × 1013 cm−2, a high uniform sheet resistance of 312.8 Ω/□, but a negative threshold for the short-gate devices fabricated on it. With the lateral stress-engineering by full removing in-situ SiN in the 600-nm SiN trench, the short-gated (70 nm) devices obtain a threshold of 0.2 V, achieving the devices operating at enhancement-mode (E-mode). Meanwhile, the novel device also can operate a large current of 610 mA/mm and a high transconductance of 394 mS/mm for the E-mode devices. Most of all, a high f T/f max of 128 GHz/255 GHz is obtained, which is the highest value among the reported E-mode AlGaN/GaN HEMTs. Besides, being together with the 211 GHz/346 GHz of f T/f max for the D-mode HEMTs fabricated on the same materials, this design of E/D-mode with the realization of f max over 200 GHz in this work is the first one that can be used in Q-band mixed-signal application with further optimization. And the minimized processing difference between the E- and D-mode designs the addition of the SiN trench, will promise an enormous competitive advantage in the fabricating costs.

Fabrication and characteristics of flexible normally-off AlGaN/GaN HEMTs

In this paper, we present a method for removing a high electron mobility transistor (HEMT) silicon substrate using mechanical grinding and deep silicon etching technology and successfully transferred the epitaxial wafer to a PET substrate to achieve the flexible normally-off HEMT. By testing the output characteristics and transfer characteristics of the Si-substrate HEMT and PET-substrate HEMT, we have demonstrated that the PET-substrate HEMT has excellent performance and successfully achieved the mechanical flexibility. Furthermore, we analyzed the physical mechanisms of the change in PET-substrate and Si-substrate HEMT characteristics, as well as flexible HEMT performance under bent and flattened states. The flexible HEMT array demonstrates significant potential in integration with other flexible devices, such as GaN-based micro-LED arrays.

Design and Simulation of High Performance Lattice Matched Double Barrier Normally Off AlInGaN/GaN HEMTs

A novel lattice matched double barrier Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.72</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.16</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.12</sub> N/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.18</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.04</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.78</sub> N/GaN normally-off high electron mobility transistor (HEMT) is designed and simulated by solving a set of thermodynamic transport equations. Using the experimentally calibrated physical models with bearing mobility degradation by surface roughness in account, the recess gate and double barrier of the proposed device achieves a maximum drain current density ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$I_{D,max}$ </tex-math></inline-formula> ) of 1149 mA/mm and a maximum transconductance ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$g_{m,max}$ </tex-math></inline-formula> ) of 358 mS/mm with a positive threshold voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{th}$ </tex-math></inline-formula> ) of 0.2 V. The small polarization charge of first barrier is responsible for positive <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{th}$ </tex-math></inline-formula> . <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$I_{DS,max}$ </tex-math></inline-formula> in the double barrier HEMT at high gate bias condition is due to injection of electrons from upper 2DEG which is almost impossible at lower gate voltage because of insufficient energy to cross the barrier. The injection of electrons is further supported by the second peak in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{g}_{m}$ </tex-math></inline-formula> curve at low gate bias <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{G} = 1\text{V}$ </tex-math></inline-formula> . The outcome of this study suggests that the proposed device will be beneficial for high-frequency and high-power electronic applications.

Simulation of Electrical Characteristics on Inhomogeneous Strains in Normally-off HEMTs with p-GaN Gate

Strain is one of the important factors affecting the two-dimensional electron gas (2DEG) transform in AlGaN/GaN material based high electron mobility transistors (HEMTs) by polarization effects. In this paper, the effects of inhomogeneous biaxial strain in different regions of the AlGaN barrier layer on electrical properties of normally-off HEMTs with p-GaN gate were discussed. The results show that biaxial strain applied in three regions has different influence on transfer, output and breakdown characteristics of the device. The strain applied in region under gate has the most significant impact on threshold voltage and drain saturation current with a decreasing of 39% and an increasing of 97% respectively as the strained lattice constant increases from 3.173061Å to 3.187229Å.While, strain applied between gate and drain electrode can improve the off-state breakdown voltage by 12% with the increasing of strained lattice constant.

A Novel GaN Metal-Insulator-Semiconductor High Electron Mobility Transistor Featuring Vertical Gate Structure.

A novel structure scheme by transposing the gate channel orientation from a long horizontal one to a short vertical one is proposed and verified by technology computer-aided design (TCAD) simulations to achieve GaN-based normally-off high electron mobility transistors (HEMTs) with reduced on-resistance and improved threshold voltage. The proposed devices exhibit high threshold voltage of 3.1 V, high peak transconductance of 213 mS, and much lower on-resistance of 0.53 mΩ·cm2 while displaying better off-state characteristics owing to more uniform electric field distribution around the recessed gate edge in comparison to the conventional lateral HEMTs. The proposed scheme provides a new technical approach to realize high-performance normally-off HEMTs.

Conduction Mechanisms at Interface of AlN/SiN Dielectric Stacks with AlGaN/GaN Heterostructures for Normally-off High Electron Mobility Transistors: Correlating Device Behavior with Nanoscale Interfaces Properties.

In this work, the conduction mechanisms at the interface of AlN/SiN dielectric stacks with AlGaN/GaN heterostructures have been studied combining different macroscopic and nanoscale characterizations on bare materials and devices. The AlN/SiN stacks grown on the recessed region of AlGaN/GaN heterostructures have been used as gate dielectric of hybrid metal-insulator-semiconductor high electron mobility transistors (MISHEMTs), showing a normally-off behavior (Vth = +1.2 V), high channel mobility (204 cm2 V-1 s-1), and very good switching behavior (ION/IOFF current ratio of (5-6) × 108 and subthreshold swing of 90 mV/dec). However, the transistors were found to suffer from a positive shift of the threshold voltage during subsequent bias sweeps, which indicates electron trapping in the dielectric stack. To get a complete understanding of the conduction mechanisms and of the charge trapping phenomena in AlN/SiN films, nanoscale current and capacitance measurements by conductive atomic force microscopy (C-AFM) and scanning capacitance microscopy (SCM) have been compared with a macroscopic temperature-dependent characterization of gate current in MIS capacitors. The nanoscale electrical analyses showed the presence of a spatially uniform distribution of electrons trapping states in the insulator and the occurrence of a density of 7 × 108 cm-2 of local and isolated current spots at high bias values. These nanoscale conductive paths can be associated with electrically active defects responsible for the trap-assisted current transport mechanism through the dielectric, observed by the temperature-dependent characterization of the gate current. The results of this study can be relevant for future applications of AlN/SiN bilayers in GaN hybrid MISHEMT technology.

1.5-V-threshold-voltage Schottky barrier normally-off AlGaN/GaN high-electron-mobility transistors with fT/fmax of 41/125 GHz

In this paper, a normally-off AlGaN/GaN high-electron-mobility transistors (HEMT) fabricated using inductively coupled plasma (ICP) CF4 plasma recessing and an implantation technique is reported. A gate-to-channel distance of ∼10 nm and an equivalent negative fluorine sheet charge density of −1.21 × 1013 cm−2 extracted using a simple threshold voltage (Vth) analytical model result in a high Vth of 1.5 V, a peak transconductance of 356 mS/mm, and a subthreshold slope of 133 mV/decade. A small degradation of channel mobility leads to a high RF performance with fT/fmax of 41/125 GHz, resulting in a record high fT × Lg product of 10.66 GHz·µm among Schottky barrier AlGaN/GaN normally-off HEMTs with Vth exceeding 1 V, to the best of our knowledge.

Study of the stability of e-mode GaN HEMTs with p-GaN gate based on combined DC and optical analysis

This paper investigates the robustness of normally-off High Electron Mobility Transistors (HEMTs) with p-GaN gate submitted to forward gate bias overstress. By means of combined DC and spectral analysis we demonstrate the following results: (i) the devices demonstrate a time-dependent failure mechanism; (ii) time to failure (TTF) can be described by a Weibull distribution with a shape factor higher than 1, suggesting a wear-out failure; (iii) the devices have an estimated 20-years lifetime for a gate voltage of 7.2V; (iv) TTF is temperature-dependent, with an activation energy of 0.5eV; (v) emission microscopy reveals the presence of hot spots, whose emission originates from yellow luminescence and/or hot electron radiation.

Step buffer layer of Al0.25Ga0.75N/Al0.08Ga0.92N on P-InAlN gate normally-off high electron mobility transistors

Normally-off AlGaN/GaN high electron mobility transistors (HEMTs) are indispensable devices for power electronics as they can greatly simplify circuit designs in a cost-effective way. In this work, the electrical characteristics of p-type InAlN gate normally-off AlGaN/GaN HEMTs with a step buffer layer of Al0.25Ga0.75N/Al0.1Ga0.9N is studied numerically. Our device simulation shows that a p-InAlN gate with a step buffer layer allows the transistor to possess normally-off behavior with high drain current and high breakdown voltage simultaneously. The gate modulation by the p-InAlN gate and the induced holes appearing beneath the gate at the GaN/Al0.25Ga0.75N interface is because a hole appearing in the p-InAlN layer can effectively vary the threshold voltage positively. The estimated threshold voltage of the normally-off HEMTs explored is 2.5 V at a drain bias of 25 V, which is 220% higher than the conventional p-AlGaN normally-off AlGaN/GaN gate injection transistor (GIT). Concurrently, the maximum current density of the explored HEMT at a drain bias of 10 V slightly decreases by about 7% (from 240 to 223 mA mm−1). At a drain bias of 15 V, the current density reached 263 mA mm−1. The explored structure is promising owing to tunable positive threshold voltage and the maintenance of similar current density; notably, its breakdown voltage significantly increases by 36% (from 800 V, GIT, to 1086 V). The engineering findings of this study indicate that novel p-InAlN for both the gate and the step buffer layer can feature a high threshold voltage, large current density and high operating voltage for advanced AlGaN/GaN HEMT devices.

Using in-process measurements of open-gate structures to evaluate threshold voltage of normally-off GaN-based high electron mobility transistors

The parameters of open-gate structures treated with different etching time were monitored during the gate recess process, and their impacts on the threshold voltage (Vth) of final fabricated AlGaN/GaN high electron mobility transistors (HEMTs) based on open-gate structures were discussed in this paper. It is found that Vth can exceed 0 V when channel resistance in the recessed region (Ron-open) increases over ∼275 Ω mm, maximum current (IDmax) decreases below ∼29 mA/mm, or recessed barrier thickness (tRB) is below ∼7.5 nm. In addition, tRB obtained by atomic force microscopy measurements and C-V measurements are also compared. Finally, theoretical common criteria based on the experimental results of this work for tRB and Ron-open were established to evaluate the Vth of a regular normally-off AlGaN/GaN HEMTs. The results indicate that these parameters of open-gate structure can be utilized to achieve normally-off HEMTs with controllable Vth.

Enhancement mode HEMT using fluorine implantation below the channel

Gallium nitride based high electron mobility transistors (HEMT) are powerful candidates for high frequency and high power applications. While switching applications demand normally-off operation, these devices are normally-on. Recent normally-off HEMTs were demonstrated by implanting fluorine above the channel, in the barrier layer. During implantation, fluorine ions penetrate into the channel and cause mobility degradation. In this paper, we propose and simulate an alternative approach in which fluorine ions are implanted below the channel of the HEMT rather than above it. The simulation tool ATLAS is calibrated using experimental data from a real HEMT device. Simulation results have shown that implanting fluorine ions below the channel is capable of achieving normally-off operation. When compared to the implantation in the barrier layer, the proposed approach offers better confinement for the two dimensional electron gas (2DEG) below the gate, eliminates the scattering of fluorine ions with channel electrons and is more efficient when it comes to the fluorine concentration required to achieve a desired threshold voltage. This technique neither affects the breakdown voltage nor the off-state current.

An analytical current-voltage characteristics model for high electron mobility transistors based on nonlinear charge-control formulation

Consideration of the nonlinear variation of the sheet carrier concentration of the two-dimensional electron gas (2-DEG) with Fermi potential in the quasi-triangular potential quantum well of the HEMT (high electron mobility transistor) structure has led to a bias-dependent effective offset distance of the 2-DEG from the heterointerface. The inclusion of the variable 2-DEG offset distance allows model characterization of the charge-control mechanism in a more consistent manner, and with greater accuracy, than the conventional linear charge-control model does. On the basis of the nonlinear charge-control formulation, the authors developed an accurate analytical drain current-voltage characteristics model for HEMT devices. This model is valid over a very wide range of operation, extending from the near-subthreshold regime to the high parasitic MESFET conduction regime to the high parasitic MESFET conduction regime. This model also includes the broadening effect of the 2-DEG quantum well in the pinchoff regime, providing a more accurate description of the current saturation mechanism. The authors demonstrate the effectiveness and accuracy of this model by comparing measured and modeled DC characteristics of normally-on as well as normally-off HEMT devices. Furthermore, the simple analytical expressions make the model very suitable for computer-aided design applications in the analysis and design of high-frequency microwave and high-speed digital HEMT devices and integrated circuits. >