Showing papers by "Xiaohua Ma published in 2019"

Perimeter Driven Transport in the p-GaN Gate as a Limiting Factor for Gate Reliability

Abstract: In this paper, we present an in-depth study of the gate leakage mechanisms and correlated breakdown of GaN-based power HEMTs with p-GaN gate, controlled by a Schottky metal/p-GaN junction. A detailed investigation of the process split and geometry dependency is done. From this study, we propose that a parasitic sidewall transistor is present, which is the cause for degradation in the p-GaN gate. The sidewall leakage has been substantiated by TCAD simulation and also by a novel method consisting of EBIC measurements directly applied on the cross section of a p-GaN gate. Based on this analysis we performed a process modification, which has led to a significant improvement in terms of gate reliability.

High Channel Conductivity, Breakdown Field Strength, and Low Current Collapse in AlGaN/GaN/Si $\delta$ -Doped AlGaN/GaN:C HEMTs

Abstract: This paper reports the AlGaN/GaN/ Si $\delta $ -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 $\delta $ -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 $\delta $ -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 $\delta $ -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 V at $\textsf {10}~\mu \text{A}$ /mm (7- $\mu \text{m}$ gate–drain spacing) and 0.53 $\text{m}\Omega ~\cdot $ cm $^{{2}}$ and the gate-to-drain electric field corresponds to 1.1 MV/cm.

Physically Transient Memristor Synapse Based on Embedding Magnesium Nanolayer in Oxide for Security Neuromorphic Electronics

Abstract: In this letter, a fully physically transient artificial synapse based on W/MgO/Mg/MgO/W memristor was realized for the first time. By embedding magnesium nanolayer in MgO switching layer, multilevel, and long-term memory with precise tuning ability was obtained. In addition, the device shows significant synaptic functions including long-term potentiation (LTP) and long-term depression (LTD). Besides, device failure can be triggered while it was immersed in deionized (DI) water for one minute at room temperature. The physically transient synaptic devices based on W/MgO/Mg/MgO/W memristor demonstrate great potential for secure neuromorphic devices, green electronics, and bioimplant electronics.

A Millimeter-Wave AlGaN/GaN HEMT Fabricated With Transitional-Recessed-Gate Technology for High-Gain and High- Linearity Applications

Abstract: A high-linearity and high-gain AlGaN/GaN HEMTs with a 100-nm gate were demonstrated. The device employs transitional recessed gate (TRG) along the gate width for millimeter wave power application. The gradually changing gate recess depth was created using transitional dosed photoetching. Accurate etching ensured that the FET-elements have a continued $\text{V}_{\mathrm {ts}}$ offset in the local equivalent threshold voltage ( $\text{V}_{\mathrm {th}}$ ) in different areas. The device exhibits a high $\text{I}_{\mathrm {d,max}}$ of 1.12 A/mm and a high peak extrinsic $\text{g}_{\mathrm {m}}$ of 374 mS/mm with an improved gate swing >2.6 V, much higher than that of Fin-HEMT. Excellent RF performance was shown, including $\text{f}_{\mathrm {T}}/\text{f}_{\mathrm {max}} =61.8$ /148.8 GHz, $\text{G}_{\mathrm {as}}/\text{G}_{\mathrm {linear}} = {9.98}$ / 12dB at 30 GHz. To the best of our knowledge, this is the best associated gain and linearity performance reported to date for AlGaN/GaN HEMTs. This letter has great potential for high gain and linearity millimeter wave power applications, which are needed for future communication systems.

Effect of thickness of metal electrode on the performance of SiNx-based resistive switching devices

Abstract: This letter studies the effect of the thickness of the top electrode on the performance of a SiNx resistive switching layer. We fabricated six devices with Ta electrodes of different thickness values (8 nm, 10 nm, 15 nm, 30 nm, 40 nm, and 50 nm) in a Ta/SiNx/Pt structure and then systematically investigated their performance. The high electrode thickness devices show stable and self-compliant bipolar resistive switching characteristics. In contrast, low electrode thickness devices display unstable RS behavior and have a high set voltage. In the low resistance state region, the Ta/SiNx/Pt devices obey Ohmic conduction, while in the high resistance state region, the conduction mechanism is Schottky emission. To explain the different RS behavior in the two device types, a nitrogen-ion-based model has been presented. According to this model, the device with a thicker top electrode has a stronger nitrogen accommodation ability, while the migration of nitrogen ions and silicon dangling bonds dominates conductive behavior.

Reduced reverse gate leakage current for GaN HEMTs with 3 nm Al/40 nm SiN passivation layer

Abstract: An obvious increase in the gate leakage current has been commonly observed in GaN HEMTs, after Plasma-enhanced chemical vapor deposition (PECVD) SiN passivation has been observed to obviously increase. This paper presents an Al/SiN stack layer passivation structure. The high gate leakage current in GaN HEMTs caused by the PECVD SiN passivation is distinctly reduced by 2 to 3 orders of magnitude by introducing a thin Al layer. It is mainly attributed to the Al layer blocking and minimizing the damage for the (Al)GaN surface in the build-up of the luminance process of PECVD SiN and then reducing the surface trap density. TEM mapping and SRIM software simulation reveal that neither damage nor inter-diffusion is demonstrated at the Al/AlGaN interface, where a continuous crystalline region is observed. The moderate current collapse suppression and 32.8% improvement in VBR are achieved in GaN HEMTs with Al/SiN passivation.

Solution-Processed Physically Transient Resistive Memory Based on Magnesium Oxide

Abstract: In this letter, physically transient resistive memory devices based on solution-processed MgO films were demonstrated for the first time. Stable and reproducible resistive switching memory performances were achieved with Mg/solution-processed MgO/W devices, which have larger OFF/ON resistance ratio ( $10^{5}$ ) than the Mg/sputtered MgO/W devices. Triggered failures can be achieved by immersing the transient devices in deionized water for 6 min at room temperature. In addition, flexible and transient memory devices were accomplished by water-assisted transfer printing method and no significant degradation is observed under a bending radius of 3 mm. The solution-processed MgO-based dissolvable devices demonstrate great potential in low-cost transient electronics, flexible memory systems, secure electronics, and biocompatible devices.

Physically Transient Memristive Synapse With Short-Term Plasticity Based on Magnesium Oxide

Abstract: In this letter, fully transient artificial synapses based on magnesium oxide memristors with short-term plasticity (STP) were proposed for the first time. Typical physiological reactions related to STP including pair-pulse facilitation and pair-pulse depression were demonstrated in such a transient synaptic device. Importantly, water-assisted transfer printing method was employed to transfer the dissolvable synaptic arrays onto bioresorbable poly (vinyl alcohol) substrate to form a fully transient system, which finally disintegrated in deionized water within 30 min. This transient synaptic emulator possesses great potential in security neuromorphic computing and environmentally resorbable applications.

Influence of Different Fin Configurations on Small-Signal Performance and Linearity for AlGaN/GaN Fin-HEMTs

Abstract: In this paper, AlGaN/GaN high-electron mobility transistors (HEMTs) with different fin configurations are fabricated and analyzed. Through S-parameter measurements and modeling of the designed devices, a detailed RF investigation on small-signal model parameters is performed under different biasing conditions. Good agreements between measured and simulated scattering parameters up to 40 GHz illustrate the validity and accuracy of the model. The influence of different fin structures on model parameters and linearity improvement is examined, and this can help to improve the frequency characteristics of fin structure by optimizing the fin length, fin width, and trench width. The significant linearity of Fin-HEMTs is confirmed by the analysis of the model parameters, which is the first time to study the small-signal characteristic of AlGaN/GaN Fin-HEMTs in detail.

Hetero-integration of quasi two-dimensional PbZr0.2Ti0.8O3 on AlGaN/GaN HEMT and non-volatile modulation of two-dimensional electron gas

Abstract: In theory, two-dimensional electron gas (2DEG) density in AlGaN/GaN heterostructures can be modulated via the polarization of a ferroelectric material. Therefore, the integration of ferroelectric materials with AlGaN/GaN high electron mobility transistors (HEMTs) is a very attractive method for exploring further applications of electronic devices. This paper reports on the integration of single crystal PbZr0.2Ti0.8O3 (PZT) thin films (20 nm) and AlGaN/GaN HEMT via a substrate transfer technique. Continuous and nonvolatile modulation of 2DEG density in the AlGaN/GaN heterojunction interface is achieved in accordance with the theory. By poling the PZT/AlGaN/GaN HEMT from 2 V to 7 V, the device exhibits different threshold voltages (from −3 V to 1 V). A modulation of 2DEG density up to 300% is achieved. Moreover, the retention characteristic of the device is excellent, and the variation of the threshold voltage is less than 0.25 V after 105 s. Compared to the conventional method of depositing PZT directly on AlGaN/GaN HEMT, the transfer technique can achieve better quality in a much thinner film and prevent the interdiffusion during growth. At the same time, the excellent quality of the transferred PZT film has a good ability to modulate the 2DEG. The generality of our approach paves the way for integrating ferroelectric materials on the GaN-based devices, and this is promising for using the device in more application fields.

Significant Degradation of AlGaN/GaN High-Electron Mobility Transistors With Fast and Thermal Neutron Irradiation

Abstract: The AlGaN/GaN high-electron mobility transistors (HEMTs) were irradiated with fast and thermal neutrons at various fluences. Output, transfer, and gate-leakage characteristics were analyzed in detail before and after irradiation. After irradiation by 14-MeV neutrons, the electrical characteristics of the devices gradually degraded as the fluence increased. After irradiation by thermal neutrons at a fluence of $2.12 \times 10^{16}$ cm−2, the electrical characteristics of devices significantly decreased. Neutrons are believed to introduce a negatively charged acceptorlike deep level in the AlGaN layer, which can deplete 2DEG in the channel, raise the energy band, and shift the threshold voltage to positive values. Such an enhanced barrier can significantly suppress the gate Schottky leakage current of AlGaN/GaN HEMTs in both the reverse and forward bias regions. After the thermal neutron radiation, the quality of Schottky contacts deteriorated significantly.

GaN/Al x Ga 1−x N/GaN heterostructure IMPATT diode for D-band applications

Abstract: In this paper, a novel structural impact ionization avalanche transit time (IMPATT) diode configured by GaN/AlxGa1−xN/GaN heterostructure is investigated at the operation frequency of D-Band. Simulation results show that, with Al composition x varies from 0.2 to 0.6, a more localized avalanche region width is obtained, the device breakdown voltage increases gradually, while the RF output power and the DC-to-RF conversion efficiency have also shown significant improvement as compared with the GaN homostructure IMPATT diode. The highest values of the RF output power density and the DC-to-RF conversion efficiency of GaN/Al0.4Ga0.6N/GaN heterostructure are obtained as 1.56 MW/cm2 and 21.99%, larger than that of 1.02 MW/cm2 and 16.37% for GaN homostructure IMPATT diode. Meanwhile, the lowest Q factor can be achieved, which implies that heterostructure IMPATT diodes exhibit better stability and higher growth rate of microwave oscillation compared with conventional IMPATT diodes.

Physically Transient Resistive Switching Memory With Material Implication Operation

Abstract: In this letter, physically transient resistive switching devices with structure of Mg/SiO2/W for logic operation were proposed for the first time. Despite the desirable nonvolatile memory characteristics, the dissolvable memory devices could be used to achieve material implication function. Meanwhile, the transient memory arrays were transferred onto a dissolvable poly (vinyl alcohol) substrate by water-assisted transfer printing technique, and the transient system was capable of dissociating in deionized water after 20 min. This transient Mg/SiO2/W device demonstrates the great potential in future in-memory logic computing systems and secure electronic applications.

Millimeter-wave AlGaN/GaN HEMTs breakdown voltage enhancement by an air-bridge recessed source field plate (RSFP)

Abstract: In this paper, an innovative air-bridge recessed source field plate is implemented into the fabrication of high voltage mm-wave GaN high-electron-mobility transistors (HEMTs). The device integrates a metal field plate (FP) and an air-bridge FP that locate over the gate region and land between the gate and drain. The reduced peak electric field contributes to a three-terminal breakdown voltage of 144 V for GaN HEMTs with LSD = 2.4 μm, which is 96 V for the normal mm-wave device. One order of magnitude lower in off-state leakage currents and well suppressed current collapse are also achieved. Particularly, slight increase in Cgs and Cgd was obtained after applying the air-bridge field plate, keeping the high frequency characteristic of mm-wave devices. The technology might attract significant attention in mm-wave GaN HEMTs.

Record combination fmax · Vbr of 25 THz·V in AlGaN/GaN HEMT with plasma treatment

Abstract: A combination of high maximum oscillation frequency (fmax) and breakdown voltage (Vbr) was achieved in AlGaN/GaN high electron mobility transistors (HEMTs) with N2O plasma treatment on the access region. The breakdown voltage is improved from 37 V to 80 V due to the formation of oxide layer in the gate region. A suppressed current collapse is obtained due to plasma treatment on the gate-source and the gate-drain regions. The effect of the present passivation method is almost the same with that of the conventional SiN passivation method, meanwhile it can avoid introducing additional parasitic capacitance due to thinner thickness. The small signal measurement shows that HEMT can yield fT and fmax of 98 and 322 GHz, respectively, higher than that of 70 and 224 GHz for the non-treated HEMT. By using the plasma treatment technique, the HEMT can simultaneously exhibit high fmax and Vbr with a record fmax⋅Vbr of 25 THz⋅V.

Overcoming Limited Resistance in 1T1R RRAM Caused by Pinch-Off Voltage During Reset Process

Abstract: This article explains the difference in the reset processes between single RRAM and 1T1R RRAM structures. In the single RRAM, as the reset voltage increases, the off-state resistance increases. This is a normal RRAM operation. However, in the 1T1R structure, the reset voltage increases while the OFF-state resistance does not increase. This may be caused by either current or voltage being insufficient to switch the RRAM to its higher resistance state. The measurements of real voltage indicate that the insufficient voltage is in fact the main cause. For the 1T1R RRAM structure, if it is to be reset to a higher resistance state, it is best done by increasing the gate voltage to increase the OFF-state resistance. Therefore, we provide a method to reduce the misjudgment in the array, and it would be beneficial for the development of high-density memory.

Millimeter-wave AlGaN/GaN HEMT breakdown voltage enhancement by a recessed float field plate

Abstract: We propose an innovative recessed float field plate (RFFP) structure for millimeter-wave AlGaN/GaN high-electron-mobility transistors (HEMTs). The reduced peak electric field contributes to a three-terminal breakdown voltage of 122–140 V for RFFP GaN HEMTs with L GD = 1.35 μm, which is found to increase from 84 to 101 V. The current collapse characteristic is also improved significantly. No obvious increase in parasite capacitance is observed after integrating the novel RFFP, thereby maintaining the high-frequency performance of GaN HEMTs. This technique is especially attractive for HEMTs of millimeter-wave frequency and above, and has great potential for applications.

Terahertz monolithic integrated waveguide transmission lines based on wide bandgap semiconductor materials

Abstract: In this paper, terahertz monolithic integrated waveguide (TMIW) transmission lines (TLs) are analyzed and fabricated based on wide bandgap semiconductor materials. The ignored parameters of TLs at low frequencies, such as the via-hole shape, the titanium/gold thickness and roughness, and the micropipe effect of material, are especially investigated based on the semiconductor fabrication technique and the terahertz-wave theory. Subsequently, the fine optimized TMIW-TL and the transition to grounded coplanar waveguide are fabricated. The measurement shows that the attenuation constant of a TL is as low as 0.19 dB/λg at 220 GHz. The proposed TMIW can provide electrical performance similar to the conventional air-filled waveguide, while it has only 10% of the volume, which indicates a great potential for future terahertz integrated systems.

Growth of Monolayer WS2 Single Crystals with Atmospheric Pressure CVD: Role of Temperature

Abstract: It has been demonstrated that the introduction of NaCl can significantly improve the quality of monolayer WS2 at the growth temperatures ranging from 700°C to 850°C by atmospheric pressure chemical vapor deposition (APCVD) without the assistant of hydrogen. Here, the influence of NaCl on the nucleation and growth of WS2 has been thoroughly investigated. The morphology and quality of WS2 grown with different temperatures are discussed by optical microscope, Raman and Photoluminescence (PL) spectra. It was found that amount of NaCl can efficiently influence the morphology and quality of WS2 crystals. PL intensity of WS2 crystal increases around three times from the center region to the edge of an individual domain, which may be attributed to the appearance of small triangle hollows formed during the growth at the edge of single crystal WS2.

Investigation of the nanochannel geometry modulation on self-heating in AlGaN/GaN Fin-HEMTs on Si

Abstract: The impact of the nanochannel geometry on the thermal performance of AlGaN/GaN Fin-HEMTs was investigated. Structures with a larger nanochannel space (Sfin) or a smaller nanochannel width (Wfin) show the suppressed self-heating effect, which was confirmed by the smaller dispersion between DC and pulsed measurements together with the reduced high temperature degradation of the drain current. A closed-form expression based on the heat flow theory was used to illustrate the coupling effect of Wfin and Sfin, and the channel temperature was mapped considering the different nanochannel geometries. As Wfin and Sfin scale down simultaneously, the nanochannel structure would present a much lower channel temperature, demonstrating that the heat generation dominated by Wfin has a greater impact on the self-heating in Fin-HEMTs.The impact of the nanochannel geometry on the thermal performance of AlGaN/GaN Fin-HEMTs was investigated. Structures with a larger nanochannel space (Sfin) or a smaller nanochannel width (Wfin) show the suppressed self-heating effect, which was confirmed by the smaller dispersion between DC and pulsed measurements together with the reduced high temperature degradation of the drain current. A closed-form expression based on the heat flow theory was used to illustrate the coupling effect of Wfin and Sfin, and the channel temperature was mapped considering the different nanochannel geometries. As Wfin and Sfin scale down simultaneously, the nanochannel structure would present a much lower channel temperature, demonstrating that the heat generation dominated by Wfin has a greater impact on the self-heating in Fin-HEMTs.