Study of the breakdown failure mechanisms for power AlGaN/GaN HEMTs implemented using a RF compatible process

The breakdown failure mechanisms for a family of power AlGaN/GaN HEMTs were studied. These devices were fabricated using a commercially available MMIC/RF technology with a semi-insulating SiC substrate. After a 10 min thermal annealing at 425 K, the transistors were subjected to temperature dependent electrical characteristics measurement. Breakdown degradation with a negative temperature coefficient of ?0.113 V/K for the devices without field plate was found. The breakdown voltage is also found to be a decreasing function of the gate length. Gate current increases simultaneously with the drain current during the drain-voltage stress test. This suggests that the probability of a direct leakage current path from gate to the 2-DEG region. The leakage current is attributed by a combination of native and generated traps/defects dominated gate tunneling, and hot electrons injected from the gate to channel. Devices with field plate show an improvement in breakdown voltage from ～40 V (with no field plate) to 138 V and with lower negative temperature coefficient. A temperature coefficient of ?0.065 V/K was observed for devices with a field plate length of 1.6 μm.     

DC and microwave characteristics of Lg 50 nm T-gate InAlN/AlN/GaN HEMT for future high power RF applications

The DC and microwave characteristics of L_g = 50 nm T-gate InAlN/AlN/GaN High Electron Mobility Transistor (HEMT) on SiC substrate with heavily doped n+ GaN source and drain regions have demonstrated using Synopsys TCAD tool. The proposed device features an AlN spacer layer, AlGaN back-barrier and SiN surface passivation. The proposed HEMT exhibits a maximum drain current density of 1.8 A/mm, peak transconductance (g_m) of 650 mS/mm and ft/fmax of 118/210 GHz. At room temperature, the measured carrier mobility, sheet charge carrier density (n_s) and breakdown voltage are 1195 cm²/Vs, 1.6 × 10¹³ cm⁻² and 18 V respectively. The superlatives of the proposed HEMTs are bewitching competitor for future monolithic microwave integrated circuits (MMIC) applications particularly in W-band (75–110 GHz) high power RF applications.     

New degradation mechanism observed for AlGaN/GaN HEMTs with sub 100 nm scale unpassivated regions around the gate periphery

AlGaN/GaN HEMTs with low gate leakage current in the μA/mm range have been fabricated with a small-unpassivated region close to the gate foot. They showed considerably higher critical voltage values (average VCR = 60 V) if subjected to step stress testing at OFF-state conditions and room temperature as compared to standard devices with conventional gate technology. This is due to the fact that electrons injected from the gate can be accumulated at the unpassivated region and thus builds up negative charge. The lower gate leakage is due to virtual gate formation, which is reducing local electric field in the vicinity of the gate. In contrast to devices with standard gate technology, degradation during step stressing is not associated with a simultaneous gate leakage and drain leakage current increase but with a strong increase of drain current at OFF-state conditions while the gate leakage is practically not affected. Then a relatively higher critical voltage of around 60 V is achieved. An abrupt increase of subthreshold drain current implies the formation of a conductive channel bypassing the gate region without influencing gate leakage. It is believed that hopping conductivity via point defects formed during device stressing creates this channel. Once this degradation mode takes place, the drain current of affected devices significantly drops. This can be explained by negative trap formation in the channel region affecting the total charge balance in 2DEG region. Electroluminescence measurements on both fresh and degraded devices showed no hot spots at OFF-state conditions. However, there is additional emission at ON-state bias, which suggests additional energetic states that lead to radiative electron transition effects in the degraded devices, most possibly defect states in the buffer.     

The role of surface barrier oxidation on AlGaN/GaN HEMTs reliability

Reliability of AlGaN/GaN HEMTs processed with different surface oxidation levels was studied using electrical and optical methods. It was found that HEMTs with more surface oxide content are more susceptible to degradation in terms of gate leakage and trapping characteristics, although this oxide layer initially passivates surface traps. In the degraded devices, trap level with activation energy of 0.45–0.47 eV was observed and attributed to surface related traps. This indicates that oxygen may play a crucial role for AlGaN/GaN HEMT reliability.     

Charge trapping related channel modulation instability in P-GaN gate HEMTs

In this paper, a study of the channel modulation instability of commercial p-GaN gate HEMTs is presented. During the gate-voltage stress test, substantial R_DS(ON) variations up to 78 mΩ (93.8%) were observed. It is found that the p-GaN/AlGaN/GaN gate structure enables the injection of holes and electrons, which can be captured by the donor/acceptor-like traps located in the AlGaN layer. Therefore, the trapped holes and electrons concurrently modulate the channel conductivity, resulting in R_DS(ON) variations. Device simulation was performed to help explain the mechanism from the perspective of energy band. In addition, results reveal that with the recommended working gate-voltage stress V_GS = 7 V, the on-state resistance, the threshold voltage and the off-state drain to source leakage current vary up to 8 mΩ (16.3%), 0.2 V (14.8%) and 12.8 μA (42.66%) within 1 h, respectively, which could raise reliability issues for the power electronics applications of p-GaN gate HEMTs.     

High-performance enhancement-mode AlGaN/GaN HEMTs using fluoride-based plasma treatment

We report a novel approach in fabricating high-performance enhancement mode (E-mode) AlGaN/GaN HEMTs. The fabrication technique is based on fluoride-based plasma treatment of the gate region in AlGaN/GaN HEMTs and post-gate rapid thermal annealing with an annealing temperature lower than 500/spl deg/C. Starting with a conventional depletion-mode HEMT sample, we found that fluoride-based plasma treatment can effectively shift the threshold voltage from -4.0 to 0.9 V. Most importantly, a zero transconductance (g/sub m/) was obtained at V/sub gs/=0 V, demonstrating for the first time true E-mode operation in an AlGaN/GaN HEMT. At V/sub gs/=0 V, the off-state drain leakage current is 28 /spl mu/A/mm at a drain-source bias of 6 V. The fabricated E-mode AlGaN/GaN HEMTs with 1 /spl mu/m-long gate exhibit a maximum drain current density of 310 mA/mm, a peak g/sub m/ of 148 mS/mm, a current gain cutoff frequency f/sub T/ of 10.1 GHz and a maximum oscillation frequency f/sub max/ of 34.3 GHz.     

Degradation of AlGaN/GaN HEMTs under elevated temperature lifetesting

Elevated temperature lifetesting was performed on 0.25 μm AlGaN/GaN HEMTs grown by MOCVD on 2-in. SiC substrates. A temperature step stress (starting at T_a of 150 °C with a step of 15 °C; ending at T_a of 240 °C; 48 h for each temperature cycle) was employed for the quick reliability evaluation of AlGaN/GaN HEMTs. It was found that the degradation of AlGaN/GaN HEMTs was initiated at ambient temperature of 195 °C. The degradation characteristics consist of a decrease of drain current and transconductance, and an increase of channel-on-resistance. However, there is no noticeable degradation of the gate diode (ideality factor, barrier height, and reverse gate leakage current). The FIB/STEM technique was used to examine the degraded devices. There is no detectable ohmic metal or gate metal interdiffusion into the epitaxial materials. Accordingly, the degradation mechanism of AlGaN/GaN HEMTs under elevated temperature lifetesting differs from that observed in GaAs and/or InP HEMTs. The reliability performance was also compared between two vendors of AlGaN/GaN epilayers. The results indicate that the reliability performance of AlGaN/GaN HEMTs could strongly depend on the material quality of AlGaN/GaN epitaxial layers on SiC substrates.     

X波段大功率GaN HEMT的研制

在研制了AlGaN/GaN HEMT外延材料的基础上,采用标准工艺制作了2.5mm大栅宽AlGaN/GaNHEMT。直流测试中,Vg=0V时器件的最大饱和电流Ids可达2.4A,最大本征跨导Gmax为520mS,夹断电压Voff为-5V;通过采用带有绝缘层的材料结构及离子注入的隔离方式,减小了器件漏电,提高了击穿电压,栅源反向电压到-20V时,栅源漏电在10-6A数量级;单胞器件测试中,Vds=34V时,器件在8GHz下连续波输出功率为16W,功率增益为6.08dB,峰值功率附加效率为43.0%;2.5mm×4四胞器件,在8GHz下,连续波输出功率42W,功率增益8dB,峰值功率附加效率34%。     

Drain-to-gate field engineering for improved frequency response of GaN-based HEMTs

We report on a novel approach for designing high-frequency AlGaN/GaN HEMTs based on gate-drain field engineering. This approach uses a drain-connected field controlling electrode (FCE). The devices with gate-to-FCE separation of 0.5–0.7 μm exhibit much smaller frequency behavior degradation with drain bias at least up to 30 V and yield RF gain and output power improvement up to ～2 times compared to conventional devices. These results show that the FCE is a powerful technique of improving the high-frequency, high power performance of GaN HEMTs at high drain biases.     

Influence of processing and annealing steps on electrical properties of InAlN/GaN high electron mobility transistor with Al2O3 gate insulation and passivation

We report on preparation and electrical characterization of InAlN/AlN/GaN metal–oxide–semiconductor high electron mobility transistors (MOS HEMTs) with Al₂O₃ gate insulation and surface passivation. About 12 nm thin high-κ dielectric film was deposited by MOCVD. Before and after the dielectric deposition, the samples were treated by different processing steps. We monitored and analyzed the steps by sequential device testing. It was found that both intentional (ex situ) and unintentional (in situ before Al₂O₃ growth) InAlN surface oxidation increases the channel sheet resistance and causes a current collapse. Post deposition annealing decreases the sheet resistance of the MOS HEMT devices and effectively suppresses the current collapse. Transistors dimensions were source-to-drain distance 8 μm and gate width 2 μm. A maximum transconductance of 110 mS/mm, a drain current of ～0.6 A/mm (V_GS = 1 V) and a gate leakage current reduction from 4 to 6 orders of magnitude compared to Schottky barrier (SB) HEMTs was achieved for MOS HEMT with 1 h annealing at 700 °C in forming gas ambient. Moreover, InAlN/GaN MOS HEMTs with deposited Al₂O₃ dielectric film were found highly thermally stable by resisting 5 h 700 °C annealing.     

The effect of surface passivation on the microwave characteristicsof undoped AlGaN/GaN HEMTs

Surface passivation of undoped AlGaN/CaN HEMT's reduces or eliminates the surface effects responsible for limiting both the RF current and breakdown voltages of the devices. Power measurements on a 2×125×0.5 μm AlGaN/GaN sapphire based HEMT demonstrate an increase in 4 GHz saturated output power from 1.0 W/mm [36% peak power-added efficiency (PAE)] to 2.0 W/mm (46% peak PAE) with 15 V applied to the drain in each case. Breakdown measurement data show a 25% average increase in breakdown voltage for 0.5 μm gate length HEMT's on the same wafer. Finally, 4 GHz power sweep data for a 2×75×0.4 μm AlGaN/GaN HEMT on sapphire processed using the Si₃N₄ passivation layer produced 4.0 W/mm saturated output power at 41% PAE (25 V drain bias). This result represents the highest reported microwave power density for undoped sapphire substrated AlGaN/GaN HEMT's     

Critical design issues for high-power GaN/AlGaN anti-serial Schottky varactor frequency triplers

In this paper the importance of a new design variable for high power anti-serial Schottky varactors, the aluminum composition of the AlGaN barrier layer, is demonstrated. AlGaN/GaN varactors containing either (1) a high-doped/low-doped GaN region or (2) just a low doped GaN region have been compared demonstrating that the selection of the device structure also depends on the amplitude of the input signal being tripled in frequency. Stronger susceptance modulation is exhibited in AlGaN/GaN ASVs made from Ga-face polar material compared to N-face polar material. Results indicate choosing the proper aluminum composition results in 27% conversion efficiency with an input signal of 5 GHz and over 7% conversion efficiency with an input signal of 60 GHz along with optimization trends. With input voltage amplitudes over 10 V an AlGaN/GaN structure with 15% Al provides greater conversion efficiency than one with 5% Al. Power absorbed in the varactor also increases as aluminum percent increases affecting reliability and power transfer. Results of a GaN ASV performing as a frequency tripler for fundamental frequencies up to 110 GHz indicate an advantage to using an AlGaN/GaN epi-structure over only a GaN epi-structure.     

Electron transport in passivated AlGaN/GaN/Si HEMTs

AlGaN/GaN/Si high electron mobility transistors (HEMTs) grown by molecular beam epitaxy are investigated using direct-current and radio-frequency measurements. As has been found, the maximum of drain current achieves 881 mA/mm with an extrinsic current gain cutoff frequency of 37 GHz for a 0.25 µm gate length. Pulsed characteristics also showed a reduction of trapping centers that improves the quality of the epilayers.     

Thermal storage effects on AlGaN/GaN HEMT

The effects of thermal storage on GaN–HEMT devices grown on SiC substrate have been investigated by DC and pulsed electrical measurements, breakdown measurements (by means of a Transmission Line Pulser, TLP), and optical and electron microscopy. After 3000 h of thermal storage testing at 300 °C, only a limited reduction of the DC drain saturation current and of the transconductance peak was observed (20% and 25% decrease, respectively). However, pulsed measurements on aged devices clearly highlight a dramatic current collapse effect that has been attributed to a creation of surface traps in the gate-to-drain and gate-to-source access region. On-state breakdown characterization carried out on aged devices did not highlight any noticeable changes with respect to the untreated devices similarly to the DC characterization. Failure analyses have demonstrated that a loss of adhesion of the passivation layer was responsible for the observed trap formation. An improved passivation deposition process was therefore developed, including a surface cleaning procedure aimed at preventing passivation detaching. The devices fabricated using this new procedure do not show any enhancement of trapping effects up to 500 h of thermal stress at 300 °C.     

A 24 W Ku band GaN based power amplifier with 9.1 dB linear gain

A Ku-band power amplifier is successfully developed with a single chip 4.8 mm AlGaN/GaN high electron mobility transistors (HEMTs). The AlGaN/GaN HEMTs device, achieved by E-beam lithography г-gate process, exhibited a gate-drain reverse breakdown voltage of larger than 100 V, a cutoff frequency of f_T=30 GHz and a maximum available gain of 13 dB at 14 GHz. The pulsed condition (100 μs pulse period and 10% duty cycle) was used to test the power characteristic of the power amplifier. At the frequency of 13.9 GHz, the developed GaN HEMTs power amplifier delivers a 43.8 dBm (24 W) saturated output power with 9.1 dB linear gain and 34.6% maximum power-added efficiency (PAE) with a drain voltage of 30 V. To our best knowledge, it is the state-of-the-art result ever reported for internal-matched 4.8 mm single chip GaN HEMTs power amplifier at Ku-band.     

Analysis of traps effect on AlGaN/GaN HEMT by luminescence techniques

The study is carried out on AlGaN/GaN HEMTs presenting current collapse effect at V_ds lower than 6 V. This effect is completely recovered by illuminating the component with light of 710 nm wavelength (1.75 eV). The spectral analysis of the light emission in the visible near infrared spectrum shows a bell-shape with superimposed distinct emission peaks. These features suggest that the electroluminescence (EL) signal is due to the direct intraband of electrons and inelastic intraband transition of electrons due to scattering by charged centres. Photoionisation experiments have been conducted to determine the light wavelengths/energies that separately change the drain current and the gate leakage current.     

Optimization design on breakdown voltage of AlGaN/GaN high-electron mobility transistor

Simulations are carried out to explore the possibility of achieving high breakdown voltage of GaN HEMT (high-electron mobility transistor). GaN cap layers with gradual increase in the doping concentration from 2×10¹⁶ to 5×10¹⁹ cm^-3 of N-type and P-type cap are investigated, respectively. Simulation results show that HEMT with P-doped GaN cap layer shows more potential to achieve higher breakdown voltage than N-doped GaN cap layer under the same doping concentration. This is because the ionized net negative space charges in P-GaN cap layer could modulate the surface electric field which makes more contribution to RESURF effect. Furthermore, a novel GaN/AlGaN/GaN HEMT with P-doped GaN buried layer in GaN buffer between gate and drain electrode is proposed. It shows enhanced performance. The breakdown voltage of the proposed structure is 640 V which is increased by 12% in comparison to UID (un-intentionally doped) GaN/AlGaN/GaN HEMT. We calculated and analyzed the distribution of electrons'' density. It is found that the depleted region is wider and electric field maximum value is induced at the left edge of buried layer. So the novel structure with P-doped GaN buried layer embedded in GaN buffer has the better improving characteristics of the power devices.     

Performance of commercial foundry-level AlGaN/GaN HEMTs after hot electron stressing

The performance degradation of commercial foundry level GaN HEMTs placed under a constant-power drain voltage step-stress test has been studied. By utilizing electroluminescence measurement techniques to optimize hot electron stress testing conditions (Meneghini, 2012), no significant permanent changes in saturation current (I_dss), transconductance (Gm), and threshold voltage (V_th) can be seen after stress testing of drain voltages from 30 V up to 200 V. We observe little permanent degradation due to hot electron effects in GaN HEMTs at these extreme operating conditions and it is inferred that other considerations, such as key dimensions in channel or peak electric field (Chynoweth, 1958; Zhang and Singh, 2001) [2,3], are more relevant to physics of failure than drain bias alone.     

栅槽刻蚀工艺对增强型GaN HEMT器件性能的影响

基于凹槽栅增强型氮化镓高电子迁移率晶体管(GaN HEMT)研究了不同的栅槽刻蚀工艺对GaN器件性能的影响。在栅槽刻蚀方面,采用了一种感应耦合等离子体(ICP)干法刻蚀技术与高温热氧化湿法刻蚀技术相结合的两步法刻蚀技术,将AlGaN势垒层全部刻蚀掉,制备出了阈值电压超过3 V的增强型Al_2O_3/AlGaN/GaN MIS-HEMT器件。相比于传统的ICP干法刻蚀技术,两步法是一种低损伤的自停止刻蚀技术,易于控制且具有高度可重复性,能够获得更高质量的刻蚀界面,所制备的器件增强型GaN MIS-HEMT器件具有阈值电压回滞小、电流开关比(I_ON/I_OFF)高、栅极泄漏电流小、击穿电压高等特性。