Implications of gate-edge electric field in AlGaN/GaN high electron mobility transistors during OFF-state degradation

Gate degradation in high electron mobility transistors (HEMTs) under OFF-state stress results from the high electric field near the gate edge. We investigate the evolution of this field over time in AlGaN/GaN HEMTs upon OFF-state stress using a combination of electroluminescence (EL) microscopy and spectroscopy. EL analysis suggests that the electric field at the sites of generated surface defects is lowered after the stress, with greater lowering at higher stress temperature. The ON-state EL spectrum remains unchanged after the stress, suggesting that the regions without generated defects are not affected during the degradation. A finite element model is employed to further demonstrate the effect of surface defects on the local electric field. A correlation is observed for the spatial distribution of the EL intensity before and after the generation of leakage sites, which provides a prescreening method to predict possible early failures on a device.     

AlGaN/GaN high electron mobility transistors with InGaN back-barriers

A GaN/ultrathin InGaN/GaN heterojunction has been used to provide a back-barrier to the electrons in an AlGaN/GaN high-electron mobility transistor (HEMT). The polarization-induced electric fields in the InGaN layer raise the conduction band in the GaN buffer with respect to the GaN channel, increasing the confinement of the two-dimensional electron gas under high electric field conditions. The enhanced confinement is especially useful in deep-submicrometer devices where an important improvement in the pinchoff and 50% increase in the output resistance have been observed. These devices also showed excellent high-frequency performance, with a current gain cut-off frequency (f/sub T/) of 153 GHz and power gain cut-off frequency (f/sub max/) of 198 GHz for a gate length of 100 nm. At a different bias, a record f/sub max/ of 230 GHz was obtained.     

Thermal effects in AlGaN/GaN/Si high electron mobility transistors

Group III-nitride compounds are of increasing interest for designing high power and high temperature transistors. A considerable progress in the growth and process technology of these devices has been achieved. However, there are still limitations concerning particularly the lack of native substrates. Comparison of the AlGaN/GaN high electron mobility transistors investigated favours the SiC substrate. Recently, encouraging results have been reported for AlGaN/GaN/Si. The crucial problem found in AlGaN/GaN transistors operating at high biases is the self-heating induced by high power dissipation in the active zone. The present work reports on a study of the self-heating in AlGaN/GaN HEMTs grown on Si(1 1 1). The electron-band parameters of the heterostructures have been calculated self-consistently by taking into account the piezoelectric and spontaneous polarizations. As an experiment support, direct-current characteristics of AlGaN/GaN/Si HEMTs have been used to derive the drain voltage-dependent temperature rise in the conductive channel. As has been found, the self-heating is relatively weak. An improvement in the electron transport is achieved by optimizing the epilayers and adjusting the electrode sizes at output of the transistors investigated.     

Erosion defect formation in Ni-gate AlGaN/GaN high electron mobility transistors

High electron mobility transistors based on Aluminum Gallium Nitride/Gallium Nitride heterostructures are poised to become the technology of choice for a wide variety of high frequency and high power applications. Their reliability in the field, particularly the reliability of the gate electrode under high reverse bias, remains an ongoing concern, however. Rapid increases in gate leakage current have been observed in devices which have undergone off-state stressing. Scanning Electron Microscopy, scanning probe microscopy, and Transmission Electron Microscopy have been used to evaluate physical changes to the structure of Ni-gated devices as the gate leakage current begins its initial increase. This evaluation indicates the formation of an interfacial defect similar to erosion under the gate observed by other authors. Defect formation appears to be dependent upon electrical field as well as temperature. Transmission Electron Microscopy has been used to demonstrate that a chemical change to the interfacial oxynitride layer present between the semiconductor and gate metal appears to occur during the formation of this defect. The interfacial layer under the gate contact transitions from a mixed oxynitride comprised of gallium and aluminum to an aluminum oxide.     

多栅指AlGaN/GaN HEMT器件自热效应研究

Self-heating in multi-finger AlGaN/GaN high-electron-mobility transistors(HEMTs) is investigated by measurements and modeling of device junction temperature under steady-state operation.Measurements are carried out using micro-Raman scattering to obtain the detailed and accurate temperature distribution of the device.The device peak temperature corresponds to the high field region at the drain side of gate edge.The channel temperature of the device is modeled using a combined electro-thermal model considering 2DEG transport characteristics and the Joule heating power distribution.The results reveal excellent correlation to the micro-Raman measurements, validating our model for the design of better cooled structures.Furthermore,the influence of layout design on the channel temperature of multi-finger AlGaN/GaN HEMTs is studied using the proposed electro-thermal model, allowing for device optimization.     

Reliability investigation of AlGaN/GaN high electron mobility transistors under reverse-bias stress

Impact of reverse-bias stress on the reliability of AlGaN/GaN high electron mobility transistors was investigated in this paper. We found that inverse piezoelectric effect could induce noisy characteristics of stress current, and the “critical voltage” increased with the drain–source bias in the step-stress experiments. Although the degradation of the gate leakage current and drain-to-source leakage current are non-recoverable, the maximum output current can recover almost completely through electron de-trapping procedure after stress. The de-trapping activation energy was estimated to be 0.30 eV by the dynamic conductance technique. The surface morphology of the electrically stressed devices was investigated after removing the gate metallization by chemical etching, and no pits or cracks under the gate contact were observed.     

Study of proton irradiation effects on AlGaN/GaN high electron mobility transistors

AlGaN/GaN high electron mobility transistors (HEMTs) were exposed to 3 MeV protons at fluences of 6 × 10¹³, 4 × 10¹⁴ and 1 × 10¹⁵ protons/cm². The drain saturation currents decreased by 20% and the maximum transconductance decreased by 5% at the highest fluence. As the fluence increased, the threshold voltage shifted more positive values. After proton irradiation, the gate leakage current increased. The Schottky barrier height changed from 0.63 eV to 0.46 eV, and the ideality factor from 2.55 to 3.98 at the highest fluence. The degradations of electrical characteristics of AlGaN/GaN HEMTs are caused by displacement damages induced by proton irradiation. The density of vacancies at different proton fluence can be calculated from SRIM. Being an acceptor-like defect, the Ga vacancy acts as a compensation center. While N vacancy acts as a donor. Adding the vacancies model into Slivaco device simulator, simulation results match well with the trends of experimental data. Hall measurement results also indicate the concentration and mobility of 2DEG decrease after proton irradiation. It is concluded that the Ga vacancies introduced maybe the primary reason for the degradation of AlGaN/GaN HEMTs performance.     

AlGaN/GaN high electron mobility transistors with selective area grown p-GaN gates

We report a selective area growth (SAG) method to define the p-GaN gate of AlGaN/GaN high electron mobility transistors (HEMTs) by metal-organic chemical vapor deposition. Compared with Schottky gate HEMTs, the SAG p-GaN gate HEMTs show more positive threshold voltage (V_th) and better gate control ability. The influence of Cp₂Mg flux of SAG p-GaN gate on the AlGaN/GaN HEMTs has also been studied. With the increasing Cp₂Mg from 0.16 μmol/min to 0.20 μmol/min, the V_th raises from -67 V to -37 V. The maximum transconductance of the SAG HEMT at a drain voltage of 10 V is 113.9 mS/mm while that value of the Schottky HEMT is 51.6 mS/mm. The SAG method paves a promising way for achieving p-GaN gate normally-off AlGaN/GaN HEMTs without dry etching damage.     

AlGaN/GaN high electron mobility transistors on Si(111) substrates

AlGaN/GaN high electron mobility transistors (HEMTs) on silicon substrates have for the first time been realized using organometallic vapor phase epitaxy (OMVPE). Using 1 Ω-cm p-Si(111), these devices exhibited static output characteristics with low output conductance and isolation approaching 80 V. Under microwave rf operation, the substrate charge becomes capacitively coupled and parasitically loads these devices thereby limiting their performance. As a result, typical 0.3 μm gate length devices show a 25 GHz cutoff frequency, with near unity f_max/f_T ratio and 0.55 W/mm output power. A small-signal equivalent circuit incorporating elements representing the parasitic substrate loading accurately models the measured S-parameters. Removal of the conductive substrate is one way to effectively eliminate this parasitic loading. Through backside processing, freestanding 0.4-mm HEMT membranes with no thermal management were demonstrated and exhibited a significant improvement in their f_max/f_T ratio up to 2.5 at the cost of lower f_T and f_max along with an almost four-fold reduction of I_dss     

Off-state breakdown and leakage current transport analysis of AlGaN/GaN high electron mobility transistors

Off-state breakdown characteristics of AlGaN/GaN high-electron-mobility transistors have been studied based on drain current injection method. It is found that at low drain current injection level, the observed premature breakdown is caused by excess gate-to-drain leakage current. Nevertheless, at high drain injection current level, buffer-leakage-dominated breakdown proceeds gate-leakage-dominated breakdown as the gate bias increases from pinch-off voltage to deep-depletion voltage. In both breakdown regions, the breakdown voltages show negative temperature coefficients. The buffer-leakage-induced breakdown should be defect-related, which is confirmed by temperature-dependent buffer leakage measurements.     

Unpassivated GaN/AlGaN/GaN power high electron mobility transistors with dispersion controlled by epitaxial layer design

In this paper, a novel GaN/AlGaN/GaN high electron mobility transistor (HEMT) is discussed. The device uses a thick GaN-cap layer (∼250 nm) to reduce the effect of surface potential fluctuations on device performance. Devices without Si₃N₄ passivation showed no dispersion with 200-ns-pulse-width gate-lag measurements. Saturated output-power density of 3.4 W/mm and peak power-added efficiency (PAE) of 32% at 10 GHz (V_DS=+15 V) were achieved from unpassivated devices on sapphire substrates. Large gate-leakage current and low breakdown voltage prevented higher drain-bias operation and are currently under investigation.     

半开态直流应力作用下AlGaN/GaN高电子迁移率晶体管的失效机制分析

本文研究了半开态直流应力条件下,AlGaN/GaN高电子迁移率晶体管的退化机制。应力实验后,器件的阈值电压电压正漂,栅漏串联电阻增大。利用数据拟合发现,沟道电流的退化量与阈值电压及栅漏串联电阻的变化量之间有密切的关系。分析表明,阈值电压的退化是引起饱和区沟道电流下降的主要因素,对于线性区电流,在应力开始的初始阶段,栅漏串联电阻的增大导致线性区电流的退化,随后沟道电流退化主要由阈值电压的退化引起。分析表明,在半开态应力作用下,栅泄露电流及热电子效应使得电子进入AlGaN层,被缺陷俘获,进而导致沟道电流退化。其中反向栅泄露电流中的电子被栅电极下AlGaN层内的缺陷俘获,导致阈值电压正漂;而热电子效应则使得栅漏串联区电阻增大。     

导致 AlGaN/GaN HEMT电学性能失效的内外因素

本文研究了不同偏置条件对AlGaN/GaN HEMT电学性能的影响。电场被认为是导致AlGaN/GaN HEMT器件电学性能退化的外因,陷阱则是内因。AlGaN/GaN HEMT器件的退化有两部分组成：可恢复退化与不可恢复退化。AlGaN/GaN HEMT器件中原本存在的陷阱与新产生的陷阱导致可恢复退化。     

MOCVD-grown AlGaN/GaN heterostructures with high electron mobility

Specific features of MOCVD growth of AlGaN/GaN heterostructures have been studied. In the structures obtained, the 2D electron gas in the channel had a density of 1.2×10¹³ cm^?2 and a mobility of 1290 cm²/(V s) at room temperature. The effect of the purity of starting components on the properties of the structure is studied.     

Degradation induced by 2-MeV alpha particles on AlGaN/GaN high electron mobility transistors

We studied the degradation of AlGaN/GaN High Electron Mobility Transistors (HEMT) after 2-MeV alpha irradiation for two different fluences, namely 10¹³α/cm² and 10¹⁴α/cm². After the exposure and depending on the irradiation fluence, we observed a drop both in drain current and transconductance, and a reduction in the leakage current of the gate diode. We attributed these effects to bulk damage, radiation-induced formation of deep-level trap sites in the channel layer, and doping compensation/removal in the barrier layer.     

Effects of post-annealing on the passivation interface characteristics of AlGaN/GaN high electron mobility transistors

We examined the effects of post-annealing in forming-gas ambient on the spin-on-dielectric (SOD)-buffered passivation as well as the conventional plasma-enhanced chemical vapor deposition (PECVD) Si₃N₄ passivation structure in association with the quantitative analysis of defects at the passivation interfaces of AlGaN/GaN high electron mobility transistors (HEMTs). Before the annealing, the interface state densities (D_it) of the PECVD Si₃N₄ are one-order higher (10¹²–10¹³ cm⁻² eV⁻¹) than those of the SOD SiO_x (10¹¹–10¹² cm⁻² eV⁻¹) as derived from C–V characterization. Clear reduction in D_it from the PECVD Si₃N₄ is extracted to a level of 10¹¹–10¹² cm⁻² eV⁻¹ with a stronger absorption from Si–N peak in Fourier transform infrared spectroscopy spectra after the post-annealing. On the other hand, negligible difference in D_it value is obtained from the SOD SiO_x. In this paper we propose that much lower measurement levels (~156 mA/mm) before the annealing and substantial recovery (~13% increase) after the annealing in maximum drain current density of the AlGaN/GaN HEMTs with Si₃N₄ passivations are due to the original higher density before the annealing and greater reduction in D_it of the PECVD Si₃N₄ after the annealing. Significant reduction after the annealing in gate–drain leakage current (from ~10⁻³ to ~10⁻⁵ A, 100-μm gate width) of the HEMTs with the Si₃N₄ passivation is also supposed to be attributed to the reduction of D_it.     

Device reliability study of AlGaN/GaN high electron mobility transistors under high gate and channel electric fields via low frequency noise spectroscopy

A set of different short term stress conditions are applied to AlGaN/GaN high electron mobility transistors and changes in the electronic behaviour of the gate stack and channel region are investigated by simultaneous gate and drain current low frequency noise measurements. Permanent degradation of gate current noise is observed during high gate reverse bias stress which is linked to defect creation in the gate edges. In the channel region a permanent degradation of drain noise is observed after a relatively high drain voltage stress in the ON-state. This is attributed to an increase in the trap density at the AlGaN/GaN interface under the gated part of the channel. It was found that self-heating alone does not cause any permanent degradation to the channel or gate stack. OFF-state stress also does not affect the gate stack or the channel.     

InGaN背势垒AlGaN/GaN HEMT中的电流崩塌研究

Current collapses were studied,which were observed in AlGaN/GaN high electron mobility transistors(HEMTs) with and without InGaN back barrier(BB) as a result of short-term bias stress.More serious drain current collapses were observed in InGaN BB AlGaN/GaN HEMTs compared with the traditional HEMTs.The results indicate that the defects and surface states induced by the InGaN BB layer may enhance the current collapse.The surface states may be the primary mechanism of the origination of current collapse in AlGaN/GaN HEMTs for short-term direct current stress.     

Impact of carbon impurities on the initial leakage current of AlGaN/GaN high electron mobility transistors

We have systematically studied the origin of high gate-leakage currents in AlGaN/GaN high electron mobility transistors (HEMTs). Devices that initially had a low gate-leakage current (good devices) are compared with ones that had a high gate-leakage current (bad devices). The apparent zero-bias Schottky barrier height of bad devices (0.4 < ϕ_B0 < 0.62 eV) was found to be lower than that of the good devices (ϕ_B0 = 0.79 eV). From transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) analysis, we found that this difference is due to the presence of carbon impurities in the nickel layer in the gate region.     

Influence of layer structure on performance of AlGaN/GaN high electron mobility transistors before and after passivation

Intentionally undoped and three different, doped layer structures are used to investigate properties of AlGaN/GaN high electron mobility transistors (HEMTs) before and after SiN passivation. For unpassivated devices, the drain current, transconductance, cutoff frequency, and microwave output-power increase with increased doping level, in spite of an increase in the gate-leakage current. After passivation, an overall performance improvement of all devices occurs. The passivation-induced sheet charge decreases from 2×10¹² cm⁻² in undoped structures to ∼0.7×10¹² cm⁻² in higher doped structures and performance improvement with passivation is less pronounced for higher doped devices. However, the output power of unpassivated and passivated devices on higher doped structures is much higher than that on the undoped-passivated counter-part. These results underline an advantage of the doped layer structure for the preparation of high-performance AlGaN/GaN HEMTs.