Transient thermal characterization of AlGaN/GaN HEMTs grown on silicon

We studied a temperature increase and a heat transfer into a substrate in a pulsed operation of 0.5 length and 150 /spl mu/m gate width AlGaN/GaN HEMTs grown on silicon. A new transient electrical characterization method is described. In combination with an optical transient interferometric mapping technique and two-dimensional thermal modeling, these methods determine the device thermal resistance to be /spl sim/70 K/W after 400 ns from the start of a pulse. We also localized the high-electron mobility transistor heat source experimentally and we extracted a thermal boundary resistance at the silicon-nitride interface of about /spl sim/7/spl times/10/sup -8/ m/sup 2/K/W. Thermal coupling at this interface may substantially influence the device thermal resistance.     

Trapping effects and microwave power performance in AlGaN/GaN HEMTs

The dc small-signal, and microwave power output characteristics of AlGaN/GaN HEMTs are presented. A maximum drain current greater than 1 A/mm and a gate-drain breakdown voltage over 80 V have been attained. For a 0.4 μm gate length, an f_T of 30 GHz and an f_max of 70 GHz have been demonstrated. Trapping effects, attributed to surface and buffer layers, and their relationship to microwave power performance are discussed. It is demonstrated that gate lag is related to surface trapping and drain current collapse is associated with the properties of the GaN buffer layer. Through a reduction of these trapping effects, a CW power density of 3.3 W/mm and a pulsed power density of 6.7 W/mm have been achieved at 3.8 GHz     

Post-process thermal treatment for microwave power improvement of AlGaN/GaN HEMTs

The effects of post-process rapid thermal annealing (RTA) treatment after device fabrication on direct current, microwave and power performances of AlGaN/GaN high electron mobility transistors (HEMTs) with a gate-length of 0.2 μm were fully investigated. By 3 min post-process RTA treatment at 350 °C under N₂ atmosphere, the direct current (DC), radio frequency (RF) small signal and power performances of AlGaN/GaN HEMTs have been much improved. The output power, power gain and power added efficiency (PAE) of GaN HEMT device with gate wide of 1 mm increase from 37.09 dBm, 6.09 dB and 42.79% to 38.22 dBm, 7.22 dB and 67.3%. The post-process RTA after device fabrication has two merits. On the one hand, it improves passivation effect of SiN_x dielectric layer on AlGaN/GaN HEMT surface, suppressing RF current dispersion. On the other hand, it helps recover dry-etch damage at the Schottky metal/AlGaN interface, leading to reduction of reverse Schottky leakage current.     

Comparison between trap and self-heating induced mobility degradation in AlGaN/GaN HEMTs

Mobility degradation due to scattering from radiation-induced defects is compared to that produced by self-heating in proton-irradiated AlGaN/GaN HEMTs using experiments and simulations. After irradiation, the mobility in the 2DEG is limited by scattering from charged traps and is temperature-limited near the gate–drain access region.     

Thermal modeling and measurements of AlGaN/GaN HEMTs including thermal boundary resistance

In this paper an analysis of thermal behavior of microwave power AlGaN/GaN HEMTs has been carried out through pulsed current–voltage (PIV) measurements and small signal [S] parameters. A special care about trapping effects has been followed where it is shown that the thermal resistance of the device can be accurately determined provided that some assumptions on the trapping behavior of the device are verified. The values obtained have been checked by three dimensional finite element (3D-FE) simulations. Finally, the thermal boundary resistance (TBR) between GaN/SiC has been extracted and compared to literature. The results we have obtained are in line with what can be found. In addition to this first set of results, a second 3D-FE analysis of a larger transistor has been performed and checked thanks to liquid crystal measurements.     

Full-Wafer Characterization of AlGaN/GaN HEMTs on Free-Standing CVD Diamond Substrates

We report on electrical characterization and uniformity measurements of the first conventionally processed AlGaN/GaN high electron mobility transistors (HEMTs) on free-standing chemical-vapor-deposited (CVD) diamond substrate wafers. DC and RF device performance is reported on HEMTs fabricated on $sim!!hbox{130-}muhbox{m}$-thick and 30-mm round CVD diamond substrates without mechanical carrying wafers. A measured $f_{T} cdot L_{G}$ product of 12.5 $hbox{GHz} cdot muhbox{m}$ is the best reported data for all GaN-on-diamond technology. X-band power performance of AlGaN/GaN HEMTs on diamond is reported to be 2.08 W/mm and 44.1% power added efficiency. This letter demonstrates the potential for GaN HEMTs to be fabricated on CVD diamond substrates utilizing contact lithography process techniques. Further optimization of the epitaxy and diamond substrate attachment process could provide for improvements in thermal spreading while preserving the electrical properties.      

Physics-based numerical simulation and device characterizations of AlGaN/GaN HEMTs with temperature effects

The research presents AlGaN/GaN HEMTs device characterizations at different temperatures using physics-based numerical simulation. Industry standard simulation tool Silvaco ATLAS is used to characterize the various electronic properties of the device. An extensive theoretical overview is done to achieve the most comprehensive values for GaN and AlGaN properties, as discussed in the paper. This research is mainly focused on simulation of temperature dependent device performances as well as on some other material properties that are not well defined in ATLAS. Energy bandgap, density of states, saturation velocities, surface traps, polarization effect, carrier lifetime and mobility, permittivity, effective Richardson's constant, and donor and acceptor energy levels are considered as critical parameters for predicting temperature effect in ALGaN/GaN HEMT. Various aspects of device performance are analyzed at high temperature along with the different bias configurations.     

增强型AlGaN/GaN HEMT器件工艺的研究进展

随着高压开关和高速射频电路的发展,增强型GaN基高电子迁移率晶体管(HEMT)成为该领域内的研究热点。增强型GaN基HEMT只有在加正栅压才有工作电流,可以大大拓展该器件在低功耗数字电路中的应用。近年来,国内外对增强型GaN基HEMT阈值电压的研究主要集中以下两个方面:在材料生长方面,通过生长薄势垒、降低Al组分、生长无极化电荷的AlGaN/GaN异质材料、生长InGaN或p-GaN盖帽层,来控制二维电子气浓度;在器件工艺方面,采用高功函数金属、MIS结构、刻蚀凹栅、F基等离子体处理,来控制表面电势,影响二维电子气浓度。从影响器件阈值电压的相关因素出发,探讨了实现和优化增强型GaN基HEMT的各种工艺方法和发展方向。     

Structure optimization of field-plate AlGaN/GaN HEMTs

AlGaN/GaN high electron mobility transistors (HEMTs) on 6H-SiC with varying field-plate length and gate-drain spacing were fabricated and analyzed. The classical small signal FET model and the well-known ColdFET method were used to extract the small signal parameters of the devices. Though the devices with field plates exhibited lower better f_T characteristic, they did demonstrate better f_max, MSG and power density performances than the conventional devices without field plate. Besides, no independence of DC characteristic on field-plate length was observed. With the increase of the field-plate length and the gate-drain spacing, the characteristic of f_T and f_max degraded due to the large parasitic effects. Loadpull method was used to measure the microwave power performance of the devices. Under the condition of continuous wave at 5.4 GHz, an output power density of 4.69 W/mm was obtained for device with field-plate length of 0.5 μm and gate-drain length of 2 μm.     

具有栅源间本征GaN调制层的AlGaN/GaN HEMT

为解决常规AlGaN/GaN高电子迁移率晶体管(HEMT)因源极电子注入栅极右侧高场区造成的雪崩击穿,并提高器件的击穿电压,提出了一种具有栅源间本征GaN (i-GaN)调制层的新型AlGaN/GaN HEMT结构.新结构器件在反向耐压时将调制层下方部分区域的二维电子气(2DEG)完全耗尽,扩展了沟道的夹断区,有效阻止了源极电子向栅极右侧高场区的注入.仿真结果表明,通过设置适当的调制层长度和厚度,器件的击穿电压可从常规结构的862 V提升至新结构的1086 V,增幅达26％.同时,GaN调制层会微幅增大器件的比导通电阻,对阈值电压也具有一定的提升作用.     

Punchthrough-Voltage Enhancement of AlGaN/GaN HEMTs Using AlGaN Double-Heterojunction Confinement

In this paper, we present an enhancement of punchthrough voltage in AlGaN/GaN high-electron-mobility-transistor devices by increasing the electron confinement in the transistor channel using an AlGaN buffer-layer structure. An optimized electron confinement results in a scaling of punchthrough voltage with device geometry and a significantly reduced subthreshold drain leakage current. These beneficial properties are pronounced even further if gate-recess technology is applied for device fabrication. Physical-based device simulations give insight in the respective electronic mechanisms.      

Characterization of the self-heating of AlGaN/GaN HEMTs during an electrical stress by using Raman spectroscopy

We report on the non-invasive measurements of the temperature in active AlGaN/GaN HEMTs grown on sapphire substrate during an electrical stress. The original study permits to highlight the drop of the self-heating in operando during the electrical stress by using Raman spectroscopy. Moreover, a correlation between the decrease of the self-heating and the fall of the drain current during the stress has been demonstrated. This study also highlights that the self-heating of the components and the influence of the ageing test on the self-heating are clearly linked to the position where temperature measurements are carried out.     

Characterization of Recessed-Gate AlGaN/GaN HEMTs as a Function of Etch Depth

Recessed-gate high-electron-mobility transistors (HEMTs) were fabricated using a Cl₂-based plasma etch to study device performance as a function of recess depth. Devices were fabricated with recess depths varying from 0 nm to 25 nm on a standard HEMT structure using a controllable, low-power etch recipe. It is shown that the threshold voltage approached zero as the recess approached the AlGaN/GaN heterojunction. At the same time, mobility decreased an order of magnitude over the etch range studied, and sheet carrier density also decreased. In addition to direct-current (DC) I–V and Hall measurements, electroluminescence was also used to characterize plasma damage in these devices.     

Current Collapse and High-Electric-Field Reliability of Unpassivated GaN/AlGaN/GaN HEMTs

Long-term ON-state and OFF-state high-electric-field stress results are presented for unpassivated GaN/AlGaN/GaN high-electron-mobility transistors on SiC substrates. Because of the thin GaN cap layer, devices show minimal current-collapse effects prior to high-electric-field stress, despite the fact that they are not passivated. This comes at the price of a relatively high gate-leakage current. Under the assumption that donor-like electron traps are present within the GaN cap, two-dimensional numerical device simulations provide an explanation for the influence of the GaN cap layer on current collapse and for the correlation between the latter and the gate-leakage current. Both ON-state and OFF-state stresses produce simultaneous current-collapse increase and gate-leakage-current decrease, which can be interpreted to be the result of gate-drain surface degradation and reduced gate electron injection. This study shows that although the thin GaN cap layer is effective in suppressing surface-related dispersion effects in virgin devices, it does not, per se, protect the device from high-electric-field degradation, and it should, to this aim, be adopted in conjunction with other technological solutions like surface passivation, prepassivation surface treatments, and/or field-plate gate     

MBE生长的跨导为186 mS/mm的AlGaN/GaN HEMT

用分子束外延 ( MBE)技术研制出了 Al Ga N/Ga N高电子迁移率晶体管 ( HEMT)材料 ,其室温迁移率为 10 35cm2 /V· s、二维电子气浓度为 1.0× 10 13 cm - 2 ;77K迁移率为 2 6 53cm2 /V· s、二维电子气浓度为 9.6× 10 12 cm- 2 。用此材料研制了栅长为 1μm、栅宽为 80μm、源 -漏间距为 4μm的 Al Ga N/Ga N HEMT,其室温最大非本征跨导为 186 m S/mm、最大漏极饱和电流密度为 92 5m A/mm、特征频率为 18.8GH z。另外 ,还研制了具有 2 0个栅指 (总栅宽为 2 0× 80μm =1.6 mm )的大尺寸器件 ,该器件的最大漏极饱和电流为 1.33A。     

DC and RF Characteristics of AlGaN/GaN/InGaN/GaN Double-Heterojunction HEMTs

We present the detailed dc and radio-frequency characteristics of an Al_0.3Ga_0.7N/GaN/In_0.1Ga_0.9 N/GaN double-heterojunction HEMT (DH-HEMT) structure. This structure incorporates a thin (3 nm) In_0.1Ga_0.9N notch layer inserted at a location that is 6-nm away from the AlGaN/GaN heterointerface. The In_0.1Ga_0.9N layer provides a unique piezoelectric polarization field which results in a higher potential barrier at the backside of the two-dimensional electron gas channel, effectively improving the carrier confinement and then reducing the buffer leakage. Both depletion-mode (D-mode) and enhancement-mode (E-mode) devices were fabricated on this new structure. Compared with the baseline AlGaN/GaN HEMTs, the DH-HEMT shows lower drain leakage current. The gate leakage current is also found to be reduced, owing to an improved surface morphology in InGaN-incorporated epitaxial structures. DC and small- and large-signal microwave characteristics, together with the linearity performances, have been investigated. The channel transit delay time analysis also revealed that there was a minor channel in the InGaN layer in which the electrons exhibited a mobility slightly lower than the GaN channel. The E-mode DH-HEMTs were also fabricated using our recently developed CF₄-based plasma treatment technique. The large-signal operation of the E-mode GaN-based HEMTs was reported for the first time. At 2 GHz, a 1times100 mum E-mode device demonstrated a maximum output power of 3.12 W/mm and a power-added efficiency of 49% with single-polarity biases (a gate bias of +0.5 V and a drain bias of 35 V). An output third-order interception point of 34.7 dBm was obtained in the E-mode HEMTs     

Reliability and degradation mechanism of AlGaN/GaN HEMTs for next generation mobile communication systems

Excellent reliability performance of AlGaN/GaN HEMTs on SiC substrates for next generation mobile communication systems has been demonstrated using DC and RF stress tests on 8 × 60 μm wide and 0.5 μm long AlGaN/GaN HEMTs at a drain voltage of V_d = 50 V. Drain current recovery measurements after stress indicate that the degradation is partly caused by slow traps generated in the SiN passivation or in the HEMT epitaxial layers. The traps in the SiN passivation layer were characterized using high and low frequency capacitance-voltage (CV) measurements of MIS test structures on thick lightly doped GaN layers.     

Ultraviolet photoresponse of ZnO nanostructured AlGaN/GaN HEMTs

We present the first active visible blind ultraviolet (UV) photodetector based on zinc oxide (ZnO) nanostructured AlGaN/GaN high electron mobility transistors (HEMTs). The ZnO nanorods (NRs) are selectively grown on the gate area by using hydrothermal method. It is shown that ZnO nanorod (NR)-gated UV detectors exhibit much superior performance in terms of response speed and recovery time to those of seed-layer-gated detectors. It is also found that the best response speed (~10 and~190 ms) and responsivity (~1.1×10⁵ A/W) were observed from detectors of the shortest gate length of 2 µm among our NR-gated devices of three different gate dimensions, and this responsivity is about one order higher than the best performance of ZnO NR-based UV detectors reported to date.     

Characterisation and modelling of parasitic effects and failure mechanisms in AlGaN/GaN HEMTs

This work is a study of the degradations of AlGaN/GaN HEMTs induced by 2000 h of ageing tests. The methodology is based on cross-characterisation analysis.The life tests (HTO 150 °C, HTO 175 °C and HTRB 175 °C and Idq 90 °C) have mainly induced a decrease of the saturation drain current, occurring during the first 50 h, followed by a stabilisation. There is a shift of the pinch-off voltage in the range of 0.1–0.2 V while the Schottky contact is rather stable after ageing. The evolution of the electrical characteristics after ageing does not depend on the bias conditions but rather more on the channel temperature. It seems to be neither field nor current driven. Low frequency drain current noise demonstrates that there is no trap creation and the weak evolution of the 1/f noise confirms that there is no degradation in the channel. Moreover, pulsed I–V measurements show a weak evolution of gate lag and drain lag rates after ageing. The same degradation mode is demonstrated for all life tests with rather high activation energy of 1.6 eV. The weak evolution of electrical characteristics observed during the life tests cannot be obviously explained by a single physical mechanism and results from a combination of trap-related effects before stabilisation.     

The effect of external stress on the electrical characteristics of AlGaN/GaN HEMTs

The electrical characteristics of AlGaN/GaN high electron mobility transistors under the application of uniform in-plane tensile and compressive stress were measured. The results demonstrate the change of the drain–source I_ds–V_ds characteristics as a function of the external stress. The output current at V_ds = 10 V increases linearly with the stress with the slope about 3 × 10⁻⁶ A MPa⁻¹. It is associated with the piezoelectric effect and kink effect. Moreover, the magnitude of the kink effect is found to be affected by the stress. It displays a linear changing trend with the slope of 3.3 × 10⁻⁴ mS MPa⁻¹ within the stress level. The energy band structure is suggested to be responsible for the dependence of the kink effect on the stress.