Gallium nitride based high power heterojunction field effecttransistors: process development and present status at UCSB

The development of GaN based devices for microwave power electronics at the University of California, Santa Barbara (UCSB), is reviewed. From 1995 to 2000, the power performance of AlGaN/GaN-on-sapphire heterojunction field effect transistors improved from 1.1 W/mm to 6.6 W/mm, respectively. Compensating the disadvantages of the low thermal conductivity of the sapphire substrate through heat management via flip chip bonding onto AlN substrates, large periphery devices with an output power of 7.6 W were demonstrated. UCSB also fabricated the first GaN based amplifier integrated circuits. Critical issues involved in the growth of high quality AlGaN/GaN heterostructures by metal-organic chemical vapor deposition and the device fabrication are discussed     

Junction Temperature Measurements and Thermal Modeling of GaInN/GaN Quantum Well Light-Emitting Diodes

We present a junction temperature analysis of GaInN/GaN quantum well (QW) light-emitting diodes (LEDs) grown on sapphire and bulk GaN substrate by micro-Raman spectroscopy. The temperature was measured up to a drive current of 250 mA (357 A/cm²). We find better cooling efficiency in dies grown on GaN substrates with a thermal resistance of 75 K/W. For dies on sapphire substrates we find values as high as 425 K/W. Poor thermal performance in the latter is attributed to the low thermal conductivity of the sapphire. Three-dimensional finite-element simulations show good agreement with the experimental results, validating our thermal model for the design of better cooled structures.     

基于76.2 mm圆片工艺的GaN HEMT可靠性评估

报道了利用76.2 mm圆片工艺实现了SiC衬底GaN HEMT微波功率管的研制,并对其进行了多项试验以评估其可靠性.器件工艺中通过引入难熔金属作器件肖特基势垒,有效提高了GaN HEMT器件肖特基势垒的热稳定性,经过500℃高温处理30 s后器件肖特基特性依然保持稳定.随后的高温工作寿命试验表明,该GaNHEMT能够...     

Direct Low‐Temperature Integration of Nanocrystalline Diamond with GaN Substrates for Improved Thermal Management of High‐Power Electronics

A novel approach for the direct synthetic diamond–GaN integration via deposition of the high‐quality nanocrystalline diamond films directly on GaN substrates at temperatures as low as 450–500 °C is reported. The low deposition temperature allows one to avoid degradation of the GaN quality, which is essential for electronic applications The specially tuned growth conditions resulted in the large crystalline diamond grain size of 100–200 nm without coarsening. Using the transient “hot disk” measurements it is demonstrated that the effective thermal conductivity of the resulting diamond/GaN composite wafers is higher than that of the original GaN substrates at elevated temperatures. The thermal crossover point is reached at ≈95–125 °C depending on the thickness of the deposited films. The developed deposition technique and obtained thermal characterization data can lead to a new method of thermal management of the high power GaN electronic and optoelectronic devices.     

应用于大功率LED器件的回路热管散热研究

为解决大功率LED的散热问题,提出一种应用于大功率LED散热的微型回路热管,研究了充液率和倾斜角度对热管冷却大功率LED的启动性能、结温和热阻等特性的影响.研究结果表明:热管的最佳充液率为60％,系统的总热阻为7.5 K/W,此时对应的热管的热阻为1.6 K/W;热管的启动时间约为6.5 min,LED的结点温度被控制在42℃以下,很好地满足了大功率LED的结温稳定性要求.     

Undoped AlGaN/GaN HEMTs for microwave power amplification

Undoped AlGaN/GaN structures are used to fabricate high electron mobility transistors (HEMTs). Using the strong spontaneous and piezoelectric polarization inherent in this crystal structure a two-dimensional electron gas (2DEG) is induced. Three-dimensional (3-D) nonlinear thermal simulations are made to determine the temperature rise from heat dissipation in various geometries. Epitaxial growth by MBE and OMVPE are described, reaching electron mobilities of 1500 and 1700 cm ²/Ns, respectively, For electron sheet density near 1×10¹³/cm², Device fabrication is described, including surface passivation used to sharply reduce the problematic current slump (dc to rf dispersion) in these HEMTs. The frequency response, reaching an intrinsic f_t of 106 GHz for 0.15 μm gates, and drain-source breakdown voltage dependence on gate length are presented. Small periphery devices on sapphire substrates have normalized microwave output power of ~4 W/mm, while large periphery devices have ~2 W/mm, both thermally limited. Performance, without and with Si₃N₄ passivation are presented. On SiC substrates, large periphery devices have electrical limits of 4 W/mm, due in part to the limited development of the substrates     

Measured microwave power performance of AlGaN/GaN MODFET

We report the first microwave power measurement on GaN FET's. At 2 GHz, a class A output power density of 1.1 W/mm with a power added efficiency of 18.6% was obtained on a 1 μm gate-length AlGaN/GaN MODFET. Mathematical simulation estimated that the transistor was operating at a channel temperature of 360°C as a result of the poor thermal conductivity of the sapphire substrate. Despite this serious heating problem, the power output density still rivals GaAs MESFET's     

高散热性能TGV转接板

随着玻璃通孔(TGV)转接板在微波系统集成中的应用越来越广泛,其微波大功率应用情况下的散热性能成为研究重点。针对TGV转接板高效散热性能的要求,进行TGV散热结构的设计和性能分析。建立TGV转接板封装集成结构的有限元模型,设计TGV转接板铜柱阵列散热结构。通过TGV工艺制作TGV高密度阵列。在4.82~14.47 W功率范围内对TGV转接板的散热性能进行测试,相应的TGV散热结构区域的热流密度为40.03~120.18 W/cm²,测得热阻芯片表面温度高达54.0~126.5℃,低于微波功率芯片最高结温150℃,可以满足大功率微波系统集成高效散热的需求。     

Thermal Boundary Resistance Between GaN and Substrate in AlGaN/GaN Electronic Devices

The influence of a thermal boundary resistance (TBR) on temperature distribution in ungated AlGaN/GaN field-effect devices was investigated using 3-D micro-Raman thermography. The temperature distribution in operating AlGaN/GaN devices on SiC, sapphire, and Si substrates was used to determine values for the TBR by comparing experimental results to finite-difference thermal simulations. While the measured TBR of about 3.3 x 10^-8 W^-1 ldr m² ldr K for devices on SiC and Si substrates has a sizeable effect on the self-heating in devices, the TBR of up to 1.2 x 10^-8 W^-1 ldr m² ldr K plays an insignificant role in devices on sapphire substrates due to the low thermal conductivity of the substrate. The determined effective TBR was found to increase with temperature at the GaN/SiC interface from 3.3 x 10^-8 W^-1 ldr m² ldr K at 150degC to 6.5 x 3.3 x 10^-8 W^-1 ldr m² ldr K at 275degC, respectively. The contribution of a low-thermal-conductivity GaN layer at the GaN/substrate interface toward the effective TBR in devices and its temperature dependence are also discussed.     

陶瓷复合FR4结构界面形貌与导热性能

基于热电分离式设计理念,将AlN陶瓷片金属化后作为微散热器嵌入FR4材料内形成了复合散热基板.采用电镜扫描、光学显微,通过冷热循环冲击试验对FR4与AlN两相界面处在高低温突变情况下的界面形貌进行了分析.利用ANSYS软件对基板进行了仿真热模拟,研究了AlN嵌入后FR4导热性能的变化规律.利用结温测试仪、功率计和半导体制冷温控台等仪器设备,通过结温测试对比研究了该复合散热结构与金属芯印刷电路板(MCPCB)对大功率LED封装散热效果的影响.结果表明,该复合散热基板在经低温-55℃,高温125℃,1 000个冷热循环后,FR4和AlN界面无剥离现象发生,在环境温度急剧变化的条件下结合力良好.同时,FR4在嵌入AlN之后,导热性能得到了明显改善,且与MCPCB相比,能更有效降低LED芯片结温.     

Silicon carbide benefits and advantages for power electronics circuits and systems

Silicon offers multiple advantages to power circuit designers, but at the same time suffers from limitations that are inherent to silicon material properties, such as low bandgap energy, low thermal conductivity, and switching frequency limitations. Wide bandgap semiconductors, such as silicon carbide (SiC) and gallium nitride (GaN), provide larger bandgaps, higher breakdown electric field, and higher thermal conductivity. Power semiconductor devices made with SiC and GaN are capable of higher blocking voltages, higher switching frequencies, and higher junction temperatures than silicon devices. SiC is by far the most advanced material and, hence, is the subject of attention from power electronics and systems designers. This paper looks at the benefits of using SiC in power electronics applications, reviews the current state of the art, and shows how SiC can be a strong and viable candidate for future power electronics and systems applications.     

Nonlinear characterization of GaN HEMT

DC I-V output, small signal and an extensive large signal characterization (load-pull measurements) of a GaN HEMT on a SiC substrate with different gate widths of 100μm and 1 mm have been carried out. From the small signal data, it has been found that the cutoff frequencies increase with gate width varying from 100μm to 1 mm, owing to the reduced contribution of the parasitic effect. The devices investigated with different gate widths are enough to work in the C band and X band. The large signal measurements include the load-pull measurements and power sweep measurements at the C band (5.5 GHz) and X band (8 GHz). When biasing the gate voltage in class AB and selecting the source impedance, the optimum load impedances seen from the device for output power and PAE were localized in the load-pull map. The results of a power sweep at an 8 GHz biased various drain voltage demonstrate that a GaN HEMT on a SiC substrate has good thermal conductivity and a high breakdown voltage, and the CW power density of 10.16 W/mm was obtained. From the results of the power sweep measurement at 5.5 GHz with different gate widths, the actual scaling rules and heat effect on the large periphery device were analyzed, although the effects are not serious.The measurement results and analyses prove that a GaN HEMT on a SiC substrate is an ideal candidate for high-power amplifier design.     

大功率LED排式热管散热器的开发及性能研究

开发了一种应用于大功率LED散热的排式热管散热器。在大空间自然对流冷却环境中,分别在0°、30°、60°、90°放置条件下对其启动性能、均温特性、散热性能进行了试验研究。试验结果表明:散热器启动性能良好,启动时间约为67min;在输入功率为30～70W的范围内,热源表面中心点温度不超过75℃;各倾角下散热器均具有较低的总热阻及扩散热阻,0°放置时总热阻最小。基于试验所得结果,通过计算LED结温论证了排式热管散热器在各倾角条件下均可满足热输出70W以下大功率LED散热的需求。     

Thermal Resistance Measurement of LED Package with Multichips

Thermal transient measurements of high power GaN-based light-emitting diodes (LEDs) with multichip designs are presented and discussed in the paper. Once transient cooling curve was obtained, the structure function theory was applied to determine the thermal resistance of packages. The total thermal resistance from junction to ambient considering optical power is 19.87 K/W, 10.78 K/W, 6.77 K/W for the one-chip, two-chip and four-chip packages, respectively. The contribution of each component to the total thermal resistance of the package can be determined from the cumulative structure function and differential structure function. The total thermal resistance of multichip packages is found to decrease with the number of chips due to parallel heat dissipation. However, the effect of the number of chips on thermal resistance of package strongly depends on the ratio of partial thermal resistance of chip and that of slug. Therefore, an important thermal design rule for packaging of high power multichip LEDs has been analogized.     

Development and characterization of the thermal behavior of packaged cascode GaN HEMTs

This study investigates the heat generation behavior of packaged normally-on multi-finger AlGaN/GaN high electron mobility transistors (HEMTs) that are cascoded with a low-voltage MOSFET (LVMOS) and a SiC Schottky barrier diode (SBD). By foremost carrying out electro-thermal simulation and related thermal measurements with infrared thermography and Raman spectroscopy for basic 5 mm GaN HEMTs, the location of hot spot in operating device can be obtained. Based on the outcome, further packaged cascode GaN HEMT is analyzed. A hybrid integration of the GaN-HEMT, LVMOS, and SiC SBD are assembled on a directly bonded copper (DBC) substrate in the four-pin metal case TO-257 package. The metal plate is used as both the source terminal and heat sink. The analytical results of thermal investigation are confirmed by comparing them with the infrared thermographic measurements and numerical results obtained from a simulation using Ansys Icepak. For a power dissipation of less than 11.8 W, the peak temperature of the GaN HEMTs is 118.7 °C, obtained from thermal measurements.     

Chip-level spray cooling of an LD-MOSFET RF power Amplifier

We report here the application of water spray cooling directly to the top surface of a lateral diffused metal oxide semiconductor field effect transistor (LD-MOSFET) in a 500-MHz RF power amplifier. With the amplifier running in Class A, spray cooling at a flow of 0.14 l/min increases the output power from 66 W to 84 W, and the power-added efficiency increases from 26% to 34%, all at 34 W input. This improvement is attributed to a large spray-induced reduction in junction temperature and total package thermal resistance. At the point of highest measured RF output and DC power dissipation, the reduction in junction temperature and total thermal resistance were estimated to be from /spl ap/214/spl deg/C to /spl ap/115/spl deg/C and from /spl ap/1.5/spl deg/C/W to /spl ap/0.6/spl deg/C/W, respectively, and the maximum spray-induced heat flux was /spl ap/162W/cm/sup 2/. In Class AB, the increase in output power and power-added efficiency are less, /spl ap/8%, but the amplifier can be driven harder before failure occurs. The maximum output in class AB is 79 W compared to 70 W without spray cooling.     

Micro heat pipes in low temperature cofire ceramic (LTCC) substrates

With projected power densities above 100 W/cm/sup 2/ for devices, new methods for thermal management from the heat generation at the die to heat removal to the ambient must be addressed. By integrating micro heat pipes directly within the ceramic substrate, effective thermal conductivity for spreading heat in both radial and axial directions was achieved. New materials and processes were developed to fabricate the unique components required to handle high thermal loads. Enhanced thermal vias to minimize the thermal impedance through the ceramic in the evaporator and condenser sections were developed, increasing the effective thermal conductivity from 2.63 to near 250 W/m-/spl deg/C. The use of an organic insert fabricated into the desired complex shape using rapid prototyping methods, coupled with the viscoelastic flow of the low temperature cofire ceramic (LTCC) during lamination, allowed complex shapes to be developed while ensuring uniform green tape density during lamination prior to tape firing. Large cavities, three-dimensional fine structures and porous wicks for capillary 3-D flow have been utilized to fabricate the heat pipes. Heat pipes and spreaders, utilizing water as the working fluid, have been demonstrated operating with power densities in excess of 300 W/cm/sup 2/.     

Self-heating in high-power AlGaN-GaN HFETs

We compare self-heating effects in AlGaN-GaN heterostructure field effect transistors (HFETs) grown on sapphire and SiC substrates. Heat dissipation strongly affects the device characteristics soon after the application of the source-drain voltage (in less than 10^-7 s). Our results show that in HFET's with the total epilayer thickness less than 1.5 μm, the thermal impedance, Θ is primarily determined by the substrate material and not by the material of the active layer. For our devices grown on 6H-SiC substrates, we measured Θ of approximately 2°C·mm/W, which was more than an order of magnitude smaller than Θ=25°C mm/W measured for similar AlGaN/GaN HFET's grown on sapphire. Our results demonstrate that AlGaN-GaN HFET's grown on SiC substrates combine advantages of superior electron transport properties in AlGaN/GaN heterostructures with excellent thermal properties of SiC, which should make these devices suitable for high-power electronic applications     

Design optimization of an integrated liquid-cooled IGBT powermodule using CFD technique

This paper presents a novel approach to optimize pin array design of an integrated, liquid-cooled, insulated gate bipolar transistor (IGBT) power module. With the aid of a computational fluid dynamics (CFD) code, the fluid field and heat transfer inside the module were analyzed, and several design options on pin arrays were examined. For IGBT die circuitry, the uniformity of temperature distribution among dies is as critical as the magnitude of the die temperature. A noticeable variation in temperature among dies can accelerate the thermal runaway and reduce the reliability of the devices. With geometrically-optimized-pin designs located both upstream and downstream of the channel, a total power dissipation of 1200 W was achieved. The maximum junction temperature was maintained at 100°C and the maximum variation among dies was controlled within 1°C. The results from this study indicated that the device junction temperatures were not only reduced in magnitude but were equalized as well. In addition, the maximum power dissipation of the module was enhanced. Comparison with other direct- (pool boiling) and indirect- (cold plate) liquid cooling techniques was also discussed     

AlGaN/GaN HEMTs on (111) silicon substrates

AlGaN/GaN HEMTs on silicon substrates have been fabricated and their static and small-signal RF characteristics investigated. The AlGaN/GaN material structures were grown on (111) p-Si by LP-MOVPE. Devices exhibit a saturation current of 0.91 A/mm, a good pinchoff and a peak extrinsic transconductance of 122 mS/mm. A unity current gain frequency of 12.5 GHz and f_max/f_T=0.83 were obtained. The highest saturation current reported so far, static output characteristics of up to 20 V and breakdown voltage at pinchoff higher than 40 V demonstrate that the devices are capable of handling ~16 W/mm static heat dissipation