Comparative high-temperature DC characterization of HEMTs with GaN and AlGaN channel layers
Comparative high-temperature DC characterization of HEMTs with GaN and AlGaN channel layers
复制标题
具有 GaN 和 AlGaN 沟道层的 HEMT 的高温直流特性对比
DOI：
--
复制
发表时间：
2010
期刊：
影响因子：
--
通讯作者：
M. Kuzuhara
中科院分区：
文献类型：
--
作者：
M. Hatano;N. Kunishio;H. Chikaoka;J. Yamazaki;Z. B. Makhzani;N. Yafune;K. Sakuno;S. Hashimoto;K. Akita;Y. Yamamoto;M. Kuzuhara
首次在AlN衬底上制备了algan通道高电子迁移率晶体管（hemt）。在VGS =2V时，最大饱和电流为0.13A/mm，最大跨导为25mS/mm。对比研究了在室温至300℃范围内，AlGaN-channel HEMT和GaN-channel HEMT的直流特性。gan沟道HEMT漏极电流温度系数约为gan沟道HEMT的一半。这些结果表明，在AlN衬底上生长的algan通道hemt是高温电子应用的有希望的候选者。高电子迁移率晶体管（hemt）在高频和高功率器件应用中引起了人们的极大兴趣。氮化镓及相关氮化物半导体有望成为高压高频HEMT器件的关键材料。最近，由于这些材料的宽带隙特性，考虑到高温下可能的器件应用，对高温器件操作的研究受到了越来越多的关注。Gaska et al.[1]报道，SiC衬底上的AlGaN/GaN hemt在将温度升高至250℃时，饱和漏极电流降低50%。他们还观察到高达300℃的稳定直流性能，没有明显的不可逆变化。Maeda等人[2]报道了SiC衬底上的AlGaN/GaN hemt具有优异的漏极电流饱和和高达400℃的足够掐断特性。他们观察到，通过将温度从25℃提高到400℃，饱和漏极电流减少了约三分之一。Daumiller at al.[3]在高达800℃的温度下测量了AlGaN/GaN hemt的I-V特性，并报告了器件在高达600℃的温度下稳定运行而没有不可逆降解。类似地，Arulkumaran等人报告了在SiC和蓝宝石衬底上制造的AlGaN/GaN hemt在500℃冷却后恢复的漏极I-V特性。Tan等人[5]报道漏极电流的温度依赖性取决于AlGaN/GaN hemt的栅极长度。为了进一步改善氮基hemt的性能限制，最近开发了algan通道hemt。Nanjo等人率先开发了al成分为0.2的AlGaNchannel hemt，其漏极电流密度为0.13A/mm。随后，同一作者报道了改进的直流性能，其中饱和漏极电流密度为0.11A/mm，最大击穿电压为1650V， Al成分为0.38[7]。Raman等人[8]报道了AlGaN通道的漏极电流密度为0.55A/mm， al成分为0.06。该器件在4GHz时的输出功率为4.5W/mm。然而，到目前为止，关于高温下AlGaNchannel hemt直流特性的研究还没有报道。在本文中，我们描述了在AlN衬底上制备的algan通道hemt的高温直流性能。估计了AlGaN通道HEMT的饱和漏极电流密度和导态电阻，并与标准AlGaN/GaN HEMT进行了比较。在室温至300℃的温度范围内，algan通道HEMT的直流性能表现出优异的热稳定性。图1显示了在独立c平面AlN衬底上制备的AlGaNchannel HEMT的原理图。采用金属-有机气相外延法（MOVPE）生长外延层。该结构由一个Al成分为0.24的未掺杂600nm AlGaN通道层和一个Al成分为0.51的未掺杂21nm AlGaN阻挡层组成。在硅片上估计生长的AlGaN/AlGaN异质结的片电阻为1740 /sq。图1 AlN衬底上Al0.51Ga0.49N/Al0.24Ga0.76N HEMT的截面结构示意图。在制造AlGaNchannel hemt的关键工艺步骤之一是源极和漏极的欧姆接触形成。初步研究表明，对于AlGaN/AlGaN异质结构，高温退火是获得良好欧姆接触的首选方法。由于Zr/Al/Mo/Au欧姆触点对AlGaN/GaN的最佳退火温度高于Ti/Al/Mo/Au[9]，我们选择Zr/Al/Mo/Au作为AlGaN通道异质结构的欧姆触点。采用电子束蒸发法依次沉积厚度分别为15、60、35和50nm的Zr、Al、Mo和Au。然后在氮气环境下，用RTA在950℃下退火30秒。蒸发Ni/Au用于肖特基栅金属化。闸门长3 m，宽515 m。为了比较，在Si衬底上制作了具有相同电极尺寸的标准AlGaN/GaN HEMT。AlGaN阻挡层的厚度为25nm， Al成分为0.25。所有器件均未使用钝化膜。图2 (a)和(b)显示了在室温（25℃）和300℃下测量的制备的algan通道HEMT的漏极I-V特性。在两种温度下均观察到优良的掐断和饱和特性。在25℃时，器件的饱和漏极电流（Id）为0.13A/mm，最大跨导（gm）为25mS/mm，阈值电压为-3.8V。估计导通电阻（RON）为60 mm。当温度升至300℃时，器件的饱和漏极电流为0.082 2a /mm，最大跨导为17mS/mm，阈值电压为-3.6V，导通电阻为103 mm。图3 (a)和(b)显示了在25℃和300℃下在Si衬底上制备的标准AlGaN/GaN HEMT的漏极I-V特性。虽然gan通道HEMT的饱和漏极电流和跨导明显优于AlGaN通道(a) (b)。图2 Al含量为0.24的AlGaN通道HEMT在RT (a)和300oC (b)下的漏极I-V特性。
AlGaN-channel high-electron mobility transistors (HEMTs) on an AlN substrate have been fabricated for the first time. A maximum saturation current of 0.13A/mm at VGS =2V and a maximum transconductance of 25mS/mm were obtained. DC characteristics of AlGaN-channel HEMT and GaN-channel HEMT were comparatively examined at temperatures ranging from RT to 300oC. The temperature coefficient of drain current for the AlGaN-channel HEMT was about one half of that for GaN-channel HEMT. These results indicate that AlGaN-channel HEMTs grown on an AlN substrate are promising candidates for high-temperature electronics applications. INTRODUCTION High-electron mobility transistors (HEMTs) are attracting great interest for high-frequency and high-power device applications. GaN and related nitride semiconductors are expected as key materials for high-voltage and highfrequency HEMT devices. Recently, reflecting the wide bandgap nature of these materials, studies on hightemperature device operation have received increased interest in view of possible device application under elevated temperatures. Gaska et al. [1] reported that AlGaN/GaN HEMTs on a SiC substrate exhibited a 50% decrease in the saturated drain current by increasing temperature up to 250oC. They also observed stable DC performance up to 300oC without noticeable irreversible change. Maeda et al. [2] reported excellent drain current saturation and sufficient pinch-off characteristics up to 400oC for AlGaN/GaN HEMTs on a SiC substrate. They observed a decrease in the saturated drain current by about one-third by increasing temperature from 25 to 400oC. Daumiller at al. [3] measured I-V characteristics of AlGaN/GaN HEMTs at temperatures up to 800oC and reported stable device operation without irreversible degradation up to 600oC. Similarly, Arulkumaran et al. [4] reported recovered drain I-V characteristics upon cooling from 500oC for AlGaN/GaN HEMTs fabricated on both SiC and sapphire substrates. Tan et al. [5] reported that the temperature dependence of the drain current is dependent on the gate length of AlGaN/GaN HEMTs. To further improve the performance limitation of nitridebased HEMTs, AlGaN-channel HEMTs have been recently developed. Nanjo et al. [6] was the first to develop AlGaNchannel HEMTs with an Al composition of 0.2, where a drain current density of 0.13A/mm was measured. Subsequently, improved DC performance was reported by the same authors, in which a saturated drain current density of 0.11A/mm and a maximum breakdown voltage of 1650V were achieved with an Al composition of 0.38 [7]. Raman et al. [8] reported a higher drain current density of 0.55A/mm with an Al composition of 0.06 for the AlGaN channel. The device delivered an output power of 4.5W/mm at 4GHz. To date, however, studies on DC characteristics of AlGaNchannel HEMTs at elevated temperatures have not been reported. In this paper, we describe high-temperature DC performance of AlGaN-channel HEMTs fabricated on an AlN substrate. Saturated drain current density and on-state resistance are estimated for the AlGaN-channel HEMT and are compared with those for the standard AlGaN/GaN HEMT. Superior thermal stability in the DC performance of AlGaN-channel HEMT is demonstrated in the temperature range from RT to 300oC. DEVICE STRUCTURE AND FABRICATION PROCESS Figure 1 shows the schematic diagram of an AlGaNchannel HEMT fabricated on a free standing C-plane AlN substrate. Epitaxial layers were grown by metal-organic vapor phase epitaxy (MOVPE). The structure consists of an undoped 600nm AlGaN channel layer with an Al composition of 0.24 and an undoped 21nm AlGaN barrier CS MANTECH Conference, May 17th-20th, 2010, Portland, Oregon, USA layer with an Al composition of 0.51. The sheet resistance of an as-grown AlGaN/AlGaN heterojunction estimated on wafer was 1740 /sq. Fig.1 Schematic cross-sectional structure of Al0.51Ga0.49N/Al0.24Ga0.76N HEMT on AlN substrate. One of the critical process steps in fabricating AlGaNchannel HEMTs is the ohmic contact formation for source and drain electrodes. Preliminary studies indicated that higher-temperature annealing is preferable to achieve good ohmic contacts for AlGaN/AlGaN heterostructures. Since the optimum annealing temperature of Zr/Al/Mo/Au ohmic contacts to AlGaN/GaN is higher than that of Ti/Al/Mo/Au [9], we have chosen Zr/Al/Mo/Au as ohmic contacts for our AlGaN channel heterostructure. Electron-beam evaporation was employed to sequentially deposit Zr, Al, Mo and Au with thicknesses of 15, 60, 35 and 50nm, respectively. Ohmic metals were then annealed by RTA at 950 for 30s under an N2 ambient. Evaporated Ni/Au was used for Schottky gate metallization. The length and width of the gate was 3 m and 515 m, respectively. For comparison, a standard AlGaN/GaN HEMT having same electrode dimensions was fabricated on a Si substrate. The thickness and Al composition for the AlGaN barrier layer were 25nm and 0.25, respectively. No passivation films were used for all the devices. RESULTS AND DISCUSSION Figures 2 (a) and (b) show drain I-V characteristics of the fabricated AlGaN-channel HEMT measured at room temperature (25oC) and at 300oC. Excellent pinch-off and saturation characteristics were observed at both temperatures. At 25oC, the device exhibited a saturated drain current (Id) of 0.13A/mm and a maximum transconductance (gm) of 25mS/mm with a threshold voltage of -3.8V. The estimated on-state resistance (RON) was 60 mm. When the temperature was raised to 300oC, the device showed a saturated drain current of 0.082A/mm, a maximum transconductance of 17mS/mm, a threshold voltage of -3.6V and an on-state resistance of 103 mm. Figures 3 (a) and (b) show drain I-V characteristics of the fabricated standard AlGaN/GaN HEMT on a Si substrate measured at 25 and 300oC. Although the saturated drain current and transconductance of the GaN-channel HEMT is evidently much better than those of the AlGaN channel (a) (b) Fig.2 Drain I-V characteristics for AlGaN-channel HEMT with Al composition of 0.24 measured at RT (a) and at 300oC (b).