1.School of Material Science and Engineering, Xiangtan University, Xiangtan 411105, China 2.Science and Technology on Reliability Physics and Application of Electronic Component Laboratory, China Electronic Product Reliability and Environmental Testing Research Institute, Guangzhou 510610, China 3.Northwest Institute of Nuclear Technology, Xi’an 710024, China 4.Key Lab of Wide Bandgap Semiconductor Material and Devices, School of Microelectronics, Xi’an University, Xi’an 710071, China
Abstract:In this paper, neutron-induced displacement damage effects of AlGaN/GaN High electron mobility ransistor (HEMT) device and heterostructure on the Xi’an pulse reactor are studied. The equivalent 1 MeV neutron fluence is 1 × 1014 n/cm2. The direct-current characteristics and low frequency noise characteristics of the HEMT deviceare used to characterize the performances before and after being irradiated by the neutrons, and then the experimental results are analyzed theoretically. The analysis results showed that the displacement damage effect caused by the neutron irradiation will introduce the bulk defects into the device. The bulk defects at the channel cause the electrical performance of the device to degrade through trapping electrons and scattering electrons, which are mainly manifested as the drift of positive threshold voltage, the decrease of output saturation drain current, and the increase of gate leakage current. In order to further confirm the effect of neutron irradiation on the defect density of the device, we implement the low-frequency noise test and the calculation of the device, and the results show that the defect density at the channel of the device increases from 1.78 × 1012 cm–3·eV–1 to 1.66 × 1014 cm–3·eV–1, which is consistent with the results of the electrical characteristics test, indicating that the new defects introduced by neutron irradiation do degrade the electrical performance of the device. At the same time, the capacitor-voltage test is also carried out to analyze the Schottky heterojunctions before and after neutron irradiation. It is found that the channel carrier concentration is significantly reduced after irradiation, and the flat band voltage also drifts positively. The analysis shows that after irradiating the device with neutrons, a large number of defects will be generated in the channel, and these defects will affect the concentration and mobility of the channel carriers, which in turn will influence the electrical performance of the device. These experimental results can be used for designing the AlGaN/GaN high electron mobility transistor for radiationhard reinforcement. Keywords:AlGaN/GaN/ neutron irradiation/ displacement damage/ 1/f noise
对中子辐照前后的AlGaN/GaN HEMT器件进行电学特性敏感参数测试分析, 发现器件的电学特性在中子辐照后有明显退化. 辐照前后的转移特性曲线和输出特性曲线如图2和图3所示. 图 2 中子辐照前后AlGaN/GaN HEMT器件转移特性曲线 Figure2. Transfer characteristic curve of AlGaN/GaN HEMT device before and after irradiation.
图 3 中子辐照前后AlGaN/GaN HEMT器件输出特性曲线 Figure3. Output characteristic curves of AlGaN/GaN HEMT devices before and after irradiation.
${V_{{\rm{th}}}}\left( x \right) = {\phi _{\rm{B}}}\left( x \right) - \frac{{q{N_{\rm{d}}}d_{\rm{d}}^2}}{{2\varepsilon \left( x \right)}} - \Delta {E_{\rm{c}}}\left( x \right) - \frac{{\sigma \left( x \right)}}{{\varepsilon \left( x \right)}},$
使用西安电子科技大学提供的低频噪声测试仪对中子辐照前后的AlGaN/GaN HEMT器件进行低频噪声测试, 测试参数如表1所示. 测试过程中选取了多个不同的栅压, 经过数据处理发现, 在辐照前后的各个器件沟道电流归一化噪声功率谱密度变化一致, 如图5所示. 图 5 AlGaN/GaN HEMT器件辐照前与辐照后沟道电流归一化噪声功率谱密度 (a) 辐照前; (b) 辐照后 Figure5. Normalized noise power spectral density of channel current in AlGaN/GaN HEMT devices before and after irradiation: (a) Before irradiation; (b) after irradiation.
其中, $ {S}_{\mathrm{V}\mathrm{f}\mathrm{b}} $只与器件界面附近的陷阱电荷、结构尺寸等参数有关, $ {G}_{\mathrm{m}} $是器件跨导. 取频率为25 Hz时的实验数据, 拟合出如图6所示的归一化沟道电流噪声功率谱密度随输出电流的变化曲线. 图 6 辐照前后归一化沟道电流噪声功率谱密度与输出电流的关系 Figure6. Normalized channel current noise power spectral density versus channel in the AlGaN/GaN HEMT devices before and after irradiation.
对实验样品进行辐照前后的C-V测试, 提取辐照前后不同频率下的C-V数据作图, 图9展示了中子辐照前后的肖特基二极管在1 MHz时电容随偏置电压的变化曲线. 图 9 中子辐照前后的肖特基二极管在1 MHz时电容随偏置电压的变化 Figure9. Variation curves of capacitance with bias voltage of Schottky diode before and after neutron irradiation at 1 MHz.
其中, $ x $是载流子距AlGaN层的距离, $ C $是在$ {V}_{\mathrm{G}\mathrm{S}} $下每单位面积的肖特基势垒层电容, A是肖特基接触的面积, $ {\varepsilon }_{\mathrm{r}} $和$ {\varepsilon }_{0} $分别是势垒层中材料的相对介电常数和真空介电常数. 基于图9和(15)式可以得到图10载流子浓度随深度的变化. 图 10 载流子浓度随深度的波动函数 Figure10. Fluctuation function of carrier concentration with depth.
从图10可以很明显看出中子辐照后载流子浓度峰值明显下降, 这说明在中子辐照后有更多的电子被俘获. 推断这是由于二维电子气沟道内的部分电子被转移到了AlGaN层的体缺陷内, 如图11所示. 这些被俘获的电子就引起了平带电压的正向漂移, 也就是发生了所谓的载流子去除效应. 图 11 二维电子气沟道内电子转移到体缺陷示意图 Figure11. Schematic diagram of electron transfer to bulk defects in the two-dimensional electron gas channel.
图 1 AlGaN/GaN HEMT器件结构图
图 2 中子辐照前后AlGaN/GaN HEMT器件转移特性曲线
图 3 中子辐照前后AlGaN/GaN HEMT器件输出特性曲线
图 4 中子辐照前后AlGaN/GaN HEMT器件栅特性曲线
图 5 AlGaN/GaN HEMT器件辐照前与辐照后沟道电流归一化噪声功率谱密度 (a) 辐照前; (b) 辐照后
图 6 辐照前后归一化沟道电流噪声功率谱密度与输出电流的关系
图 7 典型结构的AlGaN/GaN HEMT器件中噪声-缺陷源位置[20,21]
图 8 肖特基二极管器件结构图 (a)剖面图; (b)俯视图
图 9 中子辐照前后的肖特基二极管在1 MHz时电容随偏置电压的变化
图 10 载流子浓度随深度的波动函数
图 11 二维电子气沟道内电子转移到体缺陷示意图
