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Investigation of Electrical Characteristics and Trapping Effects in p-GaN Gate HEMTs Under Electron Irradiation
在电子辐照下 p-GaN 门 HEMT 的电气特性和捕获效应的研究

Zixuan Feng ⊕ ⊕ ^(o+){ }^{\oplus}, Shiwei Feng ( ( ^((){ }^{(}, Shijie Pan ( ( ^((){ }^{(}, Xuan Li ( ( ^((){ }^{(}, Binyu You, Boyang Zhang, Yaning Wang,
冯子轩 ⊕ ⊕ ^(o+){ }^{\oplus} , 冯世伟 ( ( ^((){ }^{(} , 潘世杰 ( ( ^((){ }^{(} , 李轩 ( ( ^((){ }^{(} , 游彬宇, 张博阳, 王雅宁,Yamin Zhang ⊕ ⊕ ^(o+){ }^{\oplus}, and Xiang Zheng ⊕ ⊕ ^(o+){ }^{\oplus}
张亚敏 ⊕ ⊕ ^(o+){ }^{\oplus} ，和郑翔 ⊕ ⊕ ^(o+){ }^{\oplus}

Abstract 摘要

In this work, the variations of the electrical properties and trapping effects of p-GaN gate highelectron-mobility transistors (HEMTs) under 1-MeV electron irradiation were investigated systematically. When the irradiation fluence was increased, the drain-source current and the gate leakage current also uprose, but the threshold voltage shifted toward the negative direction. Specifically, after final irradiation, traps in these devices were identified via the current-transient method, and alteration of the trapping effects near the drain and gate could be observed, respectively, by applying different filling voltages. According to the time constant spectra (TCS) and differential amplitude spectra (DAS), six types of detrapping behaviors could be identified. When compared with the results during the pristine stage, the absolute amplitudes of the traps changed after irradiation, which indicated trap densities decreased near the drain and those increased near the gate. The observed changes in the trapping behaviors are consistent with the changes in the electrical properties. To identify the activation energies and locations of traps, the current-transient response was measured at various temperatures before and after electron irradiation. The possible reason for the increased activation energies is that electron irradiation turned the Ga vacancies that decorated the dislocations into pure dislocations and increased the barrier height.
在这项工作中，系统地研究了在 1-MeV 电子辐照下 p-GaN 栅极高电子迁移率晶体管（HEMTs）的电气特性和捕获效应的变化。当辐照通量增加时，漏源电流和栅极漏电流也随之上升，但阈值电压向负方向偏移。具体而言，在最终辐照后，通过电流瞬态方法识别了这些器件中的陷阱，并通过施加不同的充电电压观察到靠近漏极和栅极的捕获效应的变化。根据时间常数谱（TCS）和微分幅度谱（DAS），可以识别出六种去捕获行为。与原始阶段的结果相比，辐照后陷阱的绝对幅度发生了变化，这表明靠近漏极的陷阱密度减少，而靠近栅极的陷阱密度增加。观察到的捕获行为变化与电气特性的变化是一致的。 为了确定陷阱的激活能和位置，在电子辐照前后在不同温度下测量了电流瞬态响应。激活能增加的可能原因是电子辐照将装饰在位错上的镓空位转变为纯位错，并增加了势垒高度。

Index Terms-Current-transient method, electron irradiation, p-GaN gate high-electron-mobility transistor (HEMT), traps.
索引词-瞬态电流法，电子辐照，p 型氮化镓门高电子迁移率晶体管（HEMT），陷阱。

I. INTRODUCTION 一. 引言

WITH the smaller size and weight of space power electronic systems, the basic characteristic requirements are gradually increasing for the power devices, such as efficiency, reliability, and controllability [1], [2], [3]. AlGaN / GaN AlGaN / GaN AlGaN//GaN\mathrm{AlGaN} / \mathrm{GaN} high-electron-mobility transistors (HEMTs) are widely used in power modules and frequency converters of terminal equipment in the aerospace and military fields [4], because of their low ON-resistance, high power density, and high-temperature resistance [5], [6], [7]. Among the available enhancement-mode solutions, p-GaN gate HEMTs have become the promising candidate devices for these application fields with their stable threshold voltage ( V TH V TH V_(TH)V_{\mathrm{TH}} ), high reliability, and good repeatability [8], [9]. Nevertheless, the performance of GaN power HEMTs will still be affected after irradiation because of the high-density defects that exist in epitaxially grown GaN materials [10]. In the space environment, electronic devices will suffer from long-term irradiation and high radiation flux, which can accumulate nonionizing radiation damage in GaN and eventually cause displacement damage [11]. Moreover, radiation may introduce mobile charges into the material, forming external current, which will result significantly device performance degradation and reduce the reliability of electronic devices [12].
随着空间电力电子系统尺寸和重量的减小，对功率器件的基本特性要求逐渐提高，例如效率、可靠性和可控性。高电子迁移率晶体管（HEMTs）因其低导通电阻、高功率密度和高温抗性，在航空航天和军事领域的终端设备的功率模块和频率转换器中被广泛使用。在现有的增强模式解决方案中，p-GaN 栅极 HEMTs 因其稳定的阈值电压、高可靠性和良好的重复性，已成为这些应用领域的有前景的候选器件。然而，由于外延生长的 GaN 材料中存在高密度缺陷，GaN 功率 HEMTs 的性能在辐照后仍会受到影响。在太空环境中，电子设备将遭受长期辐照和高辐射通量，这可能在 GaN 中积累非电离辐射损伤，并最终导致位移损伤。 此外，辐射可能会在材料中引入移动电荷，形成外部电流，这将显著降低器件性能并减少电子设备的可靠性[12]。
The irradiation of these devices in space is mainly caused by the high-energy protons and electrons that are present in the irradiation zone of the Earth. In recent years, there have been numerous reports on the effects of proton and electron irradiation on GaN-based materials [11], [13] and traditional depletion-mode AlGaN / GaN AlGaN / GaN AlGaN//GaN\mathrm{AlGaN} / \mathrm{GaN} HEMTs [14], [15], [16]. The response of GaN to irradiation damage is related to the irradiation energy, the injection dose, the carrier density, and the impurity content of GaN, which would provide possible explanations for the discrepancies between different reports [4], [17]. Hwang et al. [15] studied four different heterojunction HEMTs under 10 − MeV 10 − MeV 10-MeV10-\mathrm{MeV} electron irradiation and proposed that the electron irradiation reduced the 2-D electron gas (2DEG) mobility in the AlGaN / GaN / Si AlGaN / GaN / Si AlGaN//GaN//Si\mathrm{AlGaN} / \mathrm{GaN} / \mathrm{Si} heterojunction; this caused a reduction in the threshold voltages of the corresponding HEMTs. Chen and Liu [14] reported the effect of 1.8 − MeV 1.8 − MeV 1.8-MeV1.8-\mathrm{MeV} electron irradiation on AlGaN / GaN AlGaN / GaN AlGaN//GaN\mathrm{AlGaN} / \mathrm{GaN} HEMTs that were treated using fluorine plasma and observed
这些设备在太空中的辐照主要是由存在于地球辐照区的高能质子和电子引起的。近年来，关于质子和电子辐照对基于氮化镓（GaN）材料的影响的报告屡见不鲜[11]，[13]，以及传统的耗尽模式 AlGaN / GaN AlGaN / GaN AlGaN//GaN\mathrm{AlGaN} / \mathrm{GaN} HEMT[14]，[15]，[16]。GaN 对辐照损伤的响应与辐照能量、注入剂量、载流子密度和 GaN 的杂质含量有关，这可能为不同报告之间的差异提供了可能的解释[4]，[17]。Hwang 等人[15]研究了四种不同的异质结 HEMT 在 10 − MeV 10 − MeV 10-MeV10-\mathrm{MeV} 电子辐照下的表现，并提出电子辐照降低了 AlGaN / GaN / Si AlGaN / GaN / Si AlGaN//GaN//Si\mathrm{AlGaN} / \mathrm{GaN} / \mathrm{Si} 异质结中的二维电子气（2DEG）迁移率；这导致了相应 HEMT 的阈值电压降低。Chen 和 Liu[14]报告了 1.8 − MeV 1.8 − MeV 1.8-MeV1.8-\mathrm{MeV} 电子辐照对使用氟等离子体处理的 AlGaN / GaN AlGaN / GaN AlGaN//GaN\mathrm{AlGaN} / \mathrm{GaN} HEMT 的影响，并观察到
Fig. 1. In addition, we find that the abscissa of Fig. 2(a) should be V GS ( V ) V GS ( V ) V_(GS)(V)V_{\mathrm{GS}}(\mathrm{V}), instead of V DS ( V ) V DS ( V ) V_(DS)(V)V_{\mathrm{DS}}(\mathrm{V}). This change does not affect the content of the article because it is accepted that the transition characteristic curve is I DS ( V ) ∼ V GS ( V ) I DS ( V ) ∼ V GS ( V ) I_(DS)(V)∼V_(GS)(V)I_{\mathrm{DS}}(\mathrm{V}) \sim V_{\mathrm{GS}}(\mathrm{V}). The modified figure is in the attachment named Fig. 2-correct.
图 1。此外，我们发现图 2(a)的横坐标应该是 V GS ( V ) V GS ( V ) V_(GS)(V)V_{\mathrm{GS}}(\mathrm{V}) ，而不是 V DS ( V ) V DS ( V ) V_(DS)(V)V_{\mathrm{DS}}(\mathrm{V}) 。这个更改不会影响文章的内容，因为过渡特性曲线被接受为 I DS ( V ) ∼ V GS ( V ) I DS ( V ) ∼ V GS ( V ) I_(DS)(V)∼V_(GS)(V)I_{\mathrm{DS}}(\mathrm{V}) \sim V_{\mathrm{GS}}(\mathrm{V}) 。修改后的图在名为 Fig. 2-correct 的附件中。
that the device saturation drain current increased significantly after irradiation. Although majority of these studies reported on the changes in the electrical properties of the devices before and after irradiation, the understanding of trapping behaviors in irradiated GaN devices remains relatively limited. The changes in electrical characteristics are attributed to the trapping behaviors exhibited. Therefore, the comparison and analysis of trapping effects become imperative to deepen our understanding of these phenomena before and after electron radiation.
在辐照后，器件的饱和漏电流显著增加。尽管大多数研究报告了辐照前后器件电气特性的变化，但对辐照后氮化镓（GaN）器件中捕获行为的理解仍然相对有限。电气特性的变化归因于所表现出的捕获行为。因此，比较和分析捕获效应变得至关重要，以加深我们对电子辐照前后这些现象的理解。
When the energy of the electrons falls below 0.87 MeV , the concentration of N vacancies exceeds that of Ga vacancies. Conversely, at an electron energy level of 1.2 MeV , the concentration of Ga vacancies surpasses that of N vacancies. When the electron energy is between 0.87 and 1.2 MeV , the change pattern of the device cannot be fully determined [18]. Therefore, we choose 1 MeV for experimentation.
当电子能量低于 0.87 MeV 时，氮空位的浓度超过镓空位的浓度。相反，在电子能量为 1.2 MeV 时，镓空位的浓度超过氮空位的浓度。当电子能量在 0.87 和 1.2 MeV 之间时，设备的变化模式无法完全确定[18]。因此，我们选择 1 MeV 进行实验。
In this work, we determine the effects of 1 − MeV 1 − MeV 1-MeV1-\mathrm{MeV} electron irradiation with different fluences that ranged from 1 × 1 × 1xx1 \times 10 13 cm − 2 10 13 cm − 2 10^(13)cm^(-2)10^{13} \mathrm{~cm}^{-2} to 1 × 10 15 cm − 2 1 × 10 15 cm − 2 1xx10^(15)cm^(-2)1 \times 10^{15} \mathrm{~cm}^{-2} on p − GaN p − GaN p-GaN\mathrm{p}-\mathrm{GaN} gate AlGaN / GaN AlGaN / GaN AlGaN//GaN\mathrm{AlGaN} / \mathrm{GaN} HEMTs. In particular, we used the current-transient method to investigate the trapping effects in HEMTs, which can characterize the traps with the electrical parameters of the electronic devices themselves. The trap-related amplitude, time constants, and energy levels can be extracted from current-transient curves, accordingly indicating the effects of irradiation on trapping behaviors. We can effectively determine the trap positions by applying different filling voltages in the current-transient method and explain the possible reasons for the alteration of electrical characteristics of HEMTs.
在这项工作中，我们确定了不同辐照通量下 1 − MeV 1 − MeV 1-MeV1-\mathrm{MeV} 电子辐照对 p − GaN p − GaN p-GaN\mathrm{p}-\mathrm{GaN} 栅 AlGaN / GaN AlGaN / GaN AlGaN//GaN\mathrm{AlGaN} / \mathrm{GaN} HEMT 的影响，辐照通量范围从 1 × 1 × 1xx1 \times 10 13 cm − 2 10 13 cm − 2 10^(13)cm^(-2)10^{13} \mathrm{~cm}^{-2} 到 1 × 10 15 cm − 2 1 × 10 15 cm − 2 1xx10^(15)cm^(-2)1 \times 10^{15} \mathrm{~cm}^{-2} 。特别地，我们使用电流瞬态方法研究 HEMT 中的捕获效应，该方法可以通过电子设备本身的电气参数来表征捕获。捕获相关的幅度、时间常数和能级可以从电流瞬态曲线中提取，从而指示辐照对捕获行为的影响。我们可以通过在电流瞬态方法中施加不同的充电电压有效地确定捕获位置，并解释 HEMT 电气特性变化的可能原因。

II. EXPERIMENTAL DETAILS II. 实验细节

The devices under investigation were 15 EPC2007C transistors, which are enhancement-mode GaN power devices with Schottky p-GaN gate structures [19]. As shown in Fig. 1, the epitaxial structure of these devices consists of a ∼ 70 − nm ∼ 70 − nm ∼70-nm\sim 70-\mathrm{nm} p − GaN p − GaN p-GaN\mathrm{p}-\mathrm{GaN} cap layer, a ∼ 13 − nm ∼ 13 − nm ∼13-nm\sim 13-\mathrm{nm} AlGaN barrier layer, a ∼ 2 − μ m ∼ 2 − μ m ∼2-mum\sim 2-\mu \mathrm{m} GaN buffer layer, and a ∼ 550 − μ m Si ∼ 550 − μ m Si ∼550-mumSi\sim 550-\mu \mathrm{m} \mathrm{Si} substrate.
所调查的设备是 15 个 EPC2007C 晶体管，这些是具有肖特基 p-GaN 栅极结构的增强型 GaN 功率器件[19]。如图 1 所示，这些设备的外延结构由 ∼ 70 − nm ∼ 70 − nm ∼70-nm\sim 70-\mathrm{nm} p − GaN p − GaN p-GaN\mathrm{p}-\mathrm{GaN} 盖层、 ∼ 13 − nm ∼ 13 − nm ∼13-nm\sim 13-\mathrm{nm} AlGaN 障碍层、 ∼ 2 − μ m ∼ 2 − μ m ∼2-mum\sim 2-\mu \mathrm{m} GaN 缓冲层和 ∼ 550 − μ m Si ∼ 550 − μ m Si ∼550-mumSi\sim 550-\mu \mathrm{m} \mathrm{Si} 基底组成。
The electron irradiation procedure was performed on the 15 devices with 1 − MeV 1 − MeV 1-MeV1-\mathrm{MeV} electrons using fluences that ranged from 1 × 10 13 cm − 2 1 × 10 13 cm − 2 1xx10^(13)cm^(-2)1 \times 10^{13} \mathrm{~cm}^{-2} to 1 × 10 15 cm − 2 1 × 10 15 cm − 2 1xx10^(15)cm^(-2)1 \times 10^{15} \mathrm{~cm}^{-2} at room temperature. The total dose received by these devices was 1.61 × 10 15 cm − 2 1.61 × 10 15 cm − 2 1.61 xx10^(15)cm^(-2)1.61 \times 10^{15} \mathrm{~cm}^{-2} and the electron dose rate was kept constant at 10 11 cm − 2 ⋅ s − 1 10 11 cm − 2 ⋅ s − 1 10^(11)cm^(-2)*s^(-1)10^{11} \mathrm{~cm}^{-2} \cdot \mathrm{s}^{-1}
电子辐照程序在 15 个设备上进行，使用的电子流量范围从 1 × 10 13 cm − 2 1 × 10 13 cm − 2 1xx10^(13)cm^(-2)1 \times 10^{13} \mathrm{~cm}^{-2} 到 1 × 10 15 cm − 2 1 × 10 15 cm − 2 1xx10^(15)cm^(-2)1 \times 10^{15} \mathrm{~cm}^{-2} ，辐照时的温度为室温。这些设备接收到的总剂量为 1.61 × 10 15 cm − 2 1.61 × 10 15 cm − 2 1.61 xx10^(15)cm^(-2)1.61 \times 10^{15} \mathrm{~cm}^{-2} ，电子剂量率保持在恒定的 10 11 cm − 2 ⋅ s − 1 10 11 cm − 2 ⋅ s − 1 10^(11)cm^(-2)*s^(-1)10^{11} \mathrm{~cm}^{-2} \cdot \mathrm{s}^{-1} 。
Fig. 2. Changes in the (a) transfer curves, (b) output curves, and © gate characteristics ( I G S − V G S I G S − V G S I_(GS)-V_(GS)I_{G S}-V_{G S} ) curves of the HEMTs with respect to electron irradiation fluences ranging from 10 13 cm − 2 10 13 cm − 2 10^(13)cm^(-2)10^{13} \mathrm{~cm}^{-2} to 10 15 / cm − 2 10 15 / cm − 2 10^(15)//cm^(-2)10^{15} / \mathrm{cm}^{-2}. The inset diagram in (a) depicts the reverse drift of the threshold voltage. The inset diagram in © depicts the equivalent circuit of the gate.
图 2. HEMT 在电子辐照通量从 10 13 cm − 2 10 13 cm − 2 10^(13)cm^(-2)10^{13} \mathrm{~cm}^{-2} 到 10 15 / cm − 2 10 15 / cm − 2 10^(15)//cm^(-2)10^{15} / \mathrm{cm}^{-2} 变化时的(a) 转移曲线，(b) 输出曲线和(c) 门特性曲线（ I G S − V G S I G S − V G S I_(GS)-V_(GS)I_{G S}-V_{G S} ）。(a)中的插图描绘了阈值电压的反向漂移。(c)中的插图描绘了门的等效电路。
and 10 12 cm − 2 ⋅ s − 1 10 12 cm − 2 ⋅ s − 1 10^(12)cm^(-2)*s^(-1)10^{12} \mathrm{~cm}^{-2} \cdot \mathrm{s}^{-1}. The irradiation process was carried out in stages with the device terminals being grounded [20]. The electron beam was turned off during each measurement, and the devices were stored at room temperature for more than 30 min to ensure stable device performance [14]. After each round of irradiation, the electrical properties of these devices were characterized using a semiconductor parameter analyzer (Keithley 4200 A ) under dc (direct current). Additionally, the drain-source current transients of the devices were measured using the current-transient method [16], [21], along with their time constant spectra (TCS) and differential amplitude spectra (DAS) [22], to investigate the trapping behaviors that occurred before and after final electron irradiation.
和 10 12 cm − 2 ⋅ s − 1 10 12 cm − 2 ⋅ s − 1 10^(12)cm^(-2)*s^(-1)10^{12} \mathrm{~cm}^{-2} \cdot \mathrm{s}^{-1} 。辐照过程分阶段进行，设备端子接地[20]。在每次测量期间，电子束被关闭，设备在室温下存放超过 30 分钟，以确保设备性能稳定[14]。每轮辐照后，使用半导体参数分析仪（Keithley 4200 A）在直流（dc）下对这些设备的电气特性进行表征。此外，使用电流瞬态法[16]，[21]测量设备的漏源电流瞬态，以及它们的时间常数谱（TCS）和差分幅度谱（DAS）[22]，以研究最终电子辐照前后发生的捕获行为。

III. RESULTS AND Discussion
A. Transfer, Output, and Gate Characteristics Test
III. 结果与讨论 A. 转移、输出和门特性测试

The transfer and output characteristic curves of the devices after irradiation are shown in Fig. 2(a) and (b), respectively. All samples exhibit almost the same transfer current-voltage ( I − V ) ( I − V ) (I-V)(I-V) and output curves in the preirradiation case and show similar shift behaviors in the postirradiation case. Specifically, Fig. 2(a) shows that V TH V TH V_(TH)V_{\mathrm{TH}} drifts slightly toward the negative direction after irradiation, and Fig. 2(b) clearly shows that the saturated drain-source current ( I DS I DS I_(DS)I_{\mathrm{DS}} ) increases with increasing fluence. After irradiation with a dose of 1.61 × 10 15 cm − 2 1.61 × 10 15 cm − 2 1.61 xx10^(15)cm^(-2)1.61 \times 10^{15} \mathrm{~cm}^{-2}, the shift of V TH V TH V_(TH)V_{\mathrm{TH}} was 7.8 % 7.8 % 7.8%7.8 \% and the increase of I DS I DS I_(DS)I_{\mathrm{DS}} was 18.2 % 18.2 % 18.2%18.2 \%. The shift in V TH V TH V_(TH)V_{\mathrm{TH}} hardly has any effect on I DS I DS I_(DS)I_{\mathrm{DS}} under higher gate-source voltage ( V GS V GS V_(GS)V_{\mathrm{GS}} ) [23]. Chen and Liu [14] concluded that the increase in I DS I DS I_(DS)I_{\mathrm{DS}} could be attributed to increased electron mobility after irradiation. Studies have shown that the change in V TH V TH V_(TH)V_{\mathrm{TH}} is related to the increase in the positive charge and the reduction in the negative charge in the AlGaN layer [24], [25]. In general, the electrical properties of these devices were improved after irradiation, and this improvement could be ascribed to the reduced trap densities at the AlGaN barrier layer and the GaN buffer [16].
设备在辐照后的传输和输出特性曲线分别如图 2(a)和(b)所示。所有样品在辐照前的传输电流-电压曲线几乎相同，而在辐照后则表现出类似的偏移行为。具体而言，图 2(a)显示辐照后 V TH V TH V_(TH)V_{\mathrm{TH}} 略微向负方向漂移，图 2(b)清楚地显示饱和漏源电流（ I DS I DS I_(DS)I_{\mathrm{DS}} ）随着辐照剂量的增加而增加。在剂量为 1.61 × 10 15 cm − 2 1.61 × 10 15 cm − 2 1.61 xx10^(15)cm^(-2)1.61 \times 10^{15} \mathrm{~cm}^{-2} 的辐照后， V TH V TH V_(TH)V_{\mathrm{TH}} 的偏移为 7.8 % 7.8 % 7.8%7.8 \% ，而 I DS I DS I_(DS)I_{\mathrm{DS}} 的增加为 18.2 % 18.2 % 18.2%18.2 \% 。在较高的栅源电压（ V GS V GS V_(GS)V_{\mathrm{GS}} ）下， V TH V TH V_(TH)V_{\mathrm{TH}} 的偏移对 I DS I DS I_(DS)I_{\mathrm{DS}} 几乎没有影响[23]。陈和刘[14]得出结论， I DS I DS I_(DS)I_{\mathrm{DS}} 的增加可以归因于辐照后电子迁移率的提高。研究表明， V TH V TH V_(TH)V_{\mathrm{TH}} 的变化与 AlGaN 层中正电荷的增加和负电荷的减少有关[24]，[25]。 一般来说，这些器件的电气特性在辐照后得到了改善，这种改善可以归因于 AlGaN 势垒层和 GaN 缓冲层中陷阱密度的降低[16]。
Fig. 3. Illustration of the test waveforms used for the pulsed I − V I − V I-VI-V transfer characteristics with the (a) quasi static drain bias voltage and © quasi static gate bias voltage. Pulsed I − V I − V I-VI-V transfer characteristic curves measured under different values of (b) V DSQ V DSQ V_(DSQ)V_{\mathrm{DSQ}} and (d) V GSQ V GSQ V_(GSQ)V_{\mathrm{GSQ}}.
图 3. 用于脉冲 I − V I − V I-VI-V 传输特性的测试波形示意图，其中(a)为准静态漏极偏置电压，(c)为准静态栅极偏置电压。在不同值下测量的脉冲 I − V I − V I-VI-V 传输特性曲线，(b)为 V DSQ V DSQ V_(DSQ)V_{\mathrm{DSQ}} ，(d)为 V GSQ V GSQ V_(GSQ)V_{\mathrm{GSQ}} 。
Fig. 2© shows that the gate leakage current ( I GS I GS I_(GS)I_{\mathrm{GS}} ) increases by nearly an order of magnitude ( 109.6 % 109.6 % 109.6%109.6 \% ) when the electron fluence reaches 10 15 cm − 2 10 15 cm − 2 10^(15)cm^(-2)10^{15} \mathrm{~cm}^{-2}. The exponential rise in I GS I GS I_(GS)I_{\mathrm{GS}} versus the V GS V GS V_(GS)V_{\mathrm{GS}} shown in Fig. 2© indicates the presence of a trap-related current conduction mechanism. When V GS V GS V_(GS)V_{\mathrm{GS}} exceeds V TH , I GS V TH , I GS V_(TH),I_(GS)V_{\mathrm{TH}}, I_{\mathrm{GS}} is limited by the Schottky junction ( J S J S J_(S)J_{\mathrm{S}} ) [26]. The rise in I GS I GS I_(GS)I_{\mathrm{GS}} with increasing fluence can be attributed to lowering of the barrier height of the gate metal/p-GaN junction, which indicates that the increase of trap densities near the gate raised the probability of assisted tunneling [27]. When V GS V GS V_(GS)V_{\mathrm{GS}} is less than 0 V or when the forward bias is low, the surface current then plays a major role in I GS I GS I_(GS)I_{\mathrm{GS}}. One of the possible mechanisms for surface current is 2-D variable-range hopping (2D-VRH) assisted by a high-density surface electronic state in AlGaN [26]. By considering deep levels in AlGaN , it is likely that some of the surface hopping electrons are trapped by the deep levels. The decrease of trap densities reduces the possibility of electron capture by the deep traps, resulting in increase in I GS I GS I_(GS)I_{\mathrm{GS}} [28]. The specific details will be given in the subsequent sections.
图 2©显示，当电子通量达到 10 15 cm − 2 10 15 cm − 2 10^(15)cm^(-2)10^{15} \mathrm{~cm}^{-2} 时，栅漏电流（ I GS I GS I_(GS)I_{\mathrm{GS}} ）增加了近一个数量级（ 109.6 % 109.6 % 109.6%109.6 \% ）。图 2©中显示的 I GS I GS I_(GS)I_{\mathrm{GS}} 与 V GS V GS V_(GS)V_{\mathrm{GS}} 的指数上升表明存在与陷阱相关的电流导电机制。当 V GS V GS V_(GS)V_{\mathrm{GS}} 超过 V TH , I GS V TH , I GS V_(TH),I_(GS)V_{\mathrm{TH}}, I_{\mathrm{GS}} 时，受肖特基结（ J S J S J_(S)J_{\mathrm{S}} ）的限制[26]。随着通量的增加， I GS I GS I_(GS)I_{\mathrm{GS}} 的上升可以归因于栅金属/p-GaN 结的势垒高度降低，这表明栅附近陷阱密度的增加提高了辅助隧穿的概率[27]。当 V GS V GS V_(GS)V_{\mathrm{GS}} 小于 0 V 或当正向偏置较低时，表面电流在 I GS I GS I_(GS)I_{\mathrm{GS}} 中起主要作用。表面电流的一个可能机制是由 AlGaN 中高密度表面电子态辅助的二维可变范围跳跃（2D-VRH）[26]。考虑到 AlGaN 中的深能级，某些表面跳跃电子可能被深能级捕获。陷阱密度的减少降低了电子被深陷阱捕获的可能性，从而导致 I GS I GS I_(GS)I_{\mathrm{GS}} 的增加[28]。 具体细节将在后面的部分中提供。

B. Pulsed Current-Voltage Characteristics
B. 脉冲电流-电压特性

To verify the presence of the traps and further analyze the effects of the irradiation, pulsed current-voltage characterization was performed on unaged samples and irradiated samples at different quiescent bias points. Fig. 3(a) and © shows the testing waveforms of the pulsed I − V I − V I-VI-V transfer characterizations. The entire pulse period was 50 ms , and the measurement stage was set to have a fixed pulsewidth of 500 μ s 500 μ s 500 mus500 \mu \mathrm{s}. The impact of the measurement voltage on the trapping effect can be ignored because of the low duty cycle [16]. To ensure that the device was in the same initial condition after each detrapping transient measurement, it was recovered completely by shining a 365 − nm 365 − nm 365-nm365-\mathrm{nm} ultraviolet light of 1 W for 30 s [22], [29].
为了验证陷阱的存在并进一步分析辐照的影响，对未老化样品和在不同静态偏置点下的辐照样品进行了脉冲电流-电压特性测试。图 3(a)和(c)显示了脉冲 I − V I − V I-VI-V 传输特性的测试波形。整个脉冲周期为 50 毫秒，测量阶段设置为固定脉冲宽度 500 μ s 500 μ s 500 mus500 \mu \mathrm{s} 。由于占空比低，测量电压对捕获效应的影响可以忽略[16]。为了确保每次去捕获瞬态测量后设备处于相同的初始状态，通过照射 1 W 的 365 − nm 365 − nm 365-nm365-\mathrm{nm} 紫外光 30 秒使其完全恢复[22]，[29]。
Because the electron trapping during the filling process at ( V GSQ , V DSQ ) = ( 0 , 0 V ) V GSQ , V DSQ = ( 0 , 0 V ) (V_(GSQ),V_(DSQ))=(0,0V)\left(V_{\mathrm{GSQ}}, V_{\mathrm{DSQ}}\right)=(0,0 \mathrm{~V}) can be negligible, the alteration of I DS I DS I_(DS)I_{\mathrm{DS}}
由于在填充过程中 ( V GSQ , V DSQ ) = ( 0 , 0 V ) V GSQ , V DSQ = ( 0 , 0 V ) (V_(GSQ),V_(DSQ))=(0,0V)\left(V_{\mathrm{GSQ}}, V_{\mathrm{DSQ}}\right)=(0,0 \mathrm{~V}) 的电子捕获可以忽略不计，因此 I DS I DS I_(DS)I_{\mathrm{DS}} 的变化
Fig. 4. Biasing sequences of detrapping transient after a (a) certain drain filling voltage and © certain gate filling voltage. Schematics of the electron flow under the condition that (b) ( V GF , V DF ) = ( 0 , 25 V ) V GF , V DF = ( 0 , 25 V ) (V_(GF),V_(DF))=(0,25V)\left(V_{\mathrm{GF}}, V_{\mathrm{DF}}\right)=(0,25 \mathrm{~V}) and (d) ( V GF , V DF ) = ( 5 , 0 V ) V GF , V DF = ( 5 , 0 V ) (V_(GF),V_(DF))=(5,0V)\left(V_{\mathrm{GF}}, V_{\mathrm{DF}}\right)=(5,0 \mathrm{~V}).
图 4. 在(a)某一漏极充电电压和(c)某一栅极充电电压下，去捕获瞬态的偏置序列。电子流的示意图在(b) ( V