Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 61404038)
Received Date:28 July 2019
Accepted Date:14 October 2019
Available Online:05 December 2019
Published Online:05 January 2020
Abstract:During the service of the spacecraft, it will be disturbed by the energetic particles and rays, and thus induce total ionizing dose (TID), displacement damage (DD) or single event effect (SEE) to generate inside the electronic system, which can seriously affect the service lifetime of the electronic components. The difference in structure and types of electronic components are less sensitive to the radiation effects, but bipolar transistor is strongly sensitive to ionizing radiation effect. As a basic component of bipolar circuits, the in-depth study of bipolar transistor ionization radiation effect is of significance for engineering.It has been shown that the an amount of hydrogen can inevitably introduced from an external source during the sealing process of the devices. The KOVAR alloy is widely used as a metal cap material of bipolar transistor in the process of encapsulation. The residual gas analysis (RGA) for sealed Kovar lid packages is shown to have 1%–2% of the hydrogen in the cavity. The source of the hydrogen is generally considered to be out-gassing from the gold plating on the KOVAR. So far, the researches have focused on the study of the ionization damage effect of bipolar transistors with different structures under 60Co gamma ray irradiation. There is lack of systemic study on the comparison of transistors packaged with and without cap.In this paper, we study the influence of sealed KOVAR lid packaged on ionizing radiation damage of lateral PNP bipolar transistor (LPNP) by using 60Co gamma ray as an irradiation source. The semiconductor parameter analyzer is used to measure the electrical parameters of LPNP transistor during irradiation. The irradiation defects in LPNP transistors packaged with and without cap are characterized by deep level transient spectroscopy (DLTS). Experimental results show that the LPNP transistors packaged with and without cap have similar electrical characteristics. The base current increases with the total dose increasing, while the collector current remains almost constant. The degradation of LPNP transistor packaged with cap is more serious.According to the excess base current varying with base-emitter voltage for the LPNP transistors packaged with and without cap, the degradation of bipolar transistor packaged with cap is more serious under the same irradiation conditions. According to the analysis of DLTS, comparing with bipolar transistor packaged without cap, the signal peak at about 300 K is shifted to the left for the bipolar transistor packaged with cap. These results indicate that the LPNP transistors packaged with cap can generate more interface states during irradiation, which is attributed to a large amount of hydrogen and water vapor out-gassing from the gold plating on the KOVAR, which is released under the thermal stress. In the sealed environment, hydrogen can only diffuse into the device cavity, and is combined with the metal material in the transistor to form metal hydride. Therefore the degradation of transistor is severe under the same irradiation condition. Keywords:bipolar transistors/ ionizing radiation/ KOVAR/ interface state
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3.1.电性能测试分析
对开帽/未开帽两种类型LPNP双极型晶体管Gummel特性曲线的对比情况如图1所示. 图中给出了在剂量率为100 rad(Si)/s的60Co γ射线辐照条件下, 晶体管的基极电流(IB)和集电极电流(IC)对电离效应影响的实验结果. 图1(a)描述的是LPNP晶体管IB随发射结电压VEB的变化情况. 随着吸收剂量的增加, 两种LPNP晶体管的IB逐渐增大, 此外, 与发射结电压VEB较大时相比, 当发射结电压VEB较小时, IB增加倍数较大. 图1(b)描述的是LPNP晶体管IC随发射结电压VEB的变化情况. 随着吸收剂量的增加, 两种预处理方式的LPNP晶体管IC均无明显变化. 图 1 剂量率100 rad(Si)/s条件下γ辐射吸收剂量对开帽/未开帽处理的LPNP双极型晶体管的 (a) IB 和 (b) IC 随VEB变化曲线的影响 Figure1. Variations of (a) IB and (b) IC with base-emitter for the LPNP bipolar transistors with/without cap under dose rate of 100 rad(Si)/s with a 60Co gamma irradiation source.
本文中信号峰所处位置与文献[23]中的试验结果相似, 表明60Co γ射线辐照诱导LPNP双极型晶体管产生电离辐射损伤缺陷为界面态陷阱. 通常, 为了解释界面态陷阱的反应机制, 常采用Shaneyfelt等[24]提出的空穴/氢离子输运(HT)2模型. 该模型认为, 空穴向Si/SiO2界面传输时, 会在界面附近形成陷阱电荷. 随着空穴成为陷阱电荷或者被电子中和, 界面附近的氢原子可以在带正电的氧化物陷阱处形成氢离子, Si/SiO2界面附近的氢离子被释放, 传输到Si/SiO2界面的氢离子与界面发生相互作用, 同时生成界面态陷阱[25]. 通过图3可以看出, 在相同辐照条件下, 与开帽处理过的LPNP晶体管相比, 未开帽处理的晶体管在辐照后DLTS特征峰向左移动, 表明辐照在未开帽处理的LPNP晶体管中引入的缺陷能级位置更接近禁带中心. 由肖克莱-里德-霍尔模型[26]可知, 缺陷能级位置越接近禁带中心, 则复合效率越高, 进而对晶体管造成的损伤越严重. 此外, 缺陷能级位置及缺陷浓度均是导致晶体管性能退化的重要因素, 我们之前研究已经证实, 与缺陷浓度相比, 缺陷能级位置占主导地位, 是使晶体管电学性能退化加剧的根本原因[27]. 通过上述分析可以说明预处理过程中未开帽处理LPNP晶体管内部受到电离辐射损伤更为严重. 基于上述分析, 通过探究LPNP双极晶体管电流增益变化量(Δβ)及其电流增益倒数变化量Δ(1/β)随吸收剂量的关系, 直观反映出辐照前后晶体管电性能变化规律, 为证明开帽处理对电离辐射损伤的影响机制, 揭示器件宏观电学性能退化与微观缺陷演化的对应关系. 电流增益β是表征双极晶体管辐射损伤效应最为重要的电性能参数. 本试验采取共发射极接线方式进行测试, 当发射结正偏电压VEB = 0.65 V时, IC与IB的比值定义为电流增益, 即β = IC/IB, 电流增益变化量的表达式为: Δβ = β-β0, 电流增益倒数变化量的表达式为: Δ(1/β)=1/β-1/β0, 式中β0和β分别为晶体管辐照前和辐照后的电流增益值. 图4(a)和图4(b)分别为相同剂量率60Co γ射线辐照条件下, 开帽/未开帽处理的LPNP晶体管Δβ和Δ(1/β)随吸收剂量的变化曲线. 如图所示, 对于两种类型的LPNP型晶体管而言, 随着吸收剂量的增加, LPNP晶体管的Δβ明显下降, 晶体管Δ(1/β)逐渐升高且退化无饱和趋势. 上述结果表明, 两种类型的LPNP晶体管均发生明显的电离辐射损伤. 其中, 在相同辐照条件下, 在预加温过程中未开帽处理的晶体管Δβ和Δ(1/β)退化程度更为明显. 图 4 剂量率100 rad/s条件下γ辐射吸收剂量对开帽/未开帽处理的LPNP双极晶体管(a)电流增益变化量的影响和(b)电流增益倒数变化量的影响 Figure4. (a)The relationship between total dose and current gain for LPNP bipolar transistors with/without cap under dose rate of 100 rad (Si)/s with a 60Co gamma irradiation source. (b) The relationship between total dose and the reciprocal of current gain for LPNP bipolar transistors with/without cap under dose rate of 100 rad (Si)/s with a 60Co gamma irradiation source.