晶体取向对4H-SiC BJTs电流增益的影响

R. Ghandi, B. Bouno, M. Domeij, S. Shayestehaminzadeh, C. Zetterling, M. Ostling
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Higher quality of passivation can provide less interface traps and thereby minimizes the surface recombination current. Conventionally, vertical 4H-SiC BJTs are fabricated along the [11̲00] direction on (0001) Si-face. However due to anisotropic properties of 4H-SiC, different orientations on Si-face can also affect the base current of the BJT through variation of mobility and interface traps density distribution along each direction. In this work, single-finger small area BJTs are fabricated on (0001) Si-face along [12̲10], [011̲0], [112̲0] and [11̲00] directions. This design can provide various orientations of BJTs that corresponds to an angular range between 0 to 180 degrees relative to conventional [11̲00] direction. The goal was to find a correlation between different crystallographic orientation, mobility and interface traps density distribution through transistor characteristics and finally comparison with simulation. Fig.1 shows a cross section and top view of fabricated BJTs. 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引用次数: 0

摘要

与基于FET的器件相比,4H-SiC双极结晶体管(BJT)能够获得非常低的比导通电阻,因此被认为是高效的高功率开关器件。然而,目前高压bjt的一个缺点是相对较低的电流增益。为了降低驱动电路所需的功率,增加共发射极电流增益(β)是很重要的。4H-SiC (0001) Si-face已成为具有外延层的垂直功率bjt的有利平面,其沿c轴具有更高的迁移率并提供更高的电流增益[1]。此外,在提高SiC/SiO2界面表面钝化质量的电流增益方面也取得了重要进展[2-3]。高质量的钝化可以提供更少的界面陷阱,从而最小化表面复合电流。传统上,垂直的4H-SiC bjt是在(0001)si面上沿[11 × 00]方向制造的。然而,由于4H-SiC的各向异性,硅面的不同取向也会通过改变每个方向的迁移率和界面陷阱密度分布来影响BJT的基极电流。在这项工作中,单指小面积bjt沿[12 10],[011 0],[112 0]和[11 00]方向在(0001)si面上制作。这种设计可以提供不同的bjt方向,对应于相对于传统方向0到180度的角度范围。目的是通过晶体管特性找到不同晶体取向、迁移率和界面陷阱密度分布之间的相关性,并最终与模拟进行比较。图1显示了制造的bjt的截面和俯视图。n+发射极外延层是1.35 μ m的氮掺杂到6×1018 cm−3,并覆盖200 nm厚的2×1019 cm−3层。基底外延层为650 nm掺铝,浓度为4.3×1017 cm−3。漂移层厚度为20µm,掺杂浓度为6×1015 cm−3。采用电感耦合等离子体(ICP)腐蚀氧化掩膜形成发射极和基极。图2为表面钝化和接触金属化前沿[11 × 00]方向的最大电流增益归一化对比图。结果表明,最大电流增益与方向有关,并且当发射极边缘对准[112 * 0]方向时,bjt具有最大电流增益。在前人研究[4]的基础上,模拟了平面迁移率和界面陷阱浓度对电流增益的变化效应,如图3所示。仿真结果表明,界面氧化电荷比迁移率对电流增益的影响更大,且界面氧化电荷越低,电流增益越高。对晶体管参数(如钝化后的最大电流增益和基极电阻)的方向依赖性进行了评估,并与仿真结果进行了比较。
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Influence of crystal orientation on the current gain in 4H-SiC BJTs
The 4H-SiC bipolar junction transistors (BJT) are considered as efficient high power switching devices due to the ability of obtaining very low specific on-resistance compared to FET based devices. However, one drawback with the present high voltage BJTs is the relatively low current gain. To reduce the power required by the drive circuit, it is important to increase the common-emitter current gain (β). 4H-SiC (0001) Si-face has become a favorable plane for vertical power BJTs with epitaxial layers that shows higher mobility along the c-axis and provides higher current gain [1]. Furthermore, important progress on improving the current gain focused on the quality of surface passivation at the SiC/SiO2 interface has been reported during previous years [2–3]. Higher quality of passivation can provide less interface traps and thereby minimizes the surface recombination current. Conventionally, vertical 4H-SiC BJTs are fabricated along the [11̲00] direction on (0001) Si-face. However due to anisotropic properties of 4H-SiC, different orientations on Si-face can also affect the base current of the BJT through variation of mobility and interface traps density distribution along each direction. In this work, single-finger small area BJTs are fabricated on (0001) Si-face along [12̲10], [011̲0], [112̲0] and [11̲00] directions. This design can provide various orientations of BJTs that corresponds to an angular range between 0 to 180 degrees relative to conventional [11̲00] direction. The goal was to find a correlation between different crystallographic orientation, mobility and interface traps density distribution through transistor characteristics and finally comparison with simulation. Fig.1 shows a cross section and top view of fabricated BJTs. The n+ emitter epi-layer is 1.35 µm nitrogen doped to 6×1018 cm−3 and capped by 200-nm-thick 2×1019 cm−3 layer. The base epi-layer is 650 nm Al-doped with concentration of 4.3×1017 cm−3. The drift n− epilayer is 20 µm thick and doped to 6×1015 cm−3. Inductively coupled plasma (ICP) etching with an oxide mask was used to form emitter and base mesas. Fig.2 is a comparison of the maximum current gain with different orientations normalized to the maximum current gain along [11̲00] before surface passivation and contact metallization. The results indicate that the maximum current gain is orientation-dependent and has a maximum for BJTs with the emitter edge aligned to the [112̲0] direction. The variation effect of planar mobility and interface traps concentration on the current gain is simulated based on the previous work [4] and is illustrated in Fig.3. The simulation shows that interface oxide charges has more influence on the current gain compared to the mobility and higher current gain is attributed to lower oxide interface charges. The orientation dependence of the transistor parameters such as maximum current gain after passivation and the base resistance will be evaluated and compared with simulation.
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