CN106876458B - 一种槽栅增强型AlGaN/GaN异质结场效应晶体管 - Google Patents

一种槽栅增强型AlGaN/GaN异质结场效应晶体管 Download PDF

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CN106876458B
CN106876458B CN201710021096.3A CN201710021096A CN106876458B CN 106876458 B CN106876458 B CN 106876458B CN 201710021096 A CN201710021096 A CN 201710021096A CN 106876458 B CN106876458 B CN 106876458B
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段宝兴
郭海君
谢慎隆
袁嵩
杨银堂
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Xidian University
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Abstract

本发明公开了一种具有部分本征GaN帽层的槽栅增强型AlGaN/GaN异质结场效应晶体管。这种新型晶体管结构是在晶体管栅极边缘引入本征GaN帽层,该本征GaN帽层会降低该区域导电沟道的二维电子气浓度,实现电场调制效应。通过产生新的电场峰,降低了栅边缘的高电场,使晶体管表面的电场分布更加均匀,与传统槽栅增强型结构相比,新型结构的击穿电压和可靠性也就有了明显的提高与改善。

Description

一种槽栅增强型AlGaN/GaN异质结场效应晶体管
技术领域
本发明涉及半导体器件技术领域,特别是涉及一种槽栅增强型AlGaN/GaN异质结场效应晶体管。
背景技术
由于以Si和GaAs为代表的第一代和第二代半导体材料的局限性,第三代宽禁带半导体材料因为其优异的性能得到了飞速发展。GaN材料作为第三代半导体材料的核心之一,相比Si,GaAs和SiC特殊之处在于其所具有的极化效应。利用这种特殊性,人们研制了AlGaN/GaN高电子迁移率晶体管,AlGaN/GaN HEMTs是以AlGaN/GaN异质结材料为基础而制造的GaN基微电子器件。AlGaN/GaN异质结通过自发极化和压电极化效应在异质结界面处形成高密度二维电子气(two dimensional electron gas,2DEG),这种二维电子气具有很高的迁移率,从而使AlGaN/GaN HEMTs具有很低的导通电阻。与传统的场效应晶体管(FET)器件相比,AlGaN/GaN HEMTs具有高跨导、高饱和电流以及高截止频率等优良特性。而且,实验证明,GaN基HEMTs在1000K的高温下仍然保持着良好的直流特性,从而为高温环境应用提供了可靠的保证。
由于AlGaN/GaN异质结得天独厚的优势,AlGaN/GaN异质结材料的生长和AlGaN/GaN HEMTs器件的研制始终占据着GaN电子器件研究的主要地位。然而十几年来针对GaN基电子器件研究的大部分工作集中在耗尽型AlGaN/GaN HEMTs器件上,这是由于AlGaN/GaN异质结中强极化电荷的存在,使得制造GaN基的增强型器件变得十分困难,因此高性能增强型AlGaN/GaN HEMTs的研究具有非常重要的意义。
对于GaN基增强型器件,工艺上较为容易实现的是槽栅结构。W.B.Lanford等人通过MOCVD利用槽栅技术制得了阈值电压达0.47V的增强型器件。该器件结构自下而上包括:SiC衬底,成核层,2um厚的GaN,3nm厚的AlGaN,10nm厚的n-AlGaN,10nm厚的AlGaN。该技术通过将栅下的势垒层刻蚀一定深度,使得栅下势垒层变薄,从而使栅下2DEG浓度降低,而源漏区的载流子浓度保持较大值不变,这样既可实现器件的增强型特性,又可保证一定的电流密度。参见文献:
W.B.Lanford,T.Tanaka,Y.Otoki and I.Adesida,“Recessed-gateenhancement-mode GaN HEMT with high threshold voltage”,Electronics Letters,Vol.41,No.7,March2005.
然而,在槽栅增强型AlGaN/GaN HEMTs的栅边缘往往存在着高峰电场,其会给器件带来以下不利影响:1、会引起电子–空穴对离化,当达到GaN材料的临界击穿电场这一雪崩条件时,器件在栅电极边缘击穿。2、即使没有达到GaN材料的临界击穿电场,高电场效应仍然会使栅电极电子场致发射遂穿进入表面钝化层,这些隧穿的电子会中和AlGaN层的表面极化正电荷,而这些表面极化正电荷,直接关系到异质结界面处2DEG的浓度大小,部分表面正电荷被中和会降低高密度的2DEG浓度,从而使AlGaN/GaN HEMTs输出电流明显减小,这就是电流崩塌效应。3、使电子–空穴对的离化几率增加,电离后的空穴在纵向电场作用下进入沟道中和2DEG,也会使2DEG浓度减小,进一步减小输出电流;而且电离后的电子进入AlGaN极化层会给器件阈值电压带来不利影响,使得器件可靠性降低。
发明内容
为了解决现有技术中由于在槽栅增强型AlGaN/GaN异质结场效应晶体管的栅边缘存在高峰电场而引起的器件雪崩击穿、电流崩塌效应,阈值电压和输出电流减小,可靠性降低等一系列问题,本发明提供一种新型槽栅增强型AlGaN/GaN异质结场效应晶体管。
解决方案如下:
一种槽栅增强型AlGaN/GaN异质结场效应晶体管,包括:
半绝缘衬底;
位于所述半绝缘衬底上异质外延生长的AlN成核层;
位于所述AlN成核层上外延生长的GaN缓冲层;
位于所述GaN缓冲层上外延生长的AlGaN势垒层;
分列于所述AlGaN层上的源极、栅凹槽以及漏极;
位于所述源极与漏极之间的栅凹槽;
位于所述栅凹槽上的栅极;
其特殊之处在于:
在AlGaN势垒层上还外延生长有与栅极边缘邻接的本征GaN帽层,所述本征GaN帽层部分覆盖或者完全覆盖栅极和漏极之间的区域,其长度与沟道2DEG浓度调制需要有关。
基于上述解决方案,本发明还进一步作如下优化限定和改进:
上述本征GaN帽层是通过在AlGaN势垒层表面外延生长本征GaN层,然后刻蚀形成的。
本征GaN帽层位于栅极和漏极之间,可以部分覆盖,也可以完全覆盖。这是因为本征GaN帽层对沟道2DEG浓度调制作用的效果与其长度有关,可以灵活选择刻蚀区域,本征GaN帽层长度以不超过栅漏间距的百分之二十为佳。
上述栅凹槽是通过对本征GaN帽层和AlGaN势垒层局部刻蚀形成的。
上述栅极通过肖特基接触与所述AlGaN势垒层相连。
上述源极和所述漏极均通过欧姆接触与所述AlGaN势垒层相连。
上述外延生长的GaN缓冲层具有n型电阻特性或半绝缘特性。
上述半绝缘衬底为能够与所述AlN成核层异质外延的半绝缘材料,优选硅或碳化硅,或者采用蓝宝石衬底。
本发明的上述技术方案的有益效果如下:
在晶体管栅极边缘引入本征GaN帽层,该本征GaN帽层会降低该区域导电沟道2DEG的浓度,实现电场调制效应。通过产生新的电场峰,降低了栅边缘的高电场,使晶体管表面的电场分布更加均匀。随着本征GaN帽层长度的增加,电场调制效应增强,使得新电场峰值提高,栅边缘高峰电场下降量增加;而且由于表面电场分布更加均匀,使得器件在达到GaN材料临界击穿电场时所需要施加的漏端电压更大,击穿电压提高,器件可靠性相比于传统槽栅增强型结构,也有了明显的改善。
附图说明
图1为本发明具有部分本征GaN帽层的新型槽栅增强型AlGaN/GaN异质结场效应晶体管的示意图。
图2为传统槽栅增强型AlGaN/GaN异质结场效应晶体管结构与本发明具有本征GaN帽层的槽栅增强型AlGaN/GaN异质结场效应晶体管结构击穿时沟道电场分布与电压值对比图。
具体实施方式
为使本发明要解决的技术问题、技术方案和优点更加清楚,下面结合附图及具体实施例进行详细描述。
该实施例是一种具有部分本征GaN帽层的新型槽栅增强型AlGaN/GaN异质结场效应晶体管。其结构如图1所示,主要包括:半绝缘衬底0;位于半绝缘衬底上异质外延生长的AlN成核层1;位于AlN成核层上外延生长的GaN缓冲层2;位于GaN缓冲层上外延生长的AlGaN势垒层3;位于所述AlGaN势垒层上的栅凹槽4、漏极5以及源极6;位于栅凹槽上的栅极7;位于所述AlGaN势垒层上,与栅极边缘邻接的本征GaN帽层8。
槽栅结构是常用的实现增强型的方法。通过将栅下的势垒层刻蚀一定深度,使得栅下势垒层变薄,从而使栅下2DEG浓度降低,进而实现增强型特性。器件的阈值电压取决于刻蚀深度。
引入本征GaN帽层,使得GaN/AlGaN界面处感应出负极化电荷,这层负电荷降低了沟道2DEG浓度,产生新的电场峰,使得栅极边缘高电场降低,表面电场分布趋于均匀。随着本征GaN帽层长度的增加,电场调制效应增强,使得新电场峰值提高,栅边缘高峰电场下降量增加;而且由于表面电场分布更加均匀,使得器件在达到GaN材料临界击穿电场时所需要施加的漏端电压更大,击穿电压提高,器件可靠性相比于传统槽栅增强型结构,也有了明显的改善。
如图2所示,传统结构在栅极边缘存在高峰电场,电场分布为三角形,其击穿电压只有46V,而新型结构能够在本征GaN帽层靠近漏极一侧产生新的电场峰,有效降低了栅极边缘的高峰电场,击穿电压提高至78V,其中栅极靠近漏极边缘位置为X=3.0μm,本征GaN帽层长度为2.0μm,厚度为100nm。
其具体实现方法以感应耦合等离子体刻蚀(ICP)为例:在完成本征GaN帽层AlGaN/GaN异质结场效应晶体管的刻蚀与金属电极淀积工艺后,利用ICP在靠近栅极边缘刻蚀出本征GaN帽层。刻蚀区域可以灵活选择。
这里,本征GaN帽层的厚度,只与该帽层所对应的沟道具体要求有关,在需要减少沟道载流子浓度的地方,就应存在本征GaN帽层,帽层的厚度越大,载流子浓度减小的幅度越大,具体沟道载流子浓度的大小,主要是根据需要着重遏制的不利影响来确定,比如:
若需要一个LDD的浓度分布以提高击穿电压遏制热载流子注入效应,则可以在由栅到漏依次刻蚀不同深度,产生阶梯型本征GaN帽层。
若需要充分改善器件的击穿特性,则可以在栅漏间完全覆盖本征GaN帽层。
若需要降低器件漏极靠近栅极边缘产生的高峰电场,则可以在漏极边缘根据具体要求刻蚀产生本征GaN帽层,等等。
为获得“具有调制沟道载流子浓度的本征GaN帽层”,并不限于上述实施例采用的本征GaN帽层ICP刻蚀技术,也可以采用其他方式实现,最终应能达到相同的技术效果。
获得本征GaN帽层的刻蚀技术与方法有很多,反应离子刻蚀(RIE)、电子回旋共振等离子体刻蚀(ECR)等能够刻蚀本征GaN帽层的技术都可以应用于此方案。
以上所述的是本发明的优选实施方式,对于本技术领域的普通人员来说,基于本发明的原理,还可以进行若干改进和完善,这些改进和完善的产物也应视为本发明的保护范围。

Claims (4)

1.一种槽栅增强型AlGaN/GaN异质结场效应晶体管,包括:
半绝缘衬底;
位于所述半绝缘衬底上异质外延生长的AlN成核层;
位于所述AlN成核层上外延生长的GaN缓冲层;
位于所述GaN缓冲层上外延生长的AlGaN势垒层;
分列于所述AlGaN势垒层上的源极、栅凹槽以及漏极;
所述栅凹槽位于所述源极与漏极之间;
位于所述栅凹槽上的栅极;
其特征在于:
所述栅极通过肖特基接触与所述AlGaN势垒层相连;在AlGaN势垒层上还外延生长有与栅极边缘邻接的均一厚度的矩形本征GaN帽层,所述本征GaN帽层是通过在AlGaN势垒层表面外延生长本征GaN层,然后刻蚀形成的;所述栅凹槽是通过对本征GaN帽层和AlGaN势垒层局部刻蚀形成的;所述本征GaN帽层部分覆盖栅极和漏极之间的区域,其长度与对沟道2DEG浓度调制的需要有关;所述本征GaN帽层长度为2.0μm,厚度为100nm。
2.如权利要求1所述的槽栅增强型AlGaN/GaN异质结场效应晶体管,其特征在于:所述源极和所述漏极均通过欧姆接触与所述AlGaN势垒层相连。
3.如权利要求1所述的槽栅增强型AlGaN/GaN异质结场效应晶体管,其特征在于:所述外延生长的GaN缓冲层具有n型电阻特性。
4.如权利要求1所述的槽栅增强型AlGaN/GaN异质结场效应晶体管,其特征在于:所述半绝缘衬底的材料为硅或碳化硅,或者替换为蓝宝石衬底。
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