WANG Chang-si, XU Yue-hang, WEN Zhang, CHEN Zhi-kai, ZHAO Xiao-dong, XU Rui-min. Large-signal Characterization and Modeling for Microwave Field-Plate GaN HEMTs[J]. Journal of University of Electronic Science and Technology of China, 2017, 46(3): 485-491. DOI: 10.3969/j.issn.1001-0548.2017.03.002
Citation: WANG Chang-si, XU Yue-hang, WEN Zhang, CHEN Zhi-kai, ZHAO Xiao-dong, XU Rui-min. Large-signal Characterization and Modeling for Microwave Field-Plate GaN HEMTs[J]. Journal of University of Electronic Science and Technology of China, 2017, 46(3): 485-491. DOI: 10.3969/j.issn.1001-0548.2017.03.002

Large-signal Characterization and Modeling for Microwave Field-Plate GaN HEMTs

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  • Received Date: October 29, 2015
  • Revised Date: October 12, 2016
  • An electro-thermal large-signal model including a nonlinear thermal network for Gallium Nitride high electron mobility transistors (GaN HEMTs) with gate and source field plates is presented in this paper. This model including the nonlinear thermal network with respect to power dissipation is embedded in the improved Angelov model. Based on the electro-thermal principle, the thermal resistance and capacitance for the two field plates of the devices are identified by utilizing the electro-thermal finite element method (FEM) simulations. And the characteristics of small signal and load impedance for the two devices with different field plates have been analyzed in microwave frequency range. Accurate predictions of the quiescent currents, S-parameters up to 40 GHz, and large-signal harmonic performance for the devices with different gate peripheries have been achieved by the proposed model.
  • GaN HEMTs具有功率密度大、电子迁移率高、耐高温等优势,正被广泛应用于微波毫米波通信与雷达系统中[1-2]。为进一步提高GaN HEMTs的功率密度,在器件中引入各种场板结构成为目前研究的重点[3-5]。例如,通过将栅极向栅漏区域延伸,形成栅场板结构 (gate-field plate, G-FP),它调制了栅极下电场分布,减小了栅极下面靠近漏端的峰值电场强度,使得场板GaN HEMTs的击穿电压提高到1.9 KV[3],功率密度超过40 W/mm[4]。然而,栅场板也会增大寄生效应、栅电容及耗尽层长度,引起器件的增益下降,因此往往需要在击穿电压和增益之间折中考虑。为更好地解决该问题,斜场板、双场板或多场板等结构被用于功率器件中[4-5],在栅场板基础上,增加了源场板 (source-field plate, S-FP) 结构,能起到降低反馈电容 (Cgd) 和改善大信号增益特性的作用;该场板下的耗尽区域不会受输入信号调制的影响,能够改善器件的线性度。此外,场板结构能降低表面陷阱的影响,减小色散特性,进而起到改善跨导、增益、减小泄漏电流和增强器件的稳定性等作用[4]。因此,场板结构广泛应用于GaN HEMTs等功率器件中。

    目前,有关场板GaN HEMTs器件的模型研究主要集中在利用解析或者数值仿真法优化场板结构及分析场板对器件性能的影响[6-8];而针对不同场板GaN HEMTs的通用大信号模型研究较少,以及利用该模型对器件大信号特性的分析少有研究。

    本文针对栅、源两种场板GaN HEMTs,提出了一种包含非线性热网络的电热大信号模型。采用Ansys (v. 14) 热仿真的方法,提取了两种场板器件的热阻和热容参数,并嵌入到改进的Angelov经验模型中;分析了两种场板结构对寄生参数、小信号特性和大信号负载阻抗等的影响。最终将该大信号模型嵌入到ADS仿真软件中,完成在片测试与仿真结果对比,验证了该模型的准确性。

    虽然栅、源场板器件在结构上不同,寄生效应等也有所区别,但可以用一个通用的大信号等效电路拓扑来模拟,如图 1所示。图 2为本文所采用的电热大信号等效电路模型。它包括外部的寄生电阻、电容和电感,内部的本征电容、漏源电流和栅极电流等元件,以及表征器件自热效应的热网络参数。

    完整的模型建模流程如图 3所示,从小信号S参数测试出发,提取寄生参数和本征参数;由直流 (DC-Ⅳ) 和脉冲 (Pulsed-Ⅳ) 测试获得电流特性和色散特性;用FEM热仿真得到自热效应参数。最终将模型用符号定义器件 (symbolically-defined device, SDD) 嵌入到ADS软件中,并将仿真结果与大信号测试结果对比,完成模型的验证。

    Angelov经验模型广泛用于场效应晶体管 (field effect transistors, FETs) 大信号紧凑模型中[9-11]。由于GaN HEMTs有较明显的自热和陷阱效应,故本文在Angelov模型基础上,采用改进的Ids公式[9]

    Ids=Ipkth(1+Mipkthtanh(ψ))tanh(αVds)
    (1)

    其中,

    Ipkth=Ipk0(1+kipk0ΔTch)
    (2)
    Mipkth=(1+0.5(Mipkl1)(1+tanh(qm×(VgseffVgsm))))(1+kMΔTch)
    (3)
    ψ=Pk1th(VgseffVpk1)+Pk2th(VgseffVpk2)2+Pk3th(VgseffVpk3)3
    (4)
    Vgseff=Vgs+ksurf(VgsqVgspinchoff)(VgsVgspinchoff)+ksubs(Vdsq+Vdssub0)(VdsVdsq)
    (5)
    Pknth=(kn0+(kn0+kn1Vds)tanh(αnVds))×(1+kpnΔTch)n=1,2,3
    (6)
    ΔTch=PdissRth(Pdiss)(1exp(t/τ))
    (7)
    Rth(Pdiss)=Rth0(1+kR1exp(kR2Pdiss))
    (8)
    Cth=τ/RthPdiss
    (9)

    式中,Ipk0表示最大跨导 (gm) 处的漏源电流;Mipkth是以栅压为函数的双曲正切函数倍乘因子;Vpk1Vpk2Vpk3用于拟合HEMTs随栅压变化的非对称“钟型”gm特性;α代表饱和电压参数;Vgseff用于表征器件表面陷阱和体陷阱效应的栅电压修正参数; ΔTch 是沟道的温度变化量,用于描述器件的自热效应;其余参数为拟合系数。

    自热效应广泛存在于HEMTs功率器件中,引起沟道温度升高,电子迁移率降低,进而恶化器件的电性能[12]。为准确地表征该特性,常用由热阻 (Rth) 和热容 (Cth) 构成的热子电路来模拟自热效应[10-11]。通常,RthCth的提取采用恒温下的脉冲Ⅳ测试来完成,但热阻往往随功耗呈非线性变化,且脉冲Ⅳ测试系统往往过于复杂和耗时[13]。故本文针对两种场板GaN HEMTs器件 (如图 1所示),采用有限元热稳态和瞬态仿真的方法,提取两种结构的热参数[14]。虽然器件的沟道温度对器件的封装形式和散热系统敏感,但可以采用等效的方法来简化器件的分析。通过在衬底底部设置适当厚度的散热材料和环境温度来模拟器件实际的工作环境。例如,本文采用1 mm厚的铜块与衬底接触,边界设为环境温度300 K,在栅指上加上一定的功耗。热仿真结构及热分布如图 4a所示,通过仿真结果可以发现,热量集中在栅指上,由于外延层非常薄,可以认为其温度为沟道温度。提取的Rth图 4b所示,相比于单栅场板器件,双场板器件在散热方面更优,故Rth更小;表征热动态变化的Cth则变化不大。式 (8) 和式 (9) 用于拟合RthCth。热时间常数τ由热瞬态仿真获得。建立的非线性热网络如图 2中的虚框所示。

    多偏置小信号S参数用于提取器件的寄生和本征参数[15-16]。在0.1 ~ 40 GHz频率范围内,使用安捷伦矢量网络分析仪 (E8364B) 测量两种场板器件的S参数。利用低频段“Cold-FET”夹断状态的S参数提取器件的寄生电容;高频段“Cold-FET”正偏置条件下提取寄生电感和电阻参数;内部本征参数由“Hot-FET”S参数嵌寄生参数后获得。通过窄脉冲 (350 ns) 动态Ⅳ测试仪 (Auriga/Au4850),在低占空比 (0.1%)、不同静态偏置下,测量器件的Pulsed-Ⅳ特性,从而得到器件表面陷阱和体陷阱效应特性,并用Ids公式中的修正参数Vgseff来描述。得到器件各参数初值后,利用随机及梯度迭代算法,对参数进行整体优化。最终得到的两种结构器件寄生参数和部分本征参数如表 1图 5所示,可以看出,两种结构的寄生参数值区别较小,但本征参数值区别较大,这与双场板结构引入了更大的CgsCds的物理特性是一致的,同时,由于双场板结构在栅-漏方向延伸了耗尽层长度,所以与单场板相比,Cgd更小。图 5给出了非线性电容拟合结果,所用电容公式为[12]

    Cgs=Cgsp+Cgs0(1+tanh(ϕ1))(1+tanh(ϕ2))
    (10a)
    ϕ1=M10+M11Vgs+P111Vds
    (10b)
    ϕ2=M20+M21Vgs+M22Vds
    (10c)
    Cgd=Cgdp+Cgd0(1+tanh(ϕ3)(1+tanh(ϕ4)+2P111)
    (11a)
    ϕ3=M30M31Vds
    (11b)
    ϕ4=M40+M41VgsP111Vds
    (11c)
    参数类型
    单栅场板 (G-FP)双场板 (G-FP+S-FP)
    Rg/Ohm2.8952.768
    Rd/Ohm0.5920.583
    Rs/Ohm0.0110.012
    Lg/pH46.1952.09
    Ld/pH46.5746.66
    Ls/pH7.636.22
    Cpg/pF1.34e-46.17e-4
    Cpd/pF7.28e-47.34e-4
    Cpgd/pF0.0480.023
    Cds/pF0.0680.113
     | Show Table
    DownLoad: CSV

    将以上两种场板GaN HEMTs器件的模型参数通过ADS软件的SDD嵌入到仿真软件中[17],用于器件的电流、小信号及大信号特性仿真。利用负载牵引测试平台 (Focus/MPT-3620-TC) 测试器件的阻抗与大信号输出特性,用于与该模型的仿真结果进行对比验证,探针在片测试平台和测试装置如图 6所示。

    图 7为直流特性的仿真与测试对比图。由于自热效应的影响,在较高功耗的情况下,电流随Vds的增大出现了明显的下降 (如图 7a7b),而该模型嵌入了非线性热网络,准确地模拟了热效应。在膝点区域,仿真与实测结果略有误差,主要原因是与原始Angelov Ids公式相比,本文减少了Ids公式中的ψαVdsVgs的依赖性[9],但通过后文的模型验证可以看出,该误差的影响较小,Ids模型具有足够的精度。此外,相比于单场板器件,双场板器件的最大饱和电流大了约15 mA (如图 7c所示),这与后者具有更大的功率密度是一致的。

    图 8给出了器件在偏置电压Vds = 10 V时,gm和最大电流增益截止频率 (fT) 随栅压变化的仿真与测试结果。可以看出,两者的gm值基本相同;与双场板相比,单栅场板器件的Cgd虽然更大,但它的Cgs更小,以致于后者的fT更大一些,为46 GHz。仿真与测试结果表明,该模型能准确地预测两种场板GaN HEMTs的增益特性。

    为了验证大信号模型对小信号特性预测的一致性,对比了两种场板器件的小信号S参数仿真与测试结果,如图 9所示。可以看出,在0.1 ~ 40 GHz频率范围内,该模型能较准确地预测器件的小信号特性。

    器件输入输出负载阻抗是微波电路设计的重要参数,场板结构的不同引起的阻抗变化影响着电路匹配的难易程度。同时,准确地预测器件在不同工作状态下的阻抗特性,是最终实现最佳电路性能的关键。图 10给出了两种场板器件的负载牵引结果对比,m1m2点分别代表仿真和测试的最大输出功率 (Pout) 值。可以看出,在相同偏置状态、工作频率和输入功率的情况下,双场板器件具有更大的输出功率,而它们的最优负载阻抗点位置有所差别,其归一化幅度偏差达到0.25左右。因此,该模型准确地预测了两种场板器件的最优负载阻抗差异,能为功率放大器匹配电路设计提供较准确的指导。

    为了验证该大信号模型的大信号输出特性,在片测试了两种场板器件的Pout、增益 (Gain) 和功率附加效率 (PAE) 特性。图 11给出了工作状态为AB类、偏置电压Vgs =-2.5 V、Vds = 28 V、输入频率为8 GHz时,输出特性随输入功率变化的仿真与实测结果对比。可以看出,与单栅场板器件相比,调谐在几乎相同PAE的条件下,双场板器件的Pout和Gain均比单场板器件大了约1.4 dB,表明双场板器件具有更大的输出功率密度。仿真与实测结果吻合较好,证明该大信号模型能准确地预测两种场板器件的大信号输出特性。

    本文针对单栅场板和栅-源双场板GaN HEMTs器件,提出了一种带有非线性热网络的电热大信号模型。通过有限元热仿真方法提取了它们的热阻和热容参数,建立了一种非线性热网络,并嵌入到漏源电流经验公式中。利用该大信号模型,准确地分析和预测了两种场板GaN HEMTs的小信号特性、大信号负载阻抗及输出特性的差异。为场板GaN HEMTs器件用于微波功率放大器的电路设计与优化,提供了较为精确的大信号模型。

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