X波段AlGaN_GaNHEMTs_栅场板结构研究_英文_

Foundation Items:Project Supported by the S tate Key Development Program for Basic Research of China (2002CB311903);The Key Innovation Program of Chinese Academy of Sciences (KGCX2 SW 107)Study on the X Band Gate Field Plate forAlGaN/GaN HEMTsWang Dongdong,Liu Guoguo,Liu Dan,Li Chengzhan,Liu Xinyu(Institute o f Microelectronics,Chinese Academ y o f Sciences,Beijing 100029,China)Abstract :Conventional T shaped gate and gate field plated structures were demonstrated andcompared to each other from DC,RF and microwave power charac terization.The results show that due to gate field plate,de gradation in the value of unity current gain frequency f T and maximum frequency of oscillation f max are observed,but there is a significant improvement in knee voltage and output power density.Also,0 25 m gate length AlGa N/GaN HEMTs with varying gate field plate lengths of 0 4,0 7,0 9and 1 1 m were fabricated on SiC substrates.With the increase of field plate length within the optimum,output power density is improved drama tically,which is demonstrated by equivalent circuit analysis.At 8G Hz,a continuous wave output power density is 7 11W/m m with 35 31%po wer added efficiency and 10 25dB c orresponding linear gain a t V ds =40V for an 0 25 m 1mm de vic e with field plate length of 0 9 m.Key words :AlGaN/GaN HE MTs; gate field plate;cut off frequenc y;power densityC LC number :TN386 Document code :A Article ID :1003 353X (2008)10 0850 05EEAC C :2560SX 波段AlGaN/GaN HEMTs 栅场板结构研究王冬冬,刘果果,刘丹,李诚瞻,刘新宇(中国科学院微电子研究所,北京100029)摘要:基于SiC 衬底成功研制X 波段0 25 m 栅长带有 栅场板结构的AlGa N/Ga N HE MT,对比T 型栅结构器件,研究了 栅场板引入对器件直流、小信号及微波功率特性的影响。

结果表明, 栅场板结构减小器件截止频率及振荡频率,但明显改善器件膝点电压和输出功率密度。

针对场板长度分别为0 4、0 7、0 9、1 1 m,得出一定范围内增加场板长度,器件输出功率大幅度提高,并结合器件小信号模型提参结果分析原因。

在频率8GHz 下,总栅宽1mm,场板长度0 9 m 的器件,连续波输出功率密度7 11W/mm,功率附加效率(P AE )35 31%,相应线性增益10 25dB 。

关键词:AlGaN/Ga N HE MTs; 栅场板;截止频率;功率密度0 Introducti onAlGaN/Ga N high electron mobility transistors (HE MTs)have attracted considerable interests as power devices in microwave application,promising greater thana tenfold increase in power density as compared with Ga As de vices.Recently,the introduction of field plated structure has resulted in increasing of device breakdown voltage and dramatically enhanced power and linearity performance [1 2].AlGaN/Ga N HE MT grown on SiC substrate with field plates yielded power densities up to 40W/mm at 4GHz,which is believed to be the best po wer perfor mance to date [3].In formal reported field plated devices,the field plate was deposited on the Si N x passivation layer and技术专栏Technology Columnconnected to the source [4]or the gate [5].But for Xband,T shaped gate technique with 0 25 m gate footprint is generally applied to minimize the gate resistance.Here a structure combined field plate structure and T shaped gate is presented,which also leads to the simplification of the device design and fabrication.In this work,an X band gate field plated AlGaN/GaN HE MT structure was demonstrated.On the same wafer,conventional T shaped gate devices were also fabricated for direct comparison.With the ne w gate field plates,both DC and power performance were found to be dramatically improved.At 8GHz,saturated power density (P sa t )of 7 11W/mm with peak PAE of 41 89%with associated linear gain of 10 25dB at V ds =40V were achieved on gate field plated parison of DC,frequency and large signal power performance of gate field plated to T shaped gate was given.Power performance of devices with varying field plated length was discussed.1 D evice Structures and F abri cationFig.1sho ws two different device cross sections.The device struc tures were grown on SiC substrates by MOCVD.The epi layer c onsists of a 3 5 m Ga N buffer layer,a 110nm high mobility Ga N layer,a 1nm undoped AlN insert layer,a 23nm undoped Al 0 22Ga 0 78N barrier layer,and a 10nm undoped GaN cap layer.Average sheet resistance is about 380 /as measuredby Leightonmeasurements.(a)AlGaN/GaN HEMT wi th T s hapedgate(b)AlGa N/GaN HEM T with gate field plateFig.1 Schematic cross section of AlGaN/GaN HE MT s 图1 AlGaN/GaN HEMTs 器件结构示意图Both devices were fabricated by the sa me process.De vice fabrication started with ohmic c ontacts,which were formed by rapid thermal annealing of E beam evaporated Ti/Al/Ti/Au at 850!for 50s in N 2ing on wafer transfer length measurement (TLM)patterns,the specific contact resistivity (SCR)of ohmic contact of T shaped gates was 2 53 l0-6∀cm 2,while (2 24-2 58) l0-6∀cm 2was measured for field plated structure.Device isolation was implemented by N ion implantation.Next,100nm SiN passivation layer was deposited by PEC VD.Recessed gate with depth of 7nm was formed by ICP etch.T shaped gates and gate (Ni/Au)with varying field plated length were fabricated using E beam lithography.An overlay metallization on the ohmic c ontacts and measurement pads wasalso depositedafter gatemetallization.Total gate width and length on all the devices tested were 10 m 100 m and 0 25 m,respectively.Gate source and source drain spacing were 1 2 m and 4 m,respectively.For T shaped gate,the total width of head was 0 55 m.Field plate lengths,with respect to the gate drain edge,were 0 4,0 7,0 9and 1 1 m toward the drain.2 R esul ts and Discussions2 1 D C C harac teristicsOn wafer DC measure ments were performed using aHP4155A semiconductor parameter analyzer.The offWang Dongdon g et al:Study on the X Band Gate Field Plate for AlGaN/GaN HEMTsstate gate drain destructive breakdown voltage was changed significantly,increasing from 67V for T shaped gate to 100V for gate field plated structure as sho wn in Fig.2(a).This improve ment in breakdown voltage is due to the depletion layer formed under the field plate resulting in the reduction of the electric field strength at the gate edge toward drain [6].As shown in Fig.2(b),the breakdown voltage stops increasing with field plated length L fp beyond 0 9 m because the field distribution decays beyond the field plate edge.With increasing L fp ,the electric field waveforms at gate edge and fp edge become independent from overlapping,and finally the shape of electric field distribution almost remains unchanged,which indica tes that there is an optimum L fp of 0 9 m.Within the optimum L fp ,the breakdo wn volta ge was improved with the increase of L fp .All the discussions below are based on the optimum L fp.(a)Schottky characteristics(b)Breakdo wn voltage with different L fpFig.2 Breakdown characteristics of both T shaped and gatefield plated 0 9 m structures图2 T 型栅结构和 栅场板长度0 9 m 结构击穿特性T ypical drain c urrent volta ge(I ds -V ds )characteristics for gate field plated devices with L fp of 0 9 m and T shaped gate are sho wn in Fig 3(a).B oth gates were biased from -4to 2V in a step of 1V.As in Fig 3(a),the gate field plated HE MT e xhibits ma ximum drain current density (I max )as high a s 790mA/mm at a ga tebias of 2V and a drain bias of 11V,while I ma x of 725m A/m m measure d for T shaped gate.The knee voltages are less than 5V for gate field plate but 7 8V for T shaped ga te.The significa nt reduc tion in knee voltage clearly points out the bene fits of the ga te field plated structure.T wo possible mechanisms involved in the field plated ope ra tion are reduction of electric field at the ga te edge re sulting in a lower electron injection in surface states traps and/or modula tion of surface sta te traps in the gate drain access region [7].The DC transfe r charac teristic s are sho wn in Fig 3(b).The drain bia ses at 7V.A peak extrinsic transconduc tance (g m )of 361mS/mm for ga te field plated struc ture measured at V gs =1V.Also,T shaped gate e xhibits g m =358mS/m m.The threshold voltage of both devices is a bout -1 5V.The tra nsfer characte ristics are almost identical for both types ofdevices.(a)Drain current vol tage characteristics(b)DC transfer characteristicsFig 3 DC characteristics of T shaped and gate field platedstructures with field plate length of 0 9 m图3 T 型栅结构和 栅场板长度0 9 m 结构直流特性Wang Dongdong et al:Study on the X Band Gate Field Plate for AlGaN/GaN HE MTs2 2 R F Small Signal CharacteristicsOn wafer RF measurements were carried out withHP8510C network analyzer at 1to 40GHz.S para meters for both T shaped and gate field plated structures were measured at optimum bias points for maximum unity current gain cut off frequency (f T )and oscillation frequency.The effects of pad parasitics were de e mbedded by conventional Y parameter subtraction.Fig 4sho ws the measured short circuit current gain |h 21|and maximum available power gain (MAG )derived fro m S pa ra meter measurements as a function of frequency.The value of unity gain cut off freque ncy (f T )and f max of oscillation are determined by e xtrapolation of the |h 21|and unilateral gain at -20dB/dec,respectively.Peak f T decreases from 33 5to 28GHz,and peak f max increases from 48to 39GHz,for devices with T shaped gate and gate field plated,respectively.This is attributed to an increase in the additional feedback capacitance associated with gate [8],which degrades small signalcharacteristics.Fig 4 Short circuit current gain |h 21|and max i mum availablepower gain of both T shaped gate and gate field plated structures with L fp =0 9 m图4 T 型栅结构和 栅场板长度0 9 m AlGaN/GaNHE MTs 器件频率特性2 3 Power Characteristi c sLarge signal C W measure ments at 8GHz were performed using a focus microwave automatic load pull system.Fig 5de monstrates that,at 38V drain bias, gate field plated device yields saturated output power density (P sat )of 7 1W/mm with associated power added efficiency (P AE )of 35 3%and corresponding linear gain (G L )of 10 2dB.Whereas for the T shaped gate,P sat is 4 9W/mm with associated PAE of 30%and G L of 12 31dB.The lower P sat and P AE due to the large knee walk out voltage is the main limitation of devices with T shaped gate.However,G L of gate isabout 2dB lower than that of T shaped gate.Because the feedback capacitance associates with the field plate resulted in relatively low gain characteristics for gate field plateddevice.Fig 5 CW microwave power characterization of both gatefield plate with L fp of 0 9 m and T shaped gate AlGaN/GaN HE MTs图5 T 型栅结构和 栅场板长度0 9 m 结构AlGaN/GaN HE MT s微波功率特性Fig.6 CW microwave power performance as a function offield plate length L fp at drain bias voltage of 40V图6 AlGaN/GaN HE MTs 器件微波功率特性与场板长度L fp 的关系(V ds =40V)Moreover,it must be stressed that with the inc rease of field plated length L fp within the optimum of 0 9 m,the large signal power performance of gate field plated was improved significantly.The improve ment in breakdown voltage with increase of L fp was reflec ted in the po wer performance of the devices.At a drain bias of 40V,power densities of 5 65,6 38,7 11W/mm were measured for devices with gate field plated length of 0 4,0 7,0 9 m,respectively,as shown in Fig 6.This was due to the slight reduction of the gate resistance on the one hand.Ho wever,with the inc reaseWang Dongdon g et al:Study on the X Band Gate Field Plate for AlGaN/GaN HEMTsof L fp,lo wer linear gain,large signal gain and PAE were observed due to the increase of additional capacitance such as C gd and C gs.These were demonstrated by small signal model para meters e xtrac t.To examine the gain characteristics of both gate field plate and T shaped gate,the small signal parameters were extrac ted by GaN HE MT small signal model that includes22elements[9].The main parasitic parameter as well as intrinsic parame ters of devices are shown in Tab. 1.At drain bias of20V,compared to T shaped gate,with the field plate length increasing from 0to0 9 m,C g d increases from0 08to1 02pF and gate resistance reduces from5 52to3 15.Equivalent circuit analysis de monstrated that the introduction of gate field plate resulted in additional feedback capacitance,and reduced gate resistance which improves output power to some extent.With the increase of the field plated length,the feedback capacitance became larger,which degraded gain characteristics according to Miller effects.Tab 1 Parasitic parameters of the four kinds of devices a t V ds=20V表1 源漏偏压V ds=20V四种结构器件模型参数Normal(T gate)A(L fp=0 4 m)B(L fp=0 7 m)C(L fp=0 9 m) R g/5 523 83 493 15R d/22 352 422 45R s/5 567 36 36 36R i/1 250 810 840 64R gd/22 5819 1625 33116 23C gs/pF0 310 580 791 21C gd/pF0 080 250 591 02C ds/pF0 020 030 050 05Fig 7compares the output power performance as a function of V ds of both devices with L fp of0 9 m and T shaped gate.G L for the gate field plate structure varies from6 9to10 25dB with V ds increasing from20to 40V,while G L is nearly unchanged,about12to 12 31dB.Thus,the gain loss due to the additional gate feedback capacitance associated with the field plated is considerably compensated by increasing the drain bias volta ge.This is due to drain depletion layer e xtended toward the drain electrode with a high drain voltage, resulting in the reduction of parasitic capacitance[10].At the same time,measured P sat for gate increased as V ds increased up to45V,whereas the voltage for T shaped gate was sweeping up to less than40V and the device burned out during the measure ment,sho wing that this volta ge sweeping range was beyond the safe limit.But PAE of both devices were slightly decreased due to the increasing DC power dissipation.The influence of gate field plated structure on the feedback capacitance became less significant and eventually achieves better power performance under high voltageconditions.Fi g 7 C W microwave power performance at di fferent d rain bias voltage for both T shaped gate and gate field plates tructures图7 T型栅结构和 栅场板长度0 9 m结构器件微波功率特性与源漏偏压V ds的关系(下转第922页)Wang Dongdong et al:Study on the X Band Gate Field Plate for AlGaN/GaN HE MTs的应答器和非接触IC卡的原理与应用[M].第二版.陈大才译.北京:电子工业出版社,2002:27 97.[2]TSAI C C,DAI S B,LEE W T,et al.The RF circuit design formagnetic power and data 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[3]ALLE N P E,HOLBERG D R.C MOS模拟集成电路设计[M].第二版.冯军,李智群,译.北京:电子工业出版社, 2005:604 612.[4]RAZAVI B.模拟C MOS集成电路设计[M].陈贵灿译.西安:西安交大出版社,2003:221 232.(收稿日期:2008 0507)作者简介:杨彪(1983#),男,湖北人,研究生,研究方向为数模混合集成电路设计;田曲平(1984#),女,研究生,研究方向为英语翻译;应建华(1954#)男,四川人,副教授,目前主要从事数模混合集成电路的科研与教学工作。