GaN-Based RF Power Devices and Amplifiers
Gallium nitride power transistors can operate at millimeter wave and beyond to meet future needs of cell phones,satellites,and TV broadcasting.
By Umesh K.Mishra,Fellow IEEE,Likun Shen,Thomas E.Kazior,and Yi-Feng Wu
ABSTRACT|The rapid development of the RF power electronics requires the introduction of wide bandgap mate-rial due to its potential in high output power density,high operation voltage and high input impedance.GaN-based RF power devices have made substantial progresses in the last decade.This paper attempts to review the latest develop-ments of the GaN HEMT technologies,including material growth,processing technologies,device epitaxial structures and MMIC designs,to achieve the state-of-the-art microwave and millimeter-wave performance.The reliability and manu-facturing challenges are also discussed.
KEYWORDS|Gallium nitride;High Electron Mobility Transistors (HEMTs);microwave transistors;millimeter wave transistors; MMICs;reliability
I.INTRODUCTION
With the recent upsurge of the wireless communication market,as well as the steady but continuous progress of traditional military applications,microwave transistors are playing critical roles in many aspects of human activities. The requirements for the performance of microwave transistors are becoming more and more demanding.In the personal mobile communication applications,next generation cell phones require wider bandwidth and improved efficiency.The development of satellite com-munications and TV broadcasting requires amplifiers operating at higher frequencies(from C band to Ku band,further to Ka band)and higher power to reduce the antenna size of terminal users.The same requirement holds for broadband wireless internet connections as well because of the ever increasing speed or data transmission rate.Because of these needs,there has been significant investment in the development of high performance microwave transistors and amplifiers based on Si/SiGe, GaAs,SiC and GaN.Table1lists the major parameters of these materials and the Johnson’s figure of merit(JM) calculated to compare the power-frequency limits of different materials[1].The JM gives the power-frequency limit based solely on material properties and can be used to compare different materials for high frequency and high power applications.
The requirement for high power and high frequency requires transistors based on semiconductor ma
terials with both large breakdown voltage and high electron velocity. From this point of view,wide bandgap materials,like GaN and SiC,with higher JM are preferable.The wide bandgap results in higher breakdown voltages because the ultimate breakdown field is the field required for band-to-band impact ionization.Moreover,both have high electron saturation velocities,which allow high frequency opera-tion.The ability of GaN to form heterojunctions makes it superior compared to SiC,in spite of having similar breakdown fields and saturation electron velocities.GaN can be used to fabricate high electron mobility transistors (HEMTs)whereas SiC can only be used to fabricate metal semiconductor field effect transistors(MESFETs).The advantages of the HEMT include its high carrier concen-tration and its higher electron mobility due to reduced ionized impurity scattering.The combination of high carrier concentration and high electron mobility results in a high current density and a low channel resistance,which are especially important for high frequency operation and power switching applications.编织袋折
边器 From the amplifier point of view,GaN-based HEMTs have many advantages over existing production
Manuscript received February5,2007;revised August22,2007.
U.K.Mishra and L.Shen are with the Department of Electrical and Computer
Engineering,University of California,Santa Barbara,CA93106USA
(e-mail:mishra@ece.ucsb.edu;lkshen@ece.ucsb.edu).
移动除湿机T.E.Kazior is with the Raytheon RF Components,Andover,MA01810USA
(e-mail:Thomas_E_Kazior@raytheon).
Y.-F.Wu is with the Santa Barbara Technology Center,CREE Inc.,Goleta,
CA93117USA(e-mail:yifeng_wu@cree).
Digital Object Identifier:10.1109/JPROC.2007.911060
Vol.96,No.2,February2008|Proceedings of the IEEE287 0018-9219/$25.00Ó2007IEEE
technologies (e.g.GaAs)[2].The high output power density allows the fabrication of much smaller size devices with the same output power.Higher impedance due to the smaller size allows for easier and lower loss matching in amplifiers.The operation at high voltage due to its high breakdown electric field not only reduces the need for voltage conversion,but also provides the potential to obtain high e
fficiency,which is a critical parameter for amplifiers.The wide bandgap also enables it to operate at high temperatures.At the same time,the HEMT offers better noise performance than that of MESFET’s.
These attractive features in amplifier applications enabled by the superior semiconductor properties make the GaN-based HEMT a very promising candidate for microwave power applications.
In this article we discuss the key components of GaN HEMT technology.In Section II we review growth of high purity device layers by metal organic chemical vapor deposition (MOCVD)and molecular beam epitaxy (MBE).In Section III we present device engineering and processing technologies that are being developed to realize state-of-the-art GaN HEMT performance.The reliability and manufacturing challenges are also discussed.In Section IV,we highlight some of the GaN HEMT hybrid amplifiers and monolithic microwave integrated circuit (MMIC)that have recently been achieved.
II.GaN EPITAXIAL LAYER GROWTH
Numerous teams have been developing the MOCVD and MBE techniques for growth of group-III nitride materials such as GaN,AlN,AlGaN,and InGaN [3]–[8].In the MOCVD process,Ga,Al,and In are
supplied using corresponding metal organic compounds,usually tri-methylgallium,trimethylaluminum and timethylindium.The metal-organic compounds are then transported by a carrier gas,most often hydrogen.Thereby the concentra-tion of the compound in the carrier gas is determined by its vapor pressure.The most commonly used nitrogen source is ammonia.In the RF-MBE technique reactive nitrogen atoms and molecules are produced by passing a nitrogen flow (N 2gas)through a plasma discharge.A variant of this process uses ammonia ðNH 3Þas the nitrogen source gas
[8].The column III growth fluxes are provided by evaporation of high purity elemental sources.The growth efforts of both techniques have been focused on developing high power microwave and millimeter-wave AlGaN/GaN HEMT structures.SiC has been extensively employed as substrates due to its excellent thermal conductivity [9],while sapphire and Si are also used because of the low cost [10],[11].Device isolation from the SiC and Si substrate is provided by a resistive AlN nucleation layer,in which the growth conditions are adjusted to prevent silicon out diffusion [12].
Excellent material quality has been achieved for GaN HEMT films.The impurity concentrations in semi-insulating GaN films are below the detection limit when characterized by SIMS.AlGaN/GaN,AlN/GaN [13],GaN/AlN/GaN [14]and AlGaN/AlN/GaN [15]heterostructures with smooth and abrupt interfaces have been demonstrat-ed,leading to the formation of 2DEGs with elect
ron mobilities as high as 2000cm 2=Vs at room temperature [16].Non-uniformites of G 2%on 4-inch diameter SiC substrates are routinely achieved (for example,see Fig.1(a)V a sheet resistivity map of a GaN DHFET (Double Heterostructure Field-Effect Transistors)[17].Mercury probe capacitance-voltage (C-V)measure-ments of AlGaN/GaN HEMT structures grown on semi-insulating SiC substrates reveal high quality material.The C-V profile exhibits a sharp pinch-off and extremely low,flat capacitance at high reverse bias (equal to the capacitance of the SiC substrate)indicative of negligible GaN buffer and epi/SiC interface charge/doping [as shown in Fig.1(b)][18].
Both MOCVD and MBE techniques are capable of
growing thin layers.The use of a thin,$10A
˚,AlN interlayer between the AlGaN barrier and GaN channel has been demonstrated to reduce sheet resistance by increasing the mobility and sheet density of the HEMT structure [15].The increase in mobility is attributed to the reduction in alloy scattering and the increase in sheet charge due to the larger conduction band discontinuity at the AlGaN/GaN interface.Fig.2is an x-ray spectrum of a
250A
˚Al 0:26Ga 0:74N =10A ˚AlN/GaN HEMT grown on a SiC substrate.The presence of the thin,AlN layer enhances the strength of the Pendellosung oscillations.
Table 1Material Properties Related to the Power Performance at High Frequencies for Various
Materials
Mishra et al.:GaN-Based RF Power Devices and Amplifiers
Proceedings of the IEEE |Vol.96,No.2,February 2008
(The Pendellosung oscillations are a measure of the quality (flatness and abruptness)of the hetero-in
terface.)The AlN interlayer lowered the sheet resistance from 400to 285ohm/sq.and the mobility was increased to greater than 2000cm 2=Vs.
Using the MOCVD and MBE techniques,growers have demonstrated more complex device structures similar to GaAs pHEMTs,such as quantum well or double hetero-junction (DH)FETs.Some of these devices operate up to W-band frequencies.The quantum well or DH structures provide improved electron confinement to mitigate short channel effects associated with smaller gate lengths as well
as better substrate isolation resulting in higher gain devices and improved device efficiency.AlGaN buffer layers [19]and InGaN backside barrier layers [20]–[22]have been used to create conduction band discontinuities (double quantum wells similar to GaAs pHEMTs and InP HEMTs)that inhibit the injection of electrons into the buffer layer.Improved channel confinement/buffer iso-lation and reduced buffer leakage current by Fe,Be,or C doping of the GaN buffer layer (similar to fully depleted buried p-layers commonly used in GaAs MESFETs and Si nMOS devices)has also been demonstrated [23]–[26].Finally,highly doped cap layers are being added to the epi structure to reduce device access (source)resistance,which results in increased device gain and efficiency [19],[27].
III.ADVANCED DEVICE DESIGNS AND PROCESSING TECHNOLOGIES
While several electronic devices have been investigated (for example,HBTs [28],MESFETs [29],MISFETs [30],HEMTs [31]),most of the research work has been focused on HEMTs [including MOSHEMT [32](Metal-oxide-semiconductor HEMT)],because HEMTs have better carrier transport properties than MESFETs and the difficulty of p-doping in GaN impedes the development of bipolar transistors.A typical AlGaN/GaN HEMT is shown in Fig.3.
The polarization doping effect in GaN HEMTs was predicted by Bykhovski et al.[33].The first observation of a Two-Dimensional Electron Gas (2DEG)with a carrier concentration of the order of 1011cm À2and a room temperature mobility of 400–800cm 2=Vs in an AlGaN/GaN heterojunction was reported in 1992[31].The
first
Fig.1.(a)Sheet resistance map and (b)capacitance-voltage plot for GaN HEMT grown on a 4-inch SiC
substrate.
DC performance of AlGaN/GaN HEMT was shown in 1993with the saturation drain current of 40mA/mm [34].First RF power data of 1.1W/mm at 2GHz for an AlGaN/GaN HEMT was demonstrated in 1996[35].In the early stage of the development of the GaN devices,many AlGaN/GaN HEMTs suffered a discrepancy between the predicted output power from static I-V curves and load pull measurements of output power,referred to as B DC-to-RF dispersion.[As seen in Fig.4,current collapse occurs in the pulsed I-V measurement.It is believed to be a trap-related phenomenon where both surface and bulk traps contribute [36],[37].The existence of the dispersion has severely limited the microwave output power of GaN HEMTs,until two innovations were proposed to overcome this problem.One was the introduction of the Si x N passivation in 2000[38],[39],which effectively reduced DC-to-RF dispersion caused by surface trap states,thereby resulting in a significant increase in output power to 9and 11W/mm [40],[41].Another was the adoption of the field plate in 2003[10],[42].In addition to the traditional function of the field plate to increase the breakdown
voltage,it also reduced the dispersion beyond what Si x N passivation offered.Since then,the output power density has further increased with the help of steadily improved growth techniques,material qu钢筋保护层塑料垫块
alities,enhanced proces-sing technologies and more optimum device designs.The latest record for power density is over 40W/mm at 4GHz [43].
The trend of the GaN-based device is towards higher output power density,higher Power-Added-Efficiency (PAE),higher operation frequencies and improved reli-ability.In order to achieve these requirements,novel device designs and processing technologies are being developed.Recently,much progress has been made and will be discussed below.The first subsections focus on improvements to the performance of microwave transis-tors.The last subsection addresses the unique challenges of optimizing the device for millimeter wave applications.
A.Field-Plated GaN HEMTs
Implementing a field plate on a dielectric layer at the drain side of the GaN HEMTs has resulted in some of the most significant and exciting improvements [10],[42],[43].The performance and tradeoffs of the field plate (FP)configurations have been investigated in an attempt to extract the best gain and power characteristics.
Gate Connected FP (GC-FP):Fig.5(a)shows the cross section of a gate-connected field-plated GaN HEMT.The function of a FP is to modify the electric field profile and to decrease its peak value,hence
reducing trapping effect and increasing breakdown voltages.Initial FPs were either constructed as part of the gate or tied to the gate externally.This has been effective in improving large signal (or power)performance and enabling high voltage operation as seen in Fig.6(a)and (b)[44].Up to a certain value,the longer the FP,the more output power was achieved.
However,in this configuration the capacitance be-tween the FP and drain becomes gate-to-drain capacitance ðC gd Þ,resulting in negative Miller feedback.This causes reduction in current-gain and power-gain cutoff frequen-cies ðf t =f max Þas seen in Fig.7.
Source-Connected FP (SC-FP):A close look into the device operation reveals that,since the voltage swing across the gate and source is only 4–8V for a typical GaN HEMT,much less than the dynamic output swing up to 230V,terminating the FP to the source [shown in Fig.5(b)]also satisfies the electrostatics for it to be functional.In this configuration,the FP-to-channel capacitance becomes the drain-source capacitance,which could be absorbed in the output-tuning network.The drawback of additional C gd by the FP is hence is eliminated.Depending on the implementation,the source-connected field plate can add parasitic capacitance to the device input.However,this can also be
absorbed
Fig.3.A schematic of a typical AlGaN/GaN
祛痘除皱美白面膜素
HEMT.
Fig.4.DC and pulsed I-V characteristics of an unpassivated AlGaN/GaN
HEMT on SiC substrate.Obvious current collapse (dispersion)could be observed in the pulsed mode.
Mishra et al.:GaN-Based RF Power Devices and Amplifiers
290Proceedings of the IEEE |Vol.96,No.2,February 2008
into the input tuning circuit,at least for narrow band applications.
SC-FP,GC-FP and non-FP Devices were fabricated on the same wafer for a direct evaluation.Compared to the non-FP device,the reveres power transferðS12Þof the device with GC-FP increased by71%at4GHz,while that of the device with SC-FP actually reduced by28%.The reduction in S12for the latter is attributed to the Faraday shielding effect by the grounded field plate.As a result, at10V drain bias and4GHz the SC-FP device exhibited a maximum-stable-gain(MSG)1.3-dB higher than the non-FP device and5.2dB higher than the GC-FP device. As a result,the SC-FP devices shows a significant(95dB at4GHz)improvement in maximum stable gain,This advantage for SC-FP devices was maintained for biases from10though60V as seen in Fig.
8(a).Fig.8(b)lists the change of the capacitance components in GC-and SC-FP devices,respectively.
Large-signal performance was characterized by load-pull power measurement at4GHz.Both the GC-FP and the SC-PF devices outperformed the non-FP devices in both output power and PAE at48V and above,while the SC-FP device consistently delivered large-signal gains 5–7dB higher than that of the GC-FP device.
As successful high-voltage designs,both FP devices were able to operate at118V dc bias as shown in Fig.9, where tuning was optimized for the best combination of gain,power-added-efficiency(PAE)and output power
at Fig.5.Cross section of a GaN HEMT with(a)gate-connected field plate;(b)source-connected field
plate.
Fig.6.(a)Power density vs.drain voltage for various FP lengths.Device dimension:0:5Â246 m2.(b)Power performance of a
GaN HEMTs with gate-connected field plates,showing32.2W/mm output power at120V drain bias.
Mishra et al.:GaN-Based RF Power Devices and Amplifiers
Vol.96,No.2,February2008|Proceedings of the IEEE291
3-dB compressionðP3dBÞ.While both devices generate power densities around20W/mm,the SC-FP device distinguishes itself by7-dB higher associated gain.With the achieved large-signal gain of21dB at4GHz and the estimated voltage swing of224V,the voltage-frequency-gain product(Johnson’s voltage-frequency figure of merit [1])for the SC-FP is approaching10kV-GHz,the highest ever shown for any semiconductor device.
The above studies were for operation at C-band and below.For applications at X-band and above,dimensions for the field plates need to be reduced accordingly to manage the parasitic capacitances.
B.Deep-Recessed GaN HEMTs
SiN x passivation has been used to reduce the disper-sion,but reproducibility of breakdown voltage,gate leakage,and effectiveness of dispersion elimination is strongly process related.Recently,solutions to the dispersion problem had been addressed at the epitaxial level[45],[46].One of these approaches,which has made substantial progress,is the deep-recessed GaN HEMT using a thick cap layer to eliminate dispersion[47],as shown in Fig.10.
真空泵叶片The effect of the surface to the channel is inversely proportional to the distance between surface and channel. The thick AlGaN or GaN cap layers in the deep-recessed HEMTs increase the surface-to-channel distance,the dispersion caused by surface traps is therefore reduced or eliminated without surface passivation because now only a smaller portion of the channel charge is affected compared to the conventional AlGaN/GaN HEMTs.The graded AlGaN layer is Si-doped to compensate the negative polarization charge and prevent hole accumulation.
The processing flow was similar to that of the standard HEMT except for the deep ohmic and gate recess.A fluorine plasma treatment of the recessed surface before gate metallization was found to be very effective to reduce the gate leakage(up to two orders of magnitude)and increase breakdown voltage(9200V)[48].A record output power density P out of more than17W/mm with an associated power added efficiency(PAE)of50%was measured at V DS¼80V at4GHz(without SiN x passivation as shown in Fig.11).This is believed to be the highest power generated from a GaN transistor with-out surface passivation to date.At lower bias of30V, an excellent PAE of74%with output power density of 5.5W/mm was achieved.
In order to control the recess depth accurately and improve the manufacturability,a selective dry etch tech-nology of GaN over AlGaN using BCl3=SF6has been developed[49].The presence of fluorine d
ecreases the etch rate of AlGaN due to the formation of a
non-volatile Fig.7.f t=f max as functions of FP length L f
.
Fig.8.(a)MSG as a function of drain voltage;(b)change of the capacitance components in GC-and SC-FP devices.
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