self-heating and trapping effects on the rf performance of gan mesfets

IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 52, NO. 4, APRIL 2004 1229

Self-Heating and Trapping Effects on theRF Performance of GaN MESFETs

Syed S. Islam, Member, IEEE, and A. F. M. Anwar, Senior Member, IEEE

Abstract—RF power performances of GaN MESFETs in-corporating self-heating and trapping effects are reported. Aphysics-based large-signal model is used, which includes tem-perature dependences of transport and trapping parameters.Current collapse and dc-to-RF dispersion of output resistanceand transconductance due to traps have been accounted for in theformulation. Calculated dc and pulsed I–V characteristics are inexcellent agreement with the measured data. At 2 GHz, calculatedmaximum output power of a 0.3 m 100 m GaN MESFET is22.8 dBm at the power gain of 6.1 dB and power-added efficiencyof 28.5% are in excellent agreement with the corresponding mea-sured values of 23 dBm, 5.8 dB, and 27.5%, respectively. Betterthermal stability is observed for longer gate-length devices due tolower dissipation power density. At 2 GHz, gain compressions dueto self-heating are 2.2, 1.9, and 0.75 dB for 0.30 m 100 m,0.50 m 100 m, and 0.75 m 100 m GaN MESFETs, re-spectively. Significant increase in gain compression due to thermaleffects is reported at elevated frequencies. At 2-GHz and 10-dBmoutput power, calculated third-order intermodulations (IM3s) of0.30 m 100 m, 0.50 m 100 m, and 0.75 m 100 mGaN MESFETs are 61, 54, and 45 dBc, respectively. Forthe same devices, the IM3 increases by 9, 6, and 3 dBc due toself-heating effects, respectively. Due to self-heating effects, theoutput referred third-order intercept point decreases by 4 dBmin a 0.30 m 100 m device.

Index Terms—Current collapse, gain compression, GaN,intermodulation, large-signal model, MESFETs, self-heatingeffects, Volterra series.

I. INTRODUCTION

RADIO-FREQUENCY (RF) power amplifiers are keyelements for applications in phased-array radar and

wireless base stations. Currently, power transistors basedupon Si–LDMOS technology operating around 1-W/mmpower density are being used. In recent years, GaN-baseddevices have demonstrated superiority over Si–LDMOSin a wide variety of high-power, high-frequency, andhigh-temperature applications. The high peak and satura-tion velocity, as well as low parasitics, have resulted inof 107 GHz [0.15 m 100 m GaN/Al Ga N highelectron-mobility transistor (HEMT)] [1] and of 155 GHz(0.12 m 100 m GaN/Al Ga N HEMT) [2]. Uniquematerial properties of GaN such as high thermal conductivity

Manuscript received August 15, 2003; revised December 30, 2003.S. S. Islam is with the Department of Electrical Engineering, Rochester

Institute of Technology, Rochester, NY 14623-5603 USA (e-mail:[email protected]).

A. F. M. Anwar is with the Department of Electrical and ComputerEngineering, University of Connecticut, Storrs, CT 06269-2157 USA (e-mail:[email protected]).

Digital Object Identifier 10.1109/TMTT.2004.825662

and breakdown voltage have enabled these devices to beoperated up to 11.8 W/mm at 10 GHz using 0.3 m 100 mAlGaN–GaN HEMT [3]. Power amplifiers using GaN HEMTshave been demonstrated with output powers of 14 W at 8 GHz[4] and 36 W at 2.2 GHz [5]. The enhanced thermal stability ofGaN has also resulted in HEMTs operating up to 750 C [6].

GaN-based devices operated at high power and hightemperatures suffer from self-heating effects. Using Ramanspectroscopy, Kuball et al. [7] estimated the temperature at thegate–drain opening to be 180 C of a 4 m 200 m GaNHFET grown on Sapphire with 20 and 0 V applied on thedrain and gate, respectively. For the same bias voltages, thetemperature increase was 120 C for a 1 m 200 m GaNHFET grown on SiC. Nuttinck et al. [8] have reported thesimulation results of temperature distribution under continuousand pulsed conditions. For a 250- m-wide GaN heterostruc-ture field-effect transistor (HFET) grown on SiC, maximumtemperature of 96 C was reported in the active channel with acontinuous wave dissipation power density of 5 W/mm, which,under pulsed condition, decreased to 76 C. With sapphire asthe substrate, the maximum temperature was as high as 300 C.Using dc–I–V measurements, a decrease of maximum powerdissipation capability from 5.5 W at 150 K to 1.67 W at 293 Kfor the same device grown on SiC was also reported [8]. Inanother effort, Nuttinck et al. [9] reported approximately 25%reduction in drain current as the drain pulsewidth increasedfrom 1% to 100% at 300 K for the 250- m-wide GaN HFETgrown on SiC. The gate bias and peak drain voltage were 0and 35 V, respectively. Similar I–V characteristics are reportedunder both pulsed and continuous conditions by reducing thedevice temperature to 65 K.

The self-heating effects at a large drain bias increase the de-vice lattice temperature and reduce physical parameters such asmobility and carrier saturation velocity by increasing the car-rier phonon scattering. As a result, device parameters such astransconductance and output resistance become dependent uponthe temperature and applied electric field [10], [11]. Besides,GaN-based devices are plagued by traps that result in currentcollapse in the I–V characteristics and dc-to-RF dispersion oftransconductance and output resistance [12]–[15]. The recoveryof current collapse depends on the detrapping time constant,which determines the dispersion frequency of the transconduc-tance and output resistance. The dispersion frequency due toshallow traps in GaN increases exponentially with temperatureand can be of the order of megahertz at 600 K [16]. Therefore,an exact analysis of GaN-based devices should incorporate athermal simulation technique to calculate physical and trappingparameters at the operating temperature.

0018-9480/04$20.00 © 2004 IEEE

1230 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 52, NO. 4, APRIL 2004

TABLE ITEMPERATURE AND FIELD DEPENDENCES OF MOBILITY: COEFFICIENTS OF (2) (� IN cm /V � S, T IN K, AND E IN kV/cm) [10], [11]

Recently, an empirical GaN–AlGaN HEMT model includingthe thermal effect [8], [9] and physics-based analysis of tem-perature and size dependences of transport parameters [10],[11] have been reported. The current authors have also reporteda physics-based model to incorporate trapping effects in GaNMESFETs [16], [22]. In this paper, RF power performancesof GaN-based MESFETs have been reported, incorporatingboth trapping and thermal effects. Junction temperature iscalculated by solving Laplace’s equation. The temperatureand electric-field dependences of transport parameters areobtained by using an ensemble Monte Carlo simulation. Modelparameters are calculated by using a physics-based large-signalanalysis. RF power performances are calculated by using ageneral analysis based on the Volterra series.

II. ANALYSIS

The drain current of a GaN-based MESFET considering ve-locity saturation and the depletion layers formed in the channelat the channel/buffer interface due to the trapped carriers can beexpressed as follows [17]:

(1)

where

andis the occupied trap concentration at the n-GaNchannel/semi-insulating (SI)–GaN buffer interface, is theintrinsic carrier concentration, is the GaN dielectric constant,

is the electronic charge, is the Boltzmann constant,is the electron mobility at temperature and electric

field is the thickness of the channel, is the width ofthe device, is the gate length, is the saturation factor,and is the channel electron concentration. equals thesummation of the doping concentration in the channel andthe carriers recovered due to detrapping. is obtained byequating drain current expressions before and after saturationat .

At high electric fields, velocity saturation occurs andits magnitude decreases with increasing temperature [10],

[11]. Moreover, in short structures, transport becomes non-stationary, which makes mobility both electric-field andtemperature dependent [10], [11]. In this analysis, car-rier velocity–electric-field characteristic is assumed as

, where is theelectron velocity, is the criticalelectric field, is the saturation electron velocity, andis the low-field mobility. Differential mobility at a given tem-perature and electric field is given by[10], [11]. In this analysis, the temperature and electric-fielddependences of mobility and temperature dependence of satu-ration velocity and critical electrical field as obtained from anensemble Monte Carlo simulation [10], [11] are expressed as

(2)

(3)

and

(4)

The constants required for the determination ofare tabulated in Table I. and are estimated as

and for the determination ofin centimeters/second (3) and in kilovolts/centimeter (4),where is in kelvins [10].

Fig. 1 shows the circuit model to include the effects of car-rier trapping on frequency dispersion of output resistance andtransconductance. Following the treatment reported by Golio etal. [18], the effects of traps are incorporated through the pa-rameters and . These parameters are obtained oncethe underlying physical processes governing trap dynamics areformulated [16]. Intrinsic circuit parametersand are obtained from conventional small-signal MESFETanalysis in the absence of traps.

A. Determination of Intrinsic Model Parameters

In the saturation mode of operation, the channel canbe divided into a linear or field-dependent mobility re-gion length and saturation-velocity-limited regionlength . This is incorporated in (1) by replacing by

and by , wherewith being the potential in the channel at . The I–V

ISLAM AND ANWAR: SELF-HEATING AND TRAPPING EFFECTS ON RF PERFORMANCE OF GaN MESFETs 1231

Fig. 1. GaN MESFET. (a) Cross-sectional view. (b) Large-signalmodel. Linear model parameters are R = 6 ; L = 0:055 nH,R = 70 ; L = 0:307 nH, R = 90 ; L = 0:027 nH, andC = 0:040 pF [14], [18], [21].

relationship in the absence of trapping effects can be obtainedfrom (1) by dropping the last term in theregion and replacing by . The intrinsic transconduc-tance is determined in the absence of trapping effectsand is given by (5), shown at the bottom of this page,where and

.

and are calculated by simultaneously solving

and, which are obtained by equating drain

currents at .Intrinsic drain resistance is given by (6), shown at the

bottom of this page, whereand

. Capacitances and are given by

(7)

(8)

where is the drain-to-gate voltage. Intrinsicresistance is given by where,

and isthe gate–drain separation [19]. is the turn-off voltage.

B. Determination of Trap-Related Circuit Parameters

The trapping sub-network parameters are given by [16]

(9)

(10)

and

(11)

(5)

(6)


Fig. 2. Thermal simulation. (a) Channel temperature variation as a function ofdissipated power. (b) Analysis steps.

where is the detrapping time constant and. The 0.3 m 100 m GaN MESFET reported

by Gaquiere et al. [20] is used for simulation in this paper.The device has an active layer thickness nm, channeldoping concentration cm , and source–drainseparation m. Using cm [13],the constants of are estimated as V,

V , and V . The turn-offvoltage is estimated to be 8 V. The other circuit parame-ters are obtained from reported experimental data and are as fol-lows: nH, nH,

nH, and pF [14]–[21].The channel temperature is determined by solving Laplace’s

equation for a given power dissipation. A plot of calculatedchannel temperature as a function of dissipated power for the0.30 m 100 m GaN MESFET [20] is shown in Fig. 2(a).The sapphire substrate is assumed to be at room temperature.The calculation proceeds with an initial guess of channel tem-perature to estimate mobility, carrier velocity and critical elec-tric field for a given channel electric field [see Fig. 2(b)]. Es-timated mobility, carrier velocity, and critical electric field areused to compute drain current, power dissipation, and corre-

Fig. 3. Calculated (solid/dashed lines) and measured (symbols) I–Vcharacteristics for 0.3 �m� 100 �m [24] GaN MESFETs in the presence oflight.

sponding temperature. The calculation is repeated until a consis-tent solution for drain current, mobility, carrier velocity, criticalelectric field, and temperature is obtained. The detrapping timeconstant is then calculated by [16]

(12)

where is the electron capture cross section,is the thermal velocity, is the electron ef-

fective mass in GaN, and are the conduction andtrap energy levels, respectively, and is the effective den-sity of states for the electron in the conduction band. Themodel parameters are calculated using (5)–(11). At low fre-quency, the overall intrinsic transconductance equals ,whereas for frequencies greater than the dispersion frequency,

approaches . For fre-quencies lower than , the overall outputresistance approaches and becomesfor frequencies greater than [22]. For large-signalanalysis, , and are considered nonlinear functionsof , while is a nonlinear function of . The nonlinearfunctions are approximated up to the second-order term

, where represents andor and represents or for Volterra-series analysisof output power, power gain, and intermodulation for a giveninput power [22], [23].

III. RESULTS AND DISCUSSION

Fig. 3 shows the measured [24] and calculated dc and pulsedI–V characteristics. In the presence of light, the dc I–V measure-


Fig. 4. Variation of output power as a function of input power for L�100�mGaN MESFET at 2 GHz. Measured results are shown by circles forL = 0:3�m[20].

ments do not show any current collapse as the electrons capturedby traps located at the channel–buffer interface and the surfacetraps located in the channel between the gate–drain region havesufficient time and energy to be released. With pulsed input sig-nals, electrons captured by the buffer traps form a depletion re-gion in the channel at the channel–buffer interface. Besides, sur-face traps also capture electrons and form a virtual gate [25],which causes drain current to decrease for a given drain bias.The effect of surface traps is incorporated by considering an ad-ditional negative gate potential due to the trapped electrons inthe region between the gate and drain. The reduction in draincurrent due to the presence of surface states can be recoveredby SiN passivation. For an unpassivated structure [24], an esti-mation of the virtual gate potential is obtained by subtracting theactual gate voltage from the adjusted gate voltage in (1) requiredto obtain the dc saturation drain current. With pulse inputs and

V, virtual gate voltage is estimated as 0.75 V.The presence of buffer traps leads to an early pinchoff due

to the depletion region at the channel–buffer interface resultingin current collapse, which is partially recovered as drain bias isincreased [17]. For given gate and drain biases, drain current iscalculated using (1) by following the steps shown in Fig. 2. Theintrinsic drain voltage is obtained by subtracting from theexternally applied drain bias the voltage drops across and

. The calculated results are compared with experimental datato show good agreement.

Fig. 4 shows the variation of output power as a func-tion of input power at 2 GHz for 0.30 m 100 m,0.50 m 100 m, and 0.75 m 100 m GaN MESFETs.The bias voltages are V and V. The power

Fig. 5. Variation of power gain as a function of input power for L� 100 �mGaN MESFET at 2 GHz. Measured results are shown by circles forL = 0:3�m[20].

results have been obtained for a gate-to-source voltage higherthan that would be required to bias the device in class-A mode[20]. Measured data for m, which were obtainedby using an optimum match for the output power load andinput return loss, are plotted to show close agreement [20].The agreement between the experimental data and calculatedresults are obtained when the thermal effects are taken intoaccount. It is observed that, for a given channel electric fieldand input power, the shorter gate-length devices operate athigher temperatures, which is attributed to the existence ofhigher power densities in shorter gate-length devices dueto their reduced channel area. At elevated temperatures, theeffective mobilities of shorter gate-length devices are greatlyreduced from their room-temperature values. Due to highermobility and saturation velocity, calculated output power atisothermal condition (300 K) is higher than that obtained bytaking thermal effects into account. For a given input power,the difference between the calculated output power with andwithout taking thermal effects into account is minimum for the0.75- m gate-length device as its junction temperature deviatesthe least from ambient temperature. The difference increaseswith decreasing gate length due to higher operating temper-atures. A similar result is obtained for the calculated powergain as shown in Fig. 5. Isothermal power gain of the 0.3- mgate-length device is 9.5 dB. With thermal effects taking intoaccount, the calculated power gain is reduced by 2.2 dB. For0.50- and 0.75- m gate-length devices, the differences are 1.9and 0.75 dB, respectively. In this same figure, the measureddata for m is plotted to show excellent agreement[20].


Fig. 6. Variation of power gain as a function of gate length for an output powerof 18 dBm at 2 GHz.

Fig. 6 shows the variation of calculated power gain as a func-tion of gate length for an output power level of 18 dBm at2 GHz. As observed, at a given output power level, the gain com-pression due to thermal effects decreases with increasing gatelength. This is due to the lower operating temperature of longergate-length devices. Fig. 7 shows the calculated power-added ef-ficiency (PAE) as a function of input power with gate length asa parameter. Similar to the output power and power gain, PAEswith thermal effects taken into account are lower than those ob-tained under isothermal condition and can be correlated to thereduced mobility and saturation velocity at elevated tempera-tures.

Fig. 8 shows the variation of third-order intermodulation(IM3) as a function of output power with fundamental toneat 2 GHz. Better IM3 is obtained for shorter gate-lengthdevices due to reduced nonlinearities resulting from highertransconductance and lower capacitances. A similar result wasreported for shorter gate-length AlGaN–GaN HEMTs [26].At elevated temperatures, transconductance decreases due tolower mobility and saturation velocity and device nonlinearityincreases, resulting in higher IM3 for a given output powerlevel. At a higher output power level, nonlinearity increases,resulting in even higher IM3 due to thermal effects. A sig-nificant increase in IM3 was obtained when the fundamentaltone frequency was increased to 4 GHz [27]. This increase inIM3 is attributed to the increased leakage through capacitancesat elevated frequencies. Output referred third-order interceptpoint (OIP3), which is defined as the output power at whichthe IM3 component of the output power cross the fundamentalcomponent, as a function of gate length is shown in the insetof Fig. 8. With increasing gate length, OIP3 decreases due to

Fig. 7. Variation of PAE as a function of input power for L � 100 �m GaNMESFET at 2 GHz. Measured results are shown by circles for L = 0:3 �m[20].

Fig. 8. Variation of IM3 as a function of input power with gate length as aparameter at 2 GHz. The inset shows the OIP3 as a function of gate length.

an increase in nonlinearity, however, the self-heating effect isreduced due to lower operating temperature.


Fig. 9. Variation of power gain as a function of input power with operatingfrequency as a parameter for gate lengths. (a) L = 0:3 �m. (b) L = 0:75 �m.Room-temperature results are shown by dashed lines and thermal effects areincorporated in the results shown by solid lines.

Fig. 9 shows the variation of calculated power gain as a func-tion of input power with the operating frequency as a parameter.As observed, for the 0.3- m gate-length device, gain compres-sion due to thermal effects is 2.2 dB at 2 GHz, which increasesto 4 dB at 6 GHz. The 0.75- m gate-length device demonstrateda similar increase in gain compression due to thermal effects atelevated frequencies. This frequency dependence of gain com-pression is due to the higher power dissipation resulting fromreduced power gain for a given input power at increased oper-ating frequency.

IV. CONCLUSION

RF power performances of GaN MESFETs have been ana-lyzed considering trapping and thermal effects. A physics-basedmodel is used to correlate the performance of the device as afunction of device size and operating frequency. Short channeldevices demonstrated higher output power, power gain, andPAE, however, better thermal stability is obtained in longchannel devices. Gain compressions due to thermal effects arefound to be frequency dependent and increase significantlywith increasing frequency. Lower IM3 and higher OIP3 areobserved in shorter gate-length devices, which are degradedsignificantly due to self-heating effects.

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Syed S. Islam (S’00–M’02) received the B.S.degree in electrical and electronic engineeringfrom the Bangladesh University of Engineeringand Technology (BUET), Dhaka, Bangladesh, in1993, the M.S. degree in electrical engineering fromthe University of Saskatchewan, Saskatoon, SK,Canada, in 2000, and the Ph.D. degree in electricalengineering from the University of Connecticut,Storrs, in 2002.

He is currently an Assistant Professor with the De-partment of Electrical Engineering at Rochester In-

stitute of Technology (RIT), Rochester, NY. His research interests include mod-eling of semiconductor devices, microwave circuit design, and numerical tech-niques.

Dr. Islam is a member of the IEEE Microwave Theory and Techniques Society(IEEE MTT-S) and IEEE Electron Devices Society.

A. F. M. Anwar (S’86–M’88–SM’00) received theB.S. and M.S. degrees in electrical and electronicengineering from the Bangladesh University ofEngineering and Technology (BUET), Dhaka,Bangladesh, in 1982 and 1984, respectively, and thePh.D. degree from Clarkson University, Potsdam,NY, in 1988.

He is currently a Professor with the Departmentof Electrical and Computer Engineering, Universityof Connecticut, Storrs. His research group, in the RFMicroelectronics and Noise Laboratory, is currently

involved in the study of transport in short heterostructures and antimony basedHEMTs operating above 300 GHz. Moreover, the group is involved in mod-eling GaN-based high-power HEMTs and HBTs. He is also active in researchin the areas of CMOS-based class-E amplifiers, as well as transport dynamicsand noise in resonant tunneling diodes (RTDs) and one-dimensional (1-D) struc-tures.

Dr. Anwar is an editor for the IEEE TRANSACTIONS ON ELECTRON DEVICES.

self-heating and trapping effects on the rf performance of gan mesfets

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