Ku- and K-band GaN High Power Amplifier MMICs

Val Kaper, Scott Harris, Keith KesslerRaytheon, Andover MA 01810

vkaper@raytheon.com

Abstract: This paper describes development and characteri-zation of three Ku- and K-band GaN High Power Amplifier(HPA) MMICs. The circuits are implemented in Raytheon’sproduction-released mm-wave GaN process; they demon-strate state-of-art performance over a frequency range thatspans from 13 to 22 GHz. A low-end Ku-band HPA operatesfrom 13 to 14.5 GHz and delivers 48 Watts of output powerwith 43% PAE. A high-end Ku-band HPA operates from 15.5to 18 GHz and delivers 25 Watts of output power with 45%PAE. A K-band HPA operates from 19.5 to 22 GHz anddelivers 18 Watts of output power with 29% PAE.

Keywords: GaN; High Power Amplifier; MMIC; Ku-band;K-band.

IntroductionOwing to a unique combination of electronic (high electronsaturation velocity and mobility, and high breakdown field)and thermal (high thermal conductivity of SiC substrate)properties, GaN technology has long been recognized as anideal choice for implementation of micro- and mm-waveHPAs [1]. Specifically for various Ku- and K-band commu-nication and radar military applications, GaN HPAs offeran opportunity to provide ≈5x increase in output poweravailable from a single MMIC with similar efficiency andchip size in comparison to existing GaAs-based circuits.

In this paper, we present details of technology, circuit designand characterization of three new Ku- and K-band GaN HPAMMICs.

TechnologyThe three circuits discussed in this paper have been fabricatedusing Raytheon’s production-released mm-wave AlGaN/GaNHEMT on SiC process. This technology has found use innumerous circuit applications spanning the frequency rangefrom C- to Q-bands. A typical transistor exhibits peak extrin-sic transconductance of 410 mS/mm and the open-channelcurrent of 1.1 A/mm. A more detailed description of thetechnology can be found in [2].

Low-end Ku-band HPAA power amplifier with frequency range between 13 and 14.5GHz has been designed, fabricated and characterized. Thecircuit is a two-stage reactively-matched amplifier with anon-chip bias network. A photograph of a fabricated low-endKu-band GaN HPA is shown as an inset in Figure 1.

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Pout(@Pin=33dBm)

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Figure 1. Measured performance of the low-end Ku-bandGaN HPA as a function of frequency. Vdd=26 V, Pulse Width /Duty Cycle =100 µs / 3%, Room Temperature. The inset is aphotograph of a fabricated low-end Ku-band GaN HPA. Chip

size is 4.2 mm x 3.2 mm.

The final transistor stage is sized to produce the requiredamount of output power. The first transistor stage is sizedto provide sufficient signal drive to compress the final stagewhile consuming the least amount of DC power. Outputmatching network (a passive sub-circuit that translates 50Ohm load impedance into a specific impedance at the finalstage FET’s output resulting in optimal combination of FETpower density and drain efficiency) makes use of a noveltopology described in [3]. An on-chip bias regulator circuituses a current mirror topology with saturated mesa resistorcurrent source [4], it sets the gate DC bias voltage for theHPA in a way that the amplifiers’ performance is desensitizedfrom variations in temperature, external power supply andmanufacturing process.

The circuit performance is characterized in a standard fixturewhere a die is attached to a thermal spreader with AuSnsolder, the spreader is placed on a Cu center-block which ismounted to a cold plate. Small-signal gain, saturated outputpower and associated power-added efficiency (PAE) as func-tions of frequency, measured on a representative fixtured low-end Ku-band GaN HPA under pulsed stimulus and nominalDC bias are plotted in Figure 1. The amplifier demonstrates18.7 dB of small-signal gain, 48 Watts of saturated out-put power with associated PAE of 43% (all numbers arefrequency averages over the 13-14.5 GHz band). The peak

Distribution A: Approved for public release; distribution unlimited.

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Figure 2. Measured performance parameters of thelow-end Ku-band GaN HPA as a function of input power.Vdd=26 V, Pulse Width / Duty Cycle =100 µs / 3%, Room

Temperature.

Table 1. Drain supply voltage dependence of the low-endKu-band HPA frequency-averaged performance. Pulse Width

/ Duty Cycle =100 µs / 3%, Room Temperature.

Vdd, Small-signal Gain, Psat, PAE @Psat, Pdc@Psat,V dB Watts % Watts24 18.6 43.1 42.4 96.926 18.7 47.8 43.2 106.028 18.5 52.2 42.6 117.8

saturated output power and PAE in the band are 52.5 Wattsand 45.5% respectively.

The amplifiers’ single-tone measured transfer characteristicsat two frequency points within the band are graphed in Figure2. At 13 GHz, the output power levels at 1 dB compressionand full saturation are 43.3 and 46.3 dBm respectively withassociated PAEs of 31% and 43%. At 14 GHz, the outputpower levels at 1 dB compression and full saturation are 43.7and 47.1 dBm respectively with associated PAEs of 30% and43%.

Measured trends in the dependence of the HPA performanceon the DC drain supply voltage are tabulated in Table 1.Operating the circuit at a lower voltage makes it possible toreduce the prime (DC) power requirement at the expense ofoutput power. Conversely, the output power can be increasedby raising the drain supply voltage at the expense of higherprime power.

High-end Ku-band HPAThe second circuit to be described in this paper is a high-end Ku-band GaN HPA with an operating frequency rangebetween 15.5 and 18 GHz. The circuit is a three-stagereactively-matched amplifier. A photograph of a fabricatedhigh-end Ku-band GaN HPA is shown as an inset in Figure 3.

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Pout(@Pin=23dBm)

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SS Gain(@Pin=10dBm)

SS InpRetLoss(@Pin=10dBm)

Figure 3. Measured performance of the high-end Ku-bandGaN HPA as a function of frequency. Vdd=20 V, Pulse Width /Duty Cycle =10 µs / 10%, Room Temperature. The inset is aphotograph of a fabricated high-end Ku-band GaN HPA. Chip

size is 4.8 mm x 2.9 mm.

This circuit shares many of its attributes with the low-end Ku-band HPA presented above. A lower output power require-ment has led to reduction in the output stage FET size andnominal drain bias voltage. One more gain stage was addedto increase both small- and large-signal gains of the amplifier.

Small-signal gain, saturated output power and associatedPAE as functions of frequency, measured on a representativefixtured high-end Ku-band GaN HPA under pulsed stimulusand nominal DC bias are plotted in Figure 3. The amplifierdemonstrates 27 dB of small-signal gain, 25 Watts of satu-rated output power with associated PAE of 45% (all numbersare frequency averages over the 15.5-18 GHz band). The peaksaturated output power and PAE in the band are 26.9 Wattsand 49% respectively.

The amplifiers’ single-tone measured transfer characteristicsat two frequency points within the band are graphed in Figure4. At 16 GHz, the output power levels at 1 dB compressionand full saturation are 40.4 and 43.8 dBm respectively withassociated PAEs of 31% and 49%. At 17 GHz, the outputpower levels at 1 dB compression and full saturation are 39.9and 44.2 dBm respectively with associated PAEs of 26% and44%.

Measured trends in the dependence of the HPA performanceon the DC drain supply voltage are tabulated in Table 2.Operating the circuit at a lower voltage makes it possible toreduce the prime (DC) power requirement at the expense ofoutput power. Conversely, the output power can be increasedby raising the drain supply voltage at the expense of higherprime power.

K-band HPAThe third circuit to be described in this paper is a K-band GaNHPA with an operating frequency range between 19.5 and 22

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Figure 4. Measured performance parameters of thehigh-end Ku-band GaN HPA as a function of input power.Vdd=20 V, Pulse Width / Duty Cycle =10 µs / 10%, Room

Temperature.

Table 2. Drain supply voltage dependence of the high-endKu-band HPA frequency-averaged performance. Pulse Width

/ Duty Cycle =10 µs / 10%, Room Temperature.

Vdd, Small-signal Gain, Psat, PAE @Psat, Pdc@Psat,V dB Watts % Watts16 24.9 17.3 43 39.820 27 25.1 45.2 55.124 28.4 31 43.3 71.1

GHz. The circuit is a four-stage reactively-matched amplifier.A photograph of a fabricated K-band GaN HPA is shown asan inset in Figure 5.

This circuit shares many of its attributes with the two Ku-bandHPAs presented above. To achieve required performance athigher frequencies, the unit gate width of the output FETstage is scaled approximately inversely proportionally withfrequency.

Small-signal gain, saturated output power and associated PAEas functions of frequency, measured on a representative fix-tured K-band GaN HPA under pulsed stimulus and nominalDC bias are plotted in Figure 5. The amplifier demonstrates35 dB of small-signal gain, 18 Watts of saturated outputpower with associated PAE of 29% (all numbers are fre-quency averages over the 19.5-22 GHz band). The peaksaturated output power and PAE in the band are 20.4 Wattsand 33% respectively.

The amplifier’s single-tone measured transfer characteristicsat two frequency points within the band are graphed in Figure6. At 20 GHz, the output power levels at 1 dB compressionand full saturation are 37.6 and 42.0 dBm respectively withassociated PAEs of 15% and 27%. At 21 GHz, the outputpower levels at 1 dB compression and full saturation are 39.3and 43.1 dBm respectively with associated PAEs of 20% and33%.

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Figure 5. Measured performance of the K-band GaN HPAas a function of frequency. Vdd=20 V, Pulse Width / DutyCycle =10 µs / 10%, Room Temperature. The inset is a

photograph of a K-band GaN HPA. Chip size is 4.8 mm x 2.5mm.

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Figure 6. Measured performance parameters of theK-band GaN HPA as a function of input power. Vdd=20 V,

Pulse Width / Duty Cycle =10 µs / 10%, Room Temperature.

Measured trends in the dependence of the HPA performanceon the DC drain supply voltage are tabulated in Table 3.Operating the circuit at a lower voltage makes it possibleto reduce the prime (DC) power requirement and enhanceefficiency at the expense of output power. Conversely, theoutput power can be increased by raising the drain supplyvoltage at the expense of higher prime power and lowerefficiency.

Sensitivity of the K-band amplifier’s performance to the dutycycle of the DC / RF input stimuli is shown in Figure 7. At onefrequency in the band (20.7 GHz), as the duty cycle increasesfrom 10% to 100% (corresponding to the CW condition), thesaturated output power and efficiency are reduced by 1.2 dB

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Table 3. Drain supply voltage dependence of the K-bandHPA frequency-averaged performance. Pulse Width / Duty

Cycle =10 µs / 10%, Room Temperature.

Vdd, Small-signal Gain, Psat, PAE @Psat, Pdc@Psat,V dB Watts % Watts16 33.3 14.3 33.3 42.820 35.3 17.8 29.3 60.624 35.7 19.5 24.3 80.0

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Figure 7. Measured dependence of the K-band GaN HPA’ssaturated output power and PAE on duty cycle. Vdd=20 V,

Pulse Width =100 µs, frequency = 20.7 GHz, RoomTemperature.

and 4.2% points respectively. This degradation is largely dueto an increase in the output stage FET channel temperature.

ConclusionIn this paper, we presented information on the developmentand characterization of three new Ku- and K-band HighPower Amplifier (HPA) MMICs, implemented in Raytheon’sproduction-released mm-wave GaN technology. The circuitsdemonstrate state-of-art performance over a frequency rangethat spans from 13 to 22 GHz. Comparison of the measuredfrequency-averaged data with relevant published results onKu- and K-band GaN HPAs is tabulated in Table 4.

AcknowledgmentThe Ku-band HPAs have been developed under RaytheonIRAD funding. The K-band HPA has been developed under asub-contract from Nuvotronics,Inc (PI: Steve Huettner) on anAir Force Phase 2 SBIR topic AF103-073. The authors wouldlike to acknowledge Tom Charbonneau for layout support;Erin Bernay, Tony Puliafico, Brian Morrison and Phil Phalonfor circuit characterization.

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Table 4. Comparison with published Ku- and K-band GaNHPAs.

Reference Frequency, Psat, PAE Linear WaveformGHz Watts @Psat,% Gain, dB

Low-end 13-14.5 48 43 19 pulsedKu-band

HPA[5] 13.4-15.5 50 33 31 CW[6] 13.5-14.5 18 38 22 CW[7] 13.75-14.5 40 26 26 pulsed

13.75-14.5 25 20 24 CW[8] 14.8-15.3 62 45 10 pulsed[9] 13.4-16.5 25 30 35 CW

High-end 15.5-18 25 45 27 pulsedKu-band

HPA[10] 18-19 10 30 20 CW

K-band 19.5-22 18 29 35 pulsedHPA 19.5-22 13 24 32 CW[11] 21-24 5 47 14 CW

2. X.Zheng, J.C.Tremblay, S.Huettner, K.P.Ip, T.Papale,K.L.Lange, “Ka-Band High Power GaN SPDT SwitchMMIC,” 2013 IEEE Compound Semiconductors Inte-grated Circuit Symposium, October 2013.

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5. TGA2239-CP: 13.4-15.5 GHz 50WGaN Power Amplifier, Qorvo Datasheet,http://www.triquint.com/products/p/TGA2239-CP.

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7. CMPA1D1E025F: 25W, 13.75-14.5 GHz, 40V, Ku-bandGaN MMIC, Power Amplifier, Wolfspeed Datasheet,http://www.wolfspeed.com/media/downloads/478/CMPA1D1E025F.pdf.

8. K.Yamauchi et al, “A 45% Power Added Efficiency, Ku-band 60W GaN Power Amplifier,” 2011 IEEE MTT-SInternational Microwave Symposium, 2011.

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10. C.Friesicke et al, “A 40 dBm AlGaN/GaN HEMT PowerAmplifier MMIC for SatCom Applications at K-Band,”2016 IEEE MTT-S International Microwave Symposium,2016.

11. C.F.Campbell et al, “A K-Band 5W Doherty AmplifierMMIC Utilizing 0.15µm GaN on SiC HEMT Technol-ogy,” 2012 IEEE Compound Semiconductors IntegratedCircuit Symposium, 2012.

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