JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.16, NO.3, JUNE, 2016 ISSN(Print) 1598-1657 http://dx.doi.org/10.5573/JSTS.2016.16.3.339 ISSN(Online) 2233-4866

Manuscript received Nov. 9, 2015; accepted Dec. 22, 2015 Department of Electronic and Electrical Engineering, Sungkyunkwan University, Suwon 440-746, Korea E-mail : cspark@wavetc.com

2.6 GHz GaN-HEMT Power Amplifier MMIC for LTE Small-Cell Applications

Wonseob Lim, Hwiseob Lee, Hyunuk Kang, Wooseok Lee, Kang-Yoon Lee, Keum Cheol Hwang,

Youngoo Yang, and Cheon-Seok Park

Abstract—This paper presents a two-stage power amplifier MMIC using a 0.4 μm GaN-HEMT process. The two-stage structure provides high gain and compact circuit size using an integrated inter-stage matching network. The size and loss of the inter-stage matching network can be reduced by including bond wires as part of the matching network. The two-stage power amplifier MMIC was fabricated with a chip size of 2.0×1.9 mm2 and was mounted on a 4×4 QFN carrier for evaluation. Using a downlink LTE signal with a PAPR of 6.5 dB and a channel bandwidth of 10 MHz for the 2.6 GHz band, the power amplifier MMIC exhibited a gain of 30 dB, a drain efficiency of 32%, and an ACLR of -31.4 dBc at an average output power of 36 dBm. Using two power amplifier MMICs for the carrier and peaking amplifiers, a Doherty power amplifier was designed and implemented. At a 6 dB back-off output power level of 39 dBm, a gain of 24.7 dB and a drain efficiency of 43.5% were achieved. Index Terms—Power amplifier, MMIC, GaN-HEMT, Doherty power amplifier, LTE small cell

I. INTRODUCTION

One of the recent technology developments is the small-cell base stations that massively increase mobile data traffic. The small-cells provide low-power access points that can cover a short range from ten to several

hundred meters [1-4]. The power amplifier (PA) for these small-cell base stations must have very good performance in its output power, linearity, and efficiency, which are critical to the overall competitiveness of base transceiver systems. The first step in designing a PA for small-cell base stations is the selection of device technology.

The electrical properties of four representative semiconductor technologies for the PA design are compared in Table 1. The gallium nitride (GaN) exhibits a substantially higher breakdown field strength than silicon (Si) or gallium arsenide (GaAs) due to its wide band gap. This ensures high breakdown voltage of the device. The thermal conductivity of GaN is three times higher than that of GaAs, which is very good for high power handling with eased cooling requirements. It also has several more advantages, such as high speed, high power density, and high efficiency [5-8].

For these reasons, gallium nitride high electron

Table 1. Material properties of various semiconductors

Si GaAs SiC GaN Energy gap

(eV) 1.11 1.43 3.20 3.40

Critical electric field (MV/cm) 0.6 0.5 3.0 3.5

Charge density (#×1×1013/cm2) 0.3 0.3 0.4 1.0

Thermal conductivity (W/cm/K)

1.5 0.5 4.9 1.5

Mobility (cm2/V/s) 1,300 6,000 600 1,500

Saturation velocity

(×107cm/s) 1.0 1.3 2.0 2.7

340 WONSEOB LIM et al : 2.6 GHZ GAN-HEMT POWER AMPLIFIER MMIC FOR LTE SMALL-CELL APPLICATIONS

mobility transistors (GaN-HEMTs) have been used in high power applications at microwave frequency bands. For most cases, the PAs are designed as hybrid integrated circuits (HICs) using individually packaged transistors. However, HICs are generally bulky, so that further integration is required for small cell applications.

In this paper, a two-stage PA monolithic microwave integrated circuit (MMIC) having an average output power of 4 W was designed using a 0.4 μm GaN-HEMT process for the 2.6 GHz long-term evolution (LTE) band. The two-stage MMIC structure has the advantages of high gain and a compact inter-stage matching network. The integrated inter-stage matching network has a very small size and low loss as it includes bond wires as part of the matching elements.

Two types of evaluation boards were designed: one uses a single PA MMIC for the single-ended PA and the other uses two PA MMICs to further increase the efficiency of the Doherty power amplifier (DPA). For both cases, the PA MMICs were mounted on 4×4 quad-flat no-leads (QFN) packages. Both the single-ended PA and the DPA were evaluated using a 2.6 GHz downlink LTE signal with a peak-to-average power ratio (PAPR) of 6.5 dB and a channel bandwidth of 10 MHz.

II. MMIC DESIGN

Fig. 1 shows a schematic diagram of the proposed GaN-HEMT two-stage PA MMIC and off-chip in/out matching networks. To deliver an average output power

of 4 Watt for the downlink LTE signal, the total gate widths of 1,600 μm for the 1st-stage and 5,760 μm for the 2nd-stage were used to have optimized efficiency and linearity. Off-chip matching networks, which are based on transmission lines and surface-mountable capacitors, are located at the input and output. DC blocking capacitors, bias circuits, and R-C stabilization circuits are added to the circuit.

For the two-stage PA, the inter-stage matching network should be carefully designed [9-11]. The matching network transforms the input impedance of the 2nd-stage to the load impedance of the 1st-stage. Fig. 2 presents a schematic diagram of the inter-stage matching network used in this work. A shunt inductor is used, which includes a bond wire for the smaller size and low loss since the bond wire works as a high-Q inductor. This shunt inductor is also used to feed the VDD to the 1st-stage.

A 1 Ω series resistor is also employed to ensure stability of the circuit. To ensure accurate impedance matching, the on-chip matching network was designed using a 2.5D electromagnetic (EM) simulation tool of Momentum from Keysight’s Advanced Design System.

RFOUT

(50 Ω)

VGS1

RFIN

(50 Ω)

VGS2VDD

VDD

Second harmonic control

10 Ω

LB4

LB3

RSTB

LB2

CIM

Packaged MMIC

Off-chip M/N Off-chip M/N

12 pF

5 Ω 5 Ω 19 nH2.2 pF

0.8 pF2.8 pF LB1

Fig. 1. Schematic diagram of the two-stage PA MMIC using GaN-HEMT technology.

RSTBLB2

CIM

Second harmonic control

CIM

Fig. 2. Schematic diagram of the inter-stage matching network.

JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.16, NO.3, JUNE, 2016 341

An on-chip second harmonic short circuit is inserted at the input of the 2nd-stage to improve the efficiency. A series LC resonant circuit is applied in parallel to the input of the 2nd-stage. It gives a short circuit to the second harmonics and a capacitance to the fundamental frequency.

Fig. 3 shows simulated stability factors for the overall circuit including the package. The series resistor at the inter-stage is tuned while checking these stability factors through the broad frequency range. The inductances of the bond wires were accurately predicted using EM field simulation. Fig. 4 shows the simulated inductances of the bond wires from the schematic shown in Fig. 1. At 2.6 GHz, the simulated inductances are obtained as LB1 of 0.81 nH, LB2 and LB3 of 0.53 nH, and LB4 of 0.32 nH. Theses extracted inductances of the bond wires were applied to the circuit simulation for the overall PA MMIC design.

III. IMPLEMENTATION AND MEASUREMENT

RESULTS

Fig. 5(a) shows a chip microphotograph of the fabricated two-stage PA MMIC on a QFN package and Fig. 5(b) shows a photograph of the evaluation board. The chip was fabricated using Cree’s 0.4 µm GaN-HEMT process with a size of 2.0×1.9 mm2. A QFN package, with a size of 4.0×4.0 mm2 was used for the chip packaging. The overall circuit was implemented on a printed circuit board (PCB) using Roger’s RO4350B substrate. The implemented PA was biased with a drain voltage of 28 V and the quiescent currents are 45 mA and 55 mA for the 1st-stage and 2nd-stage, respectively.

Fig. 6 shows the simulated and measured perfor-

0 2 4 6 80

2

4

6

8

10

K factor Mu factor

Stab

ility

fact

or

Frequency (GHz)

Fig. 3. Simulated stability factors for the overall circuits.

0 2 4 6 80.0

0.2

0.4

0.6

0.8

1.0

Indu

ctan

ce (n

H)

Frequency (GHz)

LB1 LB2,LB3 LB4

Fig. 4. Simulated inductances of the bond wires.

LB3

LB3

LB4

LB2

LB1

Second harmonic control

(a)

RFOUT

RFIN

VDDVDD

VGS2VGS1

(b)

Fig. 5. Photographs of the implemented two-stage PA (a) MMIC on a package, (b) evaluation board.

342 WONSEOB LIM et al : 2.6 GHZ GAN-HEMT POWER AMPLIFIER MMIC FOR LTE SMALL-CELL APPLICATIONS

mances of the designed two-stage PA MMIC on a QFN package for the 2.6 GHz continuous wave (CW) signal. The implemented PA exhibited a high gain of above 33.0 dB, a peak output power of 42.5 dBm, and a drain efficiency (DE) of 56.0%. Minimal differences are

observed between the simulation and measurements. The measured performances of the implemented GaN-

HEMT two-stage PA MMIC for the downlink LTE signal with a channel bandwidth of 10 MHz and PAPR of 6.5 dB are shown in Fig. 7. Gain, DE, adjacent-channel leakage power ratio (ACLR), and power spectral density (PSD) are presented in Fig. 7(a) and (b). At an average output power of 36 dBm, the two-stage PA MMIC exhibited a gain of 30.0 dB, a DE of 32%, and an ACLR of -31.4 dBc.

The implemented PA MMIC was also used to implement the DPA. Fig. 8 shows a photograph of the implemented DPA using two power amplifier MMICs for the carrier and peaking amplifiers. At the input, a commercial power splitter and offset line are used for power splitting and a proper phase compensation. A quarter-wave line is deployed to obtain a load impedance modulation on the carrier amplifier.

Fig. 9 shows the measured gain and DE of the implemented DPA. A peak output power of 45.0 dBm was obtained. At a 6 dB output back-off from the peak output power, a gain of 24.7 dB and a DE of 43.5% were achieved for the 2.6 GHz CW signal. The measured performances of the GaN-HEMT two-stage PA MMIC in this work are summarized in Table 2 and are compared to those in the previously published works. Very high efficiency with high gain and small form factor was demonstrated in this work.

20 25 30 35 40 450

10

20

30

40

50

60G

ain

(dB

)

Output Power (dBm)

Simulated gain Measured gain

0

15

30

45

60

75

Simulated DE Measured DE

DE

(%)

Fig. 6. Simulated and measured performances of the designed two-stage PA for CW signal.

20 25 30 35 400

10

20

30

40

50

Gain DE

Gai

n (d

B) &

DE

(%)

Average output power (dBm)

-50

-40

-30

-20

-10

0

ACLR (lower) ACLR (upper)

AC

LR (d

Bc)

(a)

2.58 2.59 2.60 2.61 2.62-40

-20

0

20

PSD

(dB

m/3

0 kH

z)

Frequency (GHz)

-31.4 dBc

(b)

Fig. 7. Measured performances using the LTE modulated signal: (a) Gain, DE, and ACLR, (b) PSD at an average output power of 36 dBm.

RFIN RFOUT

VDD,C VDD,C

VDD,P VDD,P

VGS2,PVGS1,P

VGS2,CVGS1,CC

P

Fig. 8. Photograph of the DPA using the fabricated PA MMICs for the carrier and peaking amplifier.

JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.16, NO.3, JUNE, 2016 343

IV. CONCLUSIONS

In this paper, a GaN-HEMT two-stage PA MMIC was designed and implemented for the 2.6 GHz LTE band with an average output power of 36 dBm. By including the bond wires in the inter-stage matching network, the size and loss of the circuit were reduced. An on-chip second harmonic control circuit using a series resonance is employed in parallel with the input of the 2nd-stage for the second harmonic short to improve the efficiency.

The implemented PA MMIC was evaluated using a 2.6 GHz downlink LTE signal with a PAPR of 6.5 dB, a channel bandwidth of 10 MHz and a VDD of 28 V. The PA MMIC delivered a power gain of 30 dB, a DE of 32.0%, and an ACLR of -31.4 dBc at an average output power of 36.0 dBm.

In addition, a DPA was designed using two PA MMICs as a carrier and peaking amplifiers. The implemented DPA exhibited a gain of 24.7 dB and a DE of 43.5% at a 6 dB output back-off from the peak output power of 45 dBm. The implemented GaN-HEMT two-stage PA MMIC can

be used for driver amplifiers or even in the final stages for macro-cell or small-cell base stations.

ACKNOWLEDGMENTS

This work was supported by the Technology Innovation Program (10045892) funded By the Ministry of Trade, industry & Energy (MI, Korea).

REFERENCES

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20 25 30 35 40 45 500

10

20

30

40

50

60

Gain DE

Gai

n (d

B) &

DE

(%)

Output power (dBm)

Fig. 9. Measured performances of the DPA for CW signal.

Table 2. Performance comparison with the previous works

Ref. Freq. (GHz) Technology VDD

(V) Gain (dB)

PAVG / back-off

(dBm / dB)

DEAVG (%)

ACLR (dBc) Package Size

(mm2)

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[13] 2.65 GaN-HEMT 50.0 19.0* 29.0* / 13.5 13.5* N/A DFN 7.2×6.6

[14] 2.60 GaN-HEMT 40.0 19.0* 37.0* / 6.5 33.0* N/A QFN 4.0×3.0

GaN-HEMT 28.0 30.0 36.0 / 6.5 32.0** -31.4 QFN 4.0×4.0 This work 2.60 GaN-HEMT

DPA*** 28.0 24.7 45.0 / 6 43.5** N/A QFN / module 52.5×57.0

* approximated from the graphs, ** including DC power consumption of the 1st-stage, *** CW excitation

344 WONSEOB LIM et al : 2.6 GHZ GAN-HEMT POWER AMPLIFIER MMIC FOR LTE SMALL-CELL APPLICATIONS

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Wonseob Lim was born in Guri, Korea, in 1987. He received the B.S. degree in the department of electronic and communication engineering from Hanyang University, Ansan, Korea in 2012. He is currently working toward the Ph. D. degree in the department

of electronic and electrical engineering from Sungkyun- kwan University. His current research interests include CMOS power amplifier, high efficiency power amplifier, digital filter and mixed signal circuit design.

Hwiseob Lee was born in Incheon, Korea, in 1986. He received the B.S. degree in the department of electronic and communication engineering from Hanyang University, Ansan, Korea in 2012 and the M.S. degree in

the department of electronic and electrical engineering from Sungkyunkwan University, Suwon, Korea in 2014. He is currently working toward the Ph.D. degree in the department of electronic and electrical engineering from Sungkyunkwan University. His current research interests include high-efficiency RF power amplifiers, broadband passive circuit optimization, and wireless power transfer.

Hyunuk Kang received the B.S. degree in electronic engineering from Mokpo National Maritime University, Mokpo Korea, in 2014. He is currently working toward the M.S degree in the department of electronic and electrical engineering from

Sungkyunkwan University. His current research interests include RF power amplifier design and linearity and efficiency improvement techniques.

Wooseok Lee received the B.S. degree in electronic engineering from Chungnam University, Daejeon, Korea in 2014. He is currently working toward the M. S. degree in the department of electronic and electrical engineering from Sungkyun-

kwan University. His current research interests include high efficiency RF power amplifier, broadband power amplifier and passive circuit.

JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.16, NO.3, JUNE, 2016 345

Kang-Yoon Lee received the B.S., M.S. and Ph.D. degrees in the School of Electrical Engineering from Seoul National University, Seoul, Korea, in 1996, 1998, and 2003, respectively. From 2003 to 2005, he was with GCT Semiconductor Inc., San Jose,

CA, where he was a Manager of the Analog Division and worked on the design of CMOS frequency synthesizer for CDMA/PCS/PDC and single-chip CMOS RF chip sets for W-CDMA, WLAN, and PHS. From 2005 to 2011, he was with the Department of Electronics Engineering, Konkuk University as an Associate Professor. Since 2012, he has been with College of Information and Communi- cation Engineering, Sungkyunkwan University, where he is currently an Associate Professor. His research interests include implementation of power integrated circuits, CMOS RF transceiver, analog integrated circuits, and analog/digital mixed-mode VLSI system design.

Keum Cheol Hwang received his B.S. degree in electronics engi- neering from Pusan National University, Busan, South Korea in 2001 and M.S. and Ph.D. degrees in electrical and electronic engineering from Korea Advanced Institute of

Science and Technology (KAIST), Daejeon, South Korea in 2003 and 2006, respectively. From 2006 to 2008, he was a Senior Research Engineer at the Samsung Thales, Yongin, South Korea, where he was involved with the development of various antennas including multiband fractal antennas for communication systems and Cassegrain reflector antenna and slotted waveguide arrays for tracking radars. He was an Associate Professor in the Division of Electronics and Electrical Engineering, Dongguk University, Seoul, South Korea from 2008 to 2014. In 2015, he joined the Department of Electronic and Electrical Engineering, Sungkyunkwan University, Suwon, South Korea, where he is now an Associate Professor. His research interests include advanced electromagnetic scattering and radiation theory and applications, design of multi-band/broadband antennas and radar antennas, and optimization algorithms for electromagnetic applications. Prof. Hwang is a lifemember of KIEES, a senior member of IEEE and a member of IEICE.

Youngoo Yang was born in Hamyang, Korea, in 1969. He received the Ph.D. degree in electrical and electronic engineering from the Pohang University of Science and Technology (Postech), Pohang, Korea, in 2002. From 2002

to 2005, he was with Skyworks Solutions Inc., Newbury Park, CA, where he designed power amplifiers for various cellular handsets. Since March 2005, he has been with the School of Information and Communication Engineering, Sungkyunkwan University, Suwon, Korea, where he is currently an associate professor. His research interests include power amplifier design, RF transmitters, RFIC design, integrated circuit design for RFID/USN systems, and modeling of high power amplifiers or devices.

Cheon-Seok Park was born in Seoul, Korea, in 1960. He received the B.S. degree in electrical engineering from Seoul National University, Seoul, Korea, in 1988 and the M.S. and Ph.D. degrees in electrical and electronic engineering from the

Korea Advanced Institute of Science and Technology, Daejeon, Korea, in 1990 and 1995, respectively. He is currently a Professor with the School of Information and Communication Engineering, Sungkyunkwan University, Suwon, Korea. His research interests include design of radio-frequency power amplifiers, linearization techniques, and efficiency enhancement techniques.