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High Power and High Efficiency Ka Band Power Amplifier
Salah Din, Mike Wojtowicz, Mansoor Siddiqui
Northrop Grumman Corporation
Abstract — A 36 W Ka-band MMIC power amplifier using 0.2
um gate GaN HEMT technology is presented. The power was
measured across 27 to 30 GHz with a minimum 30% PAE. A
peak power of 40 W at 27 GHz was demonstrated. The MMIC
area is a compact 13.5 mm^2 and uses 10.67 mm device
periphery in the output stage. This demonstration is a significant
improvement in performance over current Ka-band MMIC
amplifiers.
(a)
Index Terms — Gallium Nitride, MMIC, PAE, Power Density,
power amplifiers.
I. INTRODUCTION
Ka band is in use for many applications including point to
point communications, Satellite uplink terminals, and
Electronic Warfare. The most demanding requirements in
terms of performance and cost of these platforms are imposed
on the transmitter MMICs. Good power added efficiency
(PAE), output power (Pout), die size (power density) as well
as reliability of these MMICs directly impact these
requirements. Recent published results citing a steady progress
in these parameters at Ka band are cited [1-6]. These use GaN
as well as GaAs. Described in this paper is the design and
measured results of a 2-stage Power Amplifier using the
Northrop Grumman Aerospace Systems (NGAS) 0.2 um GaN
HEMT technology that establishes a new standard of
performance achieved by a single MMIC.
II. GAN PROCESS OVERVIEW
The NGAS AlGaN/GaN GAN20 process was used for this
design. The AlGaN/GaN HEMT epitaxial layers were grown
on 100mm 4H-SiC substrates. Typical 2 dimensional electron
gas (2DEG) densities and room temperature mobilities are
1.1x10^13 cm^-2 and 1400 cm^2/V-s respectively. Devices
are fabricated with a 0.2 um T-gate, a 2 um source to drain
spacing and SiN passivation. Peak transconductance,
calculated from the DC transfer curve, and the cutoff
frequency (fT) extracted from s-parameters are 300 mS/mm
and 65 GHz respectively. Maximum drain current (I max) is 1
A/mm and typical three terminal breakdown is >90 V.
Maximum operating drain bias of 28 volts resulting in power
densities (Pout/total gate periphery) up to 4 W/mm. Wafers
are thinned to 100 um, through substrate vias formed and the
(b)
4X 8f660
16X 8f660
Fig. 1. (a) Microphotograph of Ka band PA.(b) Schematic of Ka
band PA.
backside plated with 3 um Au. The backside metal is designed
for compatibility with eutectic bonding.
III. CIRCUIT DESIGN AND ANALYSIS
The design goals for this amplifier were maximum die area
of 14 mm^2, Pout >30 Watts with greater than 28% PAE from
26 to 31 GHz. The die area constraints drove the selection of
topology as well as device selection and layout. This circuit’s
junction temperature was also allowed to accommodate nonspace use (terrestrial and airborne). The design started with
device selection and loadpull analysis. The output loss was
978-1-4799-8275-2/15/$31.00 ©2015 IEEE
30
Small Signal Gain (dB)
25
Gain, IRL, ORL (dB)
20
Stability Factor
15
s21(dB)
10
s11(dB)
5
s22(dB)
0
S11
-5
-10
Output RL (dB)
-15
S22
-20
18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
Frequency (GHz)
Input RL(dB)
Pout (dBm)
Gain (dB)
Fig.3. Simulated large signal performance at 29.5 GHz; Po>45.9
dBm and PAE~38% at 2.5 dB compression.
initially estimated to be 0.6 dB loss. With a power density of
~3.4 W/mm (design for PAE), the output stage device
periphery was selected to be 10.67 mm. Another constraint to
selecting the unit device size was a need to achieve > 9.5 dB
gain at 30 GHz. Based on the power and gain, we selected a
binary corporate topology combining 16, 8f660 um (8x82.5
um wide gate fingers) devices. The device model was derived
by scaling an available 8f600 um model.
Next a large signal load pull contour plot confirmed the
initial assumptions of Pout ranging from 3.4-4 W/mm with
PAE ~48-49% at 30 GHz at the 660um unit cell level. This
was with a drain of 28 volts and a class A/B bias Idq of 200
mA/mm. The next task was to determine the stage2 to stage1
drive ratio. PAE was a driving requirement so a 4:1 ratio was
adopted. This large ratio usually negatively impacts the
AM/PM distortion unless special care is taken in the design.
The output matching circuitry also had to accommodate the
drain current and so the metallization was increased to insure
low conductive loss. Band pass/low pass structures were used
to match the output impedance. The inter-stage, as always,
Pout (dBm)
Fig.2. Simulated Small Signal response of Amplifier
48
47
46
45
44
43
42
41
40
39
38
37
36
35
34
42
41
40
39
38
37
36
35
34
33
32
31
30
29
28
Pout
PAE
26
27
28
29
Max PAE%
Fig. 4. Small Signal performance measured on-wafer
30
Frequency (GHz)
Fig. 5. Max Pout and Max PAE variation over Freq. measured on
wafer.
was the most difficult part of the design since mutual coupling
was extensive in this dense MMIC. Low frequency
terminations were implemented at the input and output which
aided the suppression provided by the bias networks. ADS
was used in the design and Sonnet EM simulation software
was used extensively. The final schematic and micro
photograph are shown in Figure 1.
The resultant simulations are shown in Figure 2, Small
signal gain was ~18-20 dB for this 2 stage design, and
projected power was >/= 45.9 dBm, PAE>35% at 2.5 dB
compression Figure 3. The drain voltage assumed was 24-28
volts. The AM/PM simulation showed 1.5 dB/deg. max. This
would insure excellent linearity under drive. This was
because all the matching networks selected produced
minimum phase shift when terminating them with varying
source and load impedances.
978-1-4799-8275-2/15/$31.00 ©2015 IEEE
TABLE I
RECENT PUBLISHED KA BAND BENCHMARKS
Ref #
Process
Technology # Stages
44
42
40
38
36
34
32
30
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
Gain (dB)
29
34
29
27
28
30
26-30
24
22
25
13
13
28
19-22
3
3
3
2
2
3
2
GaAs
GaAs
GaN
GaN
GaN
GaN
GaN
Power
(Watts)
4.27
3.8
5
5
4
6-11
36-40
27 Ghz
30 Ghz
PAE%
Pout (Watt)
[1]
[2]
[3]
[4]
[5]
[6]
This work
Freq. (GHz)
Freq=26
Freq=27
Freq=28
Freq=29
Freq=30
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
Pin (Watt)
Fig.6. Pout (Watts) vs. Pin (Watts) of Ka band PA measured on
wafer
IV. MEASURED RESULTS
The circuit, was measured on-wafer. Using pulsed
measurement system, a small signal gain of >18 dB from 2630 GHz was measured with an input Return Loss of >7 dB
and an output return loss of >6 dB, Figure 4. The MMIC was
biased at 28 volts.
Pin vs. Pout and PAE measurements were taken on the onwafer pulsed power station. Maximum Pout and max PAE are
shown across a wafer in Figure 5. As can be seen, most sites
exceed a Pout of 34 Watts and almost all sites exceed 30 %
PAE. The results of a typical top performing site are shown in
Figure 6 where the different curves represent frequencies from
26-30 GHz. It can be seen that a Pin of 27 dBm achieves
greater than 30 watts Pout with about 18 dB gain and PAE of
>32% across the band. Plotted another way is Figures 7a,b
where Pout and PAE are plotted against power in Watts.
Measurements show that Power of > 36 Watts (45.5 dBm,
>3.4 W/mm) and PAE of >32 % across the 26-30 GHz band
with 31 dBm Pin. This we believe is a new record in Pout and
PAE achieved from a single MMIC. The peak powers
PAE (%)
32
36
30
20
24
30-34
32
Die Area
(mm^2)
16.3
9.9
4.8
14.4
--5.6-11.7
13.53
FOM (Pout/Area)
W/mm^2
0.261
0.383
1.04
0.347
--1.07-0.94
2.66-2.95
44
42
40
38
36
34
32
30
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
Freq=26
Freq=27
Freq=28
Freq=29
Freq=30
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
Pin (Watt)
Fig.7. PAE (%) vs. Pin (Watts) of Ka Band PA measured on-wafer
achieved are 26-29 GHz with >/=40 watts, with the peak
deliverance of ~43 watts at 27 GHz. The PAE at that point is
still > 30 % and reaches a peak PAE of >36% while still
delivering >40 Watts. These numbers represent the state of art
from a single MMIC at these frequencies. Accounting for the
output matching loss, this represents >4.2 W/mm from 26-29
GHz. The drain bias was set at 28 volts and dialing it down to
~24 volts should increase the PAE by 3-4 % while decreasing
the Pout by ~0.3 dB. No optimization of the gate voltages
were attempted for this first on wafer measurement.
Referring to Table1 where various previous works are
noted, a Figure of Merit (FOM) that is employed by users,
W/mm of die area, is included. Here, using 36 Watts from 2630 GHz, a FOM of 2.66 was achieved. Using 40 watts, from
26-29 GHz, a FOM of >2.95 was achieved.
A preliminary thermal analysis was conducted. The space
qualification junction temperature is 200 deg. C which
translates to >1e6 Hrs MTTF. The junction temperature with
base plate temperature of 50 deg C climbs to 208 degrees,
barely reducing the projected lifetime, and 239 degrees at a
baseplate of 70 degrees C. This still amounts to >10 year
MTTF. This assumes copper diamond heat spreader and
eutectic bonding of the MMIC on it.
978-1-4799-8275-2/15/$31.00 ©2015 IEEE
V. CONCLUSIONS AND ACKNOWLEDGEMENTS
We have presented a state of the art design that highlights
the superior capabilities of our GaN process and design
techniques. The results of this work should lead to the
widespread commercial availability of cost effective Ka Band
GaN components with output powers exceeding 25 Watts.
We would like to acknowledge Aaron Oki and the whole
GaN team in Redondo Beach for their suggestions and
expediting this lot. We would also like to thank Alex Zamora
and Rich Katz for the figures in this paper.
References
[1] F. Colomb et al., “2 and 4 watt Ka-Band GaAs PHEMT power
amplifier MMICs,” 2003 IEEE Int. Microwave Symp. Dig., pp
843-846.
[2] C. Campbell et al., “Design and Performance of a High
Efficiency Ka Band Power Amplifier MMIC,” 2010 Compound
Semiconductor Integrated Circuits Symp., CSICS. IEEE, pp. 14.
[3] C. Campbell et al., “High Efficiency Ka-band Power Amplifier
MMICs Fabricated with a 0.15um GaN on SiC HEMT process,”
2012 IEEE Int. Microwave Symp. Dig.
[4] K.S. Boutros et al., “5W GaN MMIC for Millimeter-Wave
Applications,” 2006 Compound Semiconductor IC Symp. Dig.,
pp 93-95.
[5] M. Micovic et al., “GaN MMIC Technology for Microwave and
Millimeter-wave Applications,” 2005 Compound Semiconductor
IC Symp. Dig., pp173-176.
[6] C. Campbell et al., “High Efficiency Ka-Band Gallium Nitride
Power Amplifier MMICs,” 2013 IEEE International Conference
on Microwaves, Communications, Antennas and Electronic
Systems.
978-1-4799-8275-2/15/$31.00 ©2015 IEEE