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A Review of Ku-Band GaN HEMT Power Amplifiers Development

Authors:

Abstract

This review article investigates the current status and advances in Ku-band gallium nitride (GaN) high-electron mobility transistor (HEMT) high-power amplifiers (HPAs), which are critical for satellite communications, unmanned aerial vehicle (UAV) systems, and military radar applications. The demand for high-frequency, high-power amplifiers is growing, driven by the global expansion of high-speed data communication and enhanced national security requirements. First, we compare the main GaN HEMT process technologies employed in Ku-band HPA development, categorizing the HPAs into monolithic microwave integrated circuits (MMICs) and internally matched power amplifier modules (IM-PAMs) and examining their respective characteristics. Then, by reviewing the literature, we explore design topologies, major issues like oscillation prevention and bias circuits, and heat sink technologies for thermal management. Our findings indicate that silicon carbide (SiC) substrates with gate lengths of 0.25 μm and 0.15 μm are predominantly used, with ongoing developments enabling MMICs and IM-PAMs to achieve up to 100 W output power and 30% power-added efficiency. Notably, the performance of MMIC power amplifiers is advancing more rapidly than that of IM-PAMs, highlighting MMICs as a promising direction for achieving higher efficiency and integration in future Ku-band applications. This paper can provide insights into the overall key technologies for Ku-band GaN HPA design and future development directions.
Citation: Kim, J. A Review of
Ku-Band GaN HEMT Power
Amplifiers Development.
Micromachines 2024,15, 1381.
https://doi.org/10.3390/mi15111381
Academic Editors: Zhiqun Cheng
and Zhiwei Zhang
Received: 4 October 2024
Revised: 11 November 2024
Accepted: 14 November 2024
Published: 15 November 2024
Copyright: © 2024 by the author.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Review
A Review of Ku-Band GaN HEMT Power
Amplifiers Development
Jihoon Kim
School of Electronic Engineering, Kyonggi University, Suwon-Si 16227, Republic of Korea; j7h7@kgu.ac.kr;
Tel.: +82-31-249-9803
Abstract: This review article investigates the current status and advances in Ku-band gallium nitride
(GaN) high-electron mobility transistor (HEMT) high-power amplifiers (HPAs), which are critical for
satellite communications, unmanned aerial vehicle (UAV) systems, and military radar applications.
The demand for high-frequency, high-power amplifiers is growing, driven by the global expansion of
high-speed data communication and enhanced national security requirements. First, we compare
the main GaN HEMT process technologies employed in Ku-band HPA development, categorizing
the HPAs into monolithic microwave integrated circuits (MMICs) and internally matched power
amplifier modules (IM-PAMs) and examining their respective characteristics. Then, by reviewing
the literature, we explore design topologies, major issues like oscillation prevention and bias circuits,
and heat sink technologies for thermal management. Our findings indicate that silicon carbide
(SiC) substrates with gate lengths of 0.25
µ
m and 0.15
µ
m are predominantly used, with ongoing
developments enabling MMICs and IM-PAMs to achieve up to 100 W output power and 30% power-
added efficiency. Notably, the performance of MMIC power amplifiers is advancing more rapidly
than that of IM-PAMs, highlighting MMICs as a promising direction for achieving higher efficiency
and integration in future Ku-band applications. This paper can provide insights into the overall key
technologies for Ku-band GaN HPA design and future development directions.
Keywords: Ku-band; GaN HEMT; high-power amplifier; satellite communications; MMIC; IM-PAM
1. Introduction
In recent years, the demand for satellite communication services has increased not only
in military applications but also in civilian ones, drawing significant attention to related
technology development. Satellite communication technology enables communication
between earth stations via satellites, providing various communication and broadcasting
services [
1
3
]. Figure 1shows the various applications of satellite communications [
4
].
Satellite communication can be classified into low, medium, and high orbits, including
geostationary orbits, based on altitude. Geostationary satellite communication can cover
the entire globe with just three satellites, offering the economic advantage of replacing
numerous ground base stations for wide coverage. In contrast, low-orbit satellite commu-
nication provides high-speed communication with low latency. In particular, the recent
Russian–Ukrainian war and the frequent occurrence of various disasters have highlighted
the advantages of maintaining communication during emergencies and crises, leading to
the emergence of private satellite communication companies such as Starlink. In addition
to civilian communication services, there is a growing demand for satellite radar for mil-
itary surveillance and reconnaissance. Amid the uncertainty of localized wars, not only
developed countries like the United States, European Union, Japan, and China, but also
nations worldwide are increasingly focused on developing satellite communication and
radar technology for national security [5,6].
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The frequency bands used for satellite communications are primarily divided into C-
band, X-band, Ku-band, and Ka-band. Among these, Ku-band and Ka-band oer ad-
vantages such as supporting large data transmission, providing high-resolution services
with wide bandwidth, and enabling antenna miniaturization due to their shorter wave-
lengths [4]. However, due to the high operating frequency, the performance of transceiver
modules used in towers and earth stations has been limited, making development chal-
lenging. Recent advances in complementary metal–oxidesemiconductor (CMOS) and
gallium nitride (GaN) high-electron mobility transistor (HEMT) semiconductor process
technology have, however, enabled commercialization. Ku-band is now used not only for
satellite communication but also in transmier and receiver modules (TRMs) for un-
manned aerial vehicle (UAV) communication, which is increasingly being utilized along-
side satellites due to its advantages in miniaturization and bandwidth [7].
Figure 1. Various applications of satellite communications. Figure reproduced from [4].
Table 1 compares the electrical properties of major semiconductor materials [8], while
Figure 2 illustrates the breakdown voltage characteristics as a function of cuto frequency
for semiconductor devices [9,10]. As shown in Table 1 and Figure 2, GaN HEMT semicon-
ductors have been widely studied and commercialized as high-frequency power semicon-
ductors due to their high breakdown voltage and excellent electron mobility stemming
from their wide energy band gap. High-power ampliers (HPAs) present signicant chal-
lenges in achieving performance in Ku-band satellite and UAV TRMs. To aain high out-
put power, traveling wave tube ampliers (TWTAs), which utilize vacuum tube technol-
ogy, have traditionally been employed; however, they are heavy and unstable due to their
high-voltage operation. Now, advances in GaN HEMT semiconductor process technology
are enabling the replacement of conventional TWTAs with solid-state power ampliers
(SSPAs) [11,12].
In line with this trend, research on Ku-band GaN HEMT HPAs has been actively con-
ducted by various research groups worldwide. This paper aims to summarize the current
status and major issues related to Ku-band GaN HEMT HPA technology developed in
recent years. The paper is organized as follows: In Section 2.1, we compare the processes
used to implement Ku-band GaN HEMT HPAs; in Section 2.2, we discuss the two main
approaches to implementing HPAs—monolithic microwave integrated circuits (MMICs)
and internally matched power amplier modules (IM-PAMs)—and examine their respec-
tive high-power design strategies and key issues. In Section 2.3, we review research on
additional performance enhancements beyond high power, and nally, we provide con-
clusions.
Figure 1. Various applications of satellite communications. Figure reproduced from [4].
The frequency bands used for satellite communications are primarily divided into
C-band, X-band, Ku-band, and Ka-band. Among these, Ku-band and Ka-band offer ad-
vantages such as supporting large data transmission, providing high-resolution services
with wide bandwidth, and enabling antenna miniaturization due to their shorter wave-
lengths [
4
]. However, due to the high operating frequency, the performance of transceiver
modules used in towers and earth stations has been limited, making development challeng-
ing. Recent advances in complementary metal–oxide–semiconductor (CMOS) and gallium
nitride (GaN) high-electron mobility transistor (HEMT) semiconductor process technology
have, however, enabled commercialization. Ku-band is now used not only for satellite
communication but also in transmitter and receiver modules (TRMs) for unmanned aerial
vehicle (UAV) communication, which is increasingly being utilized alongside satellites due
to its advantages in miniaturization and bandwidth [7].
Table 1compares the electrical properties of major semiconductor materials [
8
], while
Figure 2illustrates the breakdown voltage characteristics as a function of cutoff frequency
for semiconductor devices [
9
,
10
]. As shown in Table 1and Figure 2, GaN HEMT semicon-
ductors have been widely studied and commercialized as high-frequency power semicon-
ductors due to their high breakdown voltage and excellent electron mobility stemming
from their wide energy band gap. High-power amplifiers (HPAs) present significant chal-
lenges in achieving performance in Ku-band satellite and UAV TRMs. To attain high output
power, traveling wave tube amplifiers (TWTAs), which utilize vacuum tube technology,
have traditionally been employed; however, they are heavy and unstable due to their
high-voltage operation. Now, advances in GaN HEMT semiconductor process technology
are enabling the replacement of conventional TWTAs with solid-state power amplifiers
(SSPAs) [11,12].
Table 1. Comparison of the electrical properties of major semiconductor materials [8].
Property Si GaAs SiC GaN
Energy Bandgap (eV) 1.11 1.43 3.2 3.4
Critical Electric Field (MV/cm) 0.3 0.5 3.0 3.5
Charge Density (×1013/cm2)0.3 0.3 0.4 1
Mobility (cm2/V/s) 1350 8000 900 1500
Saturation Velocity (×107cm/V) 1 1.4 2 2.7
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Table 1. Comparison of the electrical properties of major semiconductor materials [8].
Property Si GaAs SiC GaN
Energy Bandgap (eV) 1.11 1.43 3.2 3.4
Critical Electric Field (MV/cm) 0.3 0.5 3.0 3.5
Charge Density (×10
13
/cm
2
) 0.3 0.3 0.4 1
Mobility (cm
2
/V/s) 1350 8000 900 1500
Saturation Velocity 10
7
cm/V) 1 1.4 2 2.7
Figure 2. Comparison of breakdown voltage and cuto frequency among various high-speed sem-
iconductor devices [9,10]. (Data from [9,10].)
2. Ku-Band GaN HEMT HPA Technology
2.1. GaN HEMT Process
To realize a GaN HEMT power amplier, it is essential to develop a GaN HEMT
process that provides the necessary design kits and fabricates the circuits or devices. Fig-
ure 3 shows the cross-sectional structure of a typical GaN HEMT [13]. GaN HEMTs with
high cuto frequency (f
T
) and maximum oscillation frequency (f
MAX
) are required, as ob-
taining gain becomes more challenging with increasing operating frequency. Therefore,
the gate length of the transistor needs to be reduced according to the scaling rule. In this
case, the breakdown voltage is also reduced proportionally, which can be a disadvantage
in power amplier design. The substrate material is also an important factor in the choice
of process. Generally, GaN HEMT processes are divided into two types: those using sili-
con carbide (SiC) substrates and those using silicon (Si) substrates. SiC substrates have
high thermal conductivity, which is benecial for dissipating heat generated by self-heat-
ing due to the high power density of GaN HEMTs. Therefore, they are often used in the
process for GaN HEMT HPAs. However, SiC is hard and dicult to process during post-
processing tasks such as dicing. In contrast, Si substrates are easier to produce using ma-
ture silicon-based semiconductor technology, are simpler to process, and are more ame-
nable to mass production, providing signicant economic advantages. However, they
have relatively poor thermal conductivity and are more susceptible to self-heating eects.
Figure 2. Comparison of breakdown voltage and cutoff frequency among various high-speed
semiconductor devices [9,10]. (Data from [9,10]).
In line with this trend, research on Ku-band GaN HEMT HPAs has been actively
conducted by various research groups worldwide. This paper aims to summarize the
current status and major issues related to Ku-band GaN HEMT HPA technology devel-
oped in recent years. The paper is organized as follows: In Section 2.1, we compare the
processes used to implement Ku-band GaN HEMT HPAs; in Section 2.2, we discuss the
two main approaches to implementing HPAs—monolithic microwave integrated circuits
(MMICs) and internally matched power amplifier modules (IM-PAMs)—and examine
their respective high-power design strategies and key issues. In Section 2.3, we review
research on additional performance enhancements beyond high power, and finally, we
provide conclusions.
2. Ku-Band GaN HEMT HPA Technology
2.1. GaN HEMT Process
To realize a GaN HEMT power amplifier, it is essential to develop a GaN HEMT process
that provides the necessary design kits and fabricates the circuits or devices. Figure 3
shows the cross-sectional structure of a typical GaN HEMT [
13
]. GaN HEMTs with high
cutoff frequency (f
T
) and maximum oscillation frequency (f
MAX
) are required, as obtaining
gain becomes more challenging with increasing operating frequency. Therefore, the gate
length of the transistor needs to be reduced according to the scaling rule. In this case,
the breakdown voltage is also reduced proportionally, which can be a disadvantage in
power amplifier design. The substrate material is also an important factor in the choice of
process. Generally, GaN HEMT processes are divided into two types: those using silicon
carbide (SiC) substrates and those using silicon (Si) substrates. SiC substrates have high
thermal conductivity, which is beneficial for dissipating heat generated by self-heating due
to the high power density of GaN HEMTs. Therefore, they are often used in the process for
GaN HEMT HPAs. However, SiC is hard and difficult to process during post-processing
tasks such as dicing. In contrast, Si substrates are easier to produce using mature silicon-
based semiconductor technology, are simpler to process, and are more amenable to mass
production, providing significant economic advantages. However, they have relatively
poor thermal conductivity and are more susceptible to self-heating effects.
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Figure 3. Cross-sectional structure of a typical GaN HEMT [13].
The most commonly used processes for GaN HEMT HPAs are the 0.15 µm and 0.25
µm GaN HEMT processes. Here, 0.15 µm and 0.25 µm refer to the gate lengths of GaN
HEMT devices. To achieve the required gain for power amplication in Ku-band, GaN
HEMTs with gate lengths of 0.25 µm or less are preferred. Recently, 0.15 µm processes
have become more popular due to their ability to achieve a high gain and high power
added eciency. Table 2 compares the device and process information for the commercial
0.25-µm GaN HEMT process, while Table 3 compares the device and process information
for the commercial 0.15-µm GaN HEMT process.
Tab le 2. Comparison of device and process information for commercial 0.25-um GaN HEMT process
[14–20].
Foundry V
DD
(V) Substrate Breakdown
Voltage (V) f
T
(GHz)
P
out
@10 GHz
(W/mm)
PAE
@10 GHz (%)
Qorvo 40 SiC 75 32 6 >60
MACOM
(Wolfspeed) 28/40 SiC >84 4.2/6.6 >55
GCS 28/48 SiC/Si 200 23 4
1
/10.8 45
1
/65
UMS 30 SiC >120 25 4.5
2
WIN semi 28/40 SiC 120 23 5/10 65/60
NXP 50 SiC >150
1
The data were measured at 15 GHz.
2
The frequency was unknown.
Tab le 3. Comparison of device and process information for commercial 0.15-um GaN HEMT process
[14–20].
Foundry V
DD
(V) Substrate Breakdown
Voltage (V)
f
T
(GHz)
P
out
@30 GHz
(W/mm)
PAE
@30 GHz (%)
Qorvo 28 SiC 50 90 4.2 >50
MACOM
(Wolfspeed) 28 SiC 84 3.75 >40
GCS 28 SiC 100 42 3 55
UMS 2025 SiC >80 4.2
2
36
3
WIN semi 28 SiC 120 35 5 50
Figure 3. Cross-sectional structure of a typical GaN HEMT [13].
The most commonly used processes for GaN HEMT HPAs are the 0.15
µ
m and
0.25 µm
GaN HEMT processes. Here, 0.15
µ
m and 0.25
µ
m refer to the gate lengths of GaN HEMT
devices. To achieve the required gain for power amplification in Ku-band, GaN HEMTs
with gate lengths of 0.25
µ
m or less are preferred. Recently, 0.15
µ
m processes have
become more popular due to their ability to achieve a high gain and high power added
efficiency. Table 2compares the device and process information for the commercial 0.25-
µ
m
GaN HEMT process, while Table 3compares the device and process information for the
commercial 0.15-µm GaN HEMT process.
Table 2. Comparison of device and process information for commercial 0.25-um GaN HEMT
process [1420].
Foundry VDD (V) Substrate Breakdown
Voltage (V) fT(GHz)
Pout
@10 GHz
(W/mm)
PAE
@10 GHz (%)
Qorvo 40 SiC 75 32 6 >60
MACOM
(Wolfspeed) 28/40 SiC >84 4.2/6.6 >55
GCS 28/48 SiC/Si 200 23 41/10.8 45 1/65
UMS 30 SiC >120 25 4.5 2
WIN semi 28/40 SiC 120 23 5/10 65/60
NXP 50 SiC >150
1The data were measured at 15 GHz. 2The frequency was unknown.
Table 3. Comparison of device and process information for commercial 0.15-um GaN HEMT process
[1420].
Foundry
V
DD
(V) Substrate
Breakdown
Voltage (V) fT(GHz)
Pout
@30 GHz
(W/mm)
PAE
@30 GHz (%)
Qorvo 28 SiC 50 90 4.2 >50
MACOM
(Wolfspeed) 28 SiC 84 3.75 >40
GCS 28 SiC 100 42 3 55
UMS 20–25 SiC >80 4.2 236 3
WIN semi 28 SiC 120 35 5 50
NXP 20–28 SiC >100
MACOM/France
(formerly OMMIC)
112 Si >50 150 4248 2
1
The foundry uses 0.1
µ
m GaN HEMT process.
2
The frequency was unknown.
3
The data were estimated from
the graphs.
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Most commercial foundries still use SiC substrates, with the exceptions of MA-
COM/France (formerly OMMIC) and GCS. The 0.25
µ
m process is based on a drain
voltage of 28–30 V. Recently, GaN HEMT processes have been developed to operate at
higher voltages of 40–50 V, depending on the foundry, leading to a nearly twofold increase
in power density. However, the f
T
is only 23 to 32 GHz, which can limit power gain at high
edge frequencies in the Ku-band. Breakdown voltages range from 75 to
200 V
. Meanwhile,
the 0.15
µ
m process utilizes a somewhat lower drain voltage of 20 to 28 V compared with
the 0.25
µ
m process, with a minimum of 12 V for OMMICs using Si substrates. Break-
down voltages for this process range from 50 to 120 V. The lower drain voltage reduces
the voltage swing of the power amplifier, which is a disadvantage for achieving high
output power. However, the f
T
performance, which is two to three times higher than
that of the 0.25
µ
m process, is favorable for obtaining high power gain and improving
efficiency. As shown in Table 3, the power-added efficiency (PAE) of over 50% at 30 GHz
led to the expectation of better power efficiency compared with the 0.25
µ
m process in the
Ku-band. MACOM/France (formerly OMMIC)‘s GaN-on-Si HEMT process has shown
output power performance comparable to other processes, even with a drive voltage of
12 V. HPA designs using cascode or stacked-field effect transistor (FET) structures that
enhance the voltage swing in this process are expected to yield higher output power and
warrant further research [21,22].
2.2. High Power Amplifier Design
There are two main ways to implement HPAs. The first is the MMIC type, where
both transistors and input/output matching circuits are designed and fabricated within a
single integrated circuit [
23
36
]. This approach reduces the overall circuit size, facilitates
mass production, and significantly minimizes parasitic components generated by the
connection between the transistor and the matching circuits [
20
,
37
]. However, achieving
a high dielectric constant (
εr
) is challenging with conventional semiconductor processes,
making it difficult to use high-Q inductors. Additionally, employing passive elements with
large inductance (L) or capacitance (C) values for matching and bias circuits increases costs
due to the larger chip size.
The second method is the microwave integrated circuit (MIC) or hybrid type, which
utilizes packaged transistors while implementing the input/output matching circuit on
the printed circuit board (PCB) [
37
]. This method leverages PCBs with low losses and high
dielectric constants to reduce matching losses by using high-Q inductors and allows for the
use of large inductors and capacitors. It also offers the advantage of being tunable after
fabrication, enabling easy modifications and optimizations [
37
]. However, this method
results in a bulkier overall circuit size, and matching is limited due to parasitic components
caused by wire bonding when connecting the transistor and the matching circuit [
37
,
38
].
As frequency increases, the impact of these parasitic components becomes more significant,
degrading performance. To address this, the internally matched HPA design is often used,
incorporating matching circuitry within the packaged transistors [3947].
Table 4summarizes the key performance characteristics of reported Ku-band GaN
HEMT HPA MMICs. As shown in Table 4, Ku-band power amplifier MMICs have been
implemented on silicon and silicon carbide substrates, with output power ranging from
7.2 W
to 93 W and gate lengths from 0.1
µ
m to 0.25
µ
m. Notably, several papers using the
0.15
µ
m process have demonstrated output powers exceeding 40 W while achieving power-
added efficiencies (PAEs) of 30–40% [
24
,
36
]. In references [
29
,
30
], high efficiencies close to
40% were also reported at output powers above 10 W. GaN HEMT HPAs implemented with
0.25
µ
m processes exhibit somewhat lower power efficiency but achieve high output powers
of over 40 W, as seen in references [
28
,
32
]. A recent paper [
31
] utilized a 0.2
µ
m internal
process to achieve output power close to 100 W in MMIC form. This demonstrates that
MMIC technology has advanced to the point where it can achieve performance comparable
to that of existing IM-PAMs, and it is expected to replace TWTAs in the Ku-band.
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Table 4. Summary of the key performance characteristics of reported Ku-band GaN HEMT HPA
MMICs.
Reference Gate Length (µm) Substrate VDD (V) Pout (W) PAE (%)
[23] 0.25 24 20 16
[24] 0.15 SiC 28 47.5 36.2
[25] 0.25 SiC 25 7.2–9.5 35
[26] 0.25 30 25 26–30
[27] 0.15 SiC 25 7.9 35
[28] 0.25 SiC 40 63 30
[29] 0.15 SiC 28 8.3–13.2 35.7–45.4
[30] 0.15 SiC 28 16–25 30–40 1
[31] 0.20 SiC 28 79–93 28.7–31.5
[32] 0.25 SiC 28 40 17
[33] 0.1 Si 11 8.9 27
[34] 0.1 Si 12 8.9–15.8 30–41 1
[35] 0.1 Si 9 10 35
[36] 0.15 28 40–50 36
1The data were estimated from the graphs.
Table 5compares HPA MMIC papers with output powers exceeding 20 W from a
circuit design perspective. All references adopted a multistage amplifier approach. Except
for [
32
], which was designed as a four-stage amplifier, the other references used a three-
stage configuration to drive the main power cell. For output powers up to 20 W, the main
power cell combined the currents of eight GaN HEMTs in parallel, resulting in a total
gate width of approximately 10 mm. For output powers above 40 W, the standard design
approach employed sixteen GaN HEMTs in parallel, with a total gate width ranging from
10 to 25 mm. Notably, Ref. [
31
] presents a power cell with a total gate width of about 10 mm
that achieved an output power of nearly 100 W, demonstrating excellent power density.
Table 5. Comparison of reported HPA MMICs above 20 W from a circuit design perspective.
Reference
BW (GHz) Output
Power (W)
# of GaN HEMTs
in the Final Stage
Total Gate Width
(mm)
# of Stages
[23] 13.75–14.5 20 8 9.6 3
[26] 13–18 25 8 8.64 3
[28] 12.7–13.25 63 16 3
[30] 13–17 16–25 8 7.68 3
[31] 14–18 79–93 16 10.88 3
[32] 15.25–16.25 40 16 25.6 4
[36] 13–15.5 40–50 16 15.36 3
Figure 4a,b show the HPA structure and schematic commonly used in the aforemen-
tioned papers. The HPA consists of a two-way GaN HEMT power cell in the first stage, a
four-way configuration in the second stage, and an eight-way configuration in the third
stage. In contrast, an HPA with a 16-way GaN HEMT as the main power cell replicates the
previously described 2-4-8 structure, tying the inputs and outputs together.
As shown in the schematic of Figure 4b, an RC stabilization circuit with resistors and
capacitors connected in parallel is often employed to enhance the stability of the input
side of the circuit. Due to the large size of the transistor, substantial DC currents and RF
signals are input and output. This increases the risk of oscillation, particularly if the signal
diverges to one side. To mitigate even-mode and odd-mode oscillations, large resistors are
typically connected to the gate and drain (see the beige boxes in Figure 4b) [48].
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stage. In contrast, an HPA with a 16-way GaN HEMT as the main power cell replicates the
previously described 2-4-8 structure, tying the inputs and outputs together.
(a)
(b)
Figure 4. (a) Structure and (b) circuit schematic of conventional GaN HEMT HPA MMIC. Figures
reproduced or reworked with permission from ref. [33]. Copyright 2023 MDPI.
As shown in the schematic of Figure 4b, an RC stabilization circuit with resistors and
capacitors connected in parallel is often employed to enhance the stability of the input
Figure 4. (a) Structure and (b) circuit schematic of conventional GaN HEMT HPA MMIC. Figures
reproduced or reworked with permission from ref. [33]. Copyright 2023 MDPI.
Another design consideration is the source via the structure of the GaN HEMT. There
are two main types of source vias: the outer source via (OSV), where only the outermost
part of the source finger is connected, and the individual source via (ISV), which connects
ground vias for each of the source fingers. Figure 5a,b illustrate the layouts of a 4
×
50
µ
m
GaN HEMT device implemented in OSV and ISV configurations, respectively. In general,
Micromachines 2024,15, 1381 8 of 15
larger and more numerous ground source vias reduce the parasitic inductance caused by
the source via structure, which improves load-pull characteristics and enhances device
performance by dissipating heat generated by self-heating more effectively. However, the
ISV structure can excessively increase the size of the main power cell, posing challenges for
MMIC manufacturing. Therefore, it is common to use an ISV structure for the drive stage
and an OSV for the main stage [32].
Micromachines 2024, 15, x FOR PEER REVIEW 8 of 15
side of the circuit. Due to the large size of the transistor, substantial DC currents and RF
signals are input and output. This increases the risk of oscillation, particularly if the signal
diverges to one side. To mitigate even-mode and odd-mode oscillations, large resistors
are typically connected to the gate and drain (see the beige boxes in Figure 4b) [48].
Another design consideration is the source via the structure of the GaN HEMT. There
are two main types of source vias: the outer source via (OSV), where only the outermost
part of the source nger is connected, and the individual source via (ISV), which connects
ground vias for each of the source ngers. Figure 5a,b illustrate the layouts of a 4 × 50 µm
GaN HEMT device implemented in OSV and ISV congurations, respectively. In general,
larger and more numerous ground source vias reduce the parasitic inductance caused by
the source via structure, which improves load-pull characteristics and enhances device
performance by dissipating heat generated by self-heating more eectively. However, the
ISV structure can excessively increase the size of the main power cell, posing challenges
for MMIC manufacturing. Therefore, it is common to use an ISV structure for the drive
stage and an OSV for the main stage [32].
(a) (b)
Figure 5. (a) OSV and (b) ISV layouts of a 4 × 50 µm GaN HEMT.
Additionally, careful design is essential for the bias circuit. As shown in Figure 4b, a
virtual bias mimic circuit with open stubs and RCs in series on the opposite side of the
actual bias input is used to ensure that each connected transistor presents the same im-
pedance to the gate bias supply (see the green boxes in Figure 4b). The drain bias circuit
also requires careful design to ensure that each transistor receives a drain voltage through
a line that is in phase with the others (see the gray boxes in Figure 4b).
Table 6 compares Ku-band GaN HEMT HPAs implemented using the MIC approach.
Compared with the MMIC schemes in Table 4, more 0.25 µm processes were utilized, with
the average output power ranging from 50 to 120 W, which was higher than that of the
MMIC schemes. However, the PAEs fell within the 20 to 30 percent range. All MIC power
ampliers in Table 6 were implemented in the IM-PAM type.
Table 6. Summary of the key performance characteristics of reported Ku-band GaN HEMT HPA
MICs.
Reference Gate Length (µm) Substrate V
DD
(V) Pout (W) PAE (%)
[39] 24 63 32
[40] 24 50 29
[41] 0.25 SiC 24 63 25
[42] 0.25 SiC 40 50 23
[43] 0.25 SiC 40 5766
[44] 0.25 SiC 50 50
[45] 0.15 24 100 32
1
[46] 24 80 2328
1
[47] 0.15 30 120 31
1
The data were estimated from the graphs.
Figure 5. (a) OSV and (b) ISV layouts of a 4 ×50 µm GaN HEMT.
Additionally, careful design is essential for the bias circuit. As shown in Figure 4b,
a virtual bias mimic circuit with open stubs and RCs in series on the opposite side of
the actual bias input is used to ensure that each connected transistor presents the same
impedance to the gate bias supply (see the green boxes in Figure 4b). The drain bias circuit
also requires careful design to ensure that each transistor receives a drain voltage through
a line that is in phase with the others (see the gray boxes in Figure 4b).
Table 6compares Ku-band GaN HEMT HPAs implemented using the MIC approach.
Compared with the MMIC schemes in Table 4, more 0.25
µ
m processes were utilized, with
the average output power ranging from 50 to 120 W, which was higher than that of the
MMIC schemes. However, the PAEs fell within the 20 to 30 percent range. All MIC power
amplifiers in Table 6were implemented in the IM-PAM type.
Table 6. Summary of the key performance characteristics of reported Ku-band GaN HEMT
HPA MICs.
Reference
Gate Length (
µ
m)
Substrate VDD (V) Pout (W) PAE (%)
[39] 24 63 32
[40] 24 50 29
[41] 0.25 SiC 24 63 25
[42] 0.25 SiC 40 50 23
[43] 0.25 SiC 40 57–66
[44] 0.25 SiC 50 50
[45] 0.15 24 100 32 1
[46] 24 80 23–28 1
[47] 0.15 30 120 31
1The data were estimated from the graphs.
Table 7compares the HPA MIC references of Table 6from a circuit design perspective.
The GaN dies used in the surveyed papers were primarily the CGHV1J070D, sold by
MACOM (formerly Wolfspeed), and one developed in-house by Mitsubishi, Japan. The
number of GaN HEMTs in the main power stage varied from 12 to 64, depending on
the output power, with a total gate width approximately in the range of 15 to 30 mm.
It is evident that MIC power amplifiers are primarily developed to achieve high output
power in a narrow band. Due to the size constraints imposed by PCB implementation of
input–output matching circuits, a single-stage design was often utilized, which necessitates
a separate high-input driving power in the system configuration.
Micromachines 2024,15, 1381 9 of 15
Table 7. Comparison of reported HPA MICs from a circuit design perspective.
Reference
BW (GHz) Pout (W) # of GaN HEMTs
in the Final Stage
Total Gate Width
(mm)
# of Stages
[41]113.75–14.5 63 24 28.8 1
[42] 13.75–14.5 50 12 14.4 2
[43]216.2–16.8 57–66 12 14.4 1
[44]212.4–13.8 50 12 14.4 1
[45]1100 48 28.8 1
[46]113.75–14.5 80 48 1
[47]111.7–12.2 120 64 30.72 31
1
These references used the same GaN HEMT die (CGHV1J070D by MACOM).
2
These references used GaN
HEMT dies developed in-house by Mitsubishi, Japan. 3The data were estimated from the figures.
Figure 6illustrates a typical example of a Ku-band MIC power amplifier implemented
using an internal matching approach. To compactly implement an impedance transformer
that transformed the low impedance of a large GaN HEMT die into a high impedance,
the PCB connected to the GaN HEMT die was made from a material with a very high
dielectric constant, as shown in Figure 6. This design prevented the feeding line widths
from becoming excessively large, keeping the overall module size manageable. The power
dividing and combining components that followed were implemented using materials
with lower dielectric constants. In [
41
], PCBs with
εr
values of 38.5 and 9.8 were used,
while [
43
] also utilized PCBs with
εr
values of 40 and 9.8. When transforming impedance,
it is common to employ a multi-step impedance conversion to achieve a gradual transition
and avoid a high Q-factor on the Smith chart [43].
Micromachines 2024, 15, x FOR PEER REVIEW 9 of 15
Table 7 compares the HPA MIC references of Table 6 from a circuit design perspec-
tive. The GaN dies used in the surveyed papers were primarily the CGHV1J070D, sold by
MACOM (formerly Wolfspeed), and one developed in-house by Mitsubishi, Japan. The
number of GaN HEMTs in the main power stage varied from 12 to 64, depending on the
output power, with a total gate width approximately in the range of 15 to 30 mm. It is
evident that MIC power ampliers are primarily developed to achieve high output power
in a narrow band. Due to the size constraints imposed by PCB implementation of input–
output matching circuits, a single-stage design was often utilized, which necessitates a
separate high-input driving power in the system conguration.
Table 7. Comparison of reported HPA MICs from a circuit design perspective.
Reference BW (GHz) Pout (W) # of GaN HEMTs
in the Final Stage
Total Gate Width
(mm) # of Stages
[41]
1
13.7514.5 63 24 28.8 1
[42] 13.7514.5 50 12 14.4 2
[43]
2
16.216.8 5766 12 14.4 1
[44]
2
12.413.8 50 12 14.4 1
[45]
1
100 48 28.8 1
[46]
1
13.7514.5 80 48 1
[47]
1
11.712.2 120 64 30.72
3
1
1
These references used the same GaN HEMT die (CGHV1J070D by MACOM).
2
These references
used GaN HEMT dies developed in-house by Mitsubishi, Japan.
3
The data were estimated from the
gures.
Figure 6 illustrates a typical example of a Ku-band MIC power amplier imple-
mented using an internal matching approach. To compactly implement an impedance
transformer that transformed the low impedance of a large GaN HEMT die into a high
impedance, the PCB connected to the GaN HEMT die was made from a material with a
very high dielectric constant, as shown in Figure 6. This design prevented the feeding line
widths from becoming excessively large, keeping the overall module size manageable.
The power dividing and combining components that followed were implemented using
materials with lower dielectric constants. In [41], PCBs with 𝜀 values of 38.5 and 9.8 were
used, while [43] also utilized PCBs with 𝜀 values of 40 and 9.8. When transforming im-
pedance, it is common to employ a multi-step impedance conversion to achieve a gradual
transition and avoid a high Q-factor on the Smith chart [43].
Figure 6. Design example of a Ku-band MIC HPA implemented with an internal matching approach.
Figures reproduced with permission from ref. [43]. Copyright 2018 MDPI.
Additionally, the GaN HEMT die and PCB were connected via wire bonding, and this
was advantageous to minimize the bonding length and maximize the number of bonds
to reduce parasitic inductance. Figure 7a shows an example photograph of an actual
fabricated Ku-band GaN HEMT IM-PAM, and Figure 7b presents a photo of a GaN HEMT
power amplifier module featuring extensive wire bonding.
Micromachines 2024,15, 1381 10 of 15
Micromachines2024,15,xFORPEERREVIEW10of15
Figure6.DesignexampleofaKu-bandMICHPAimplementedwithaninternalmatchingapproach.
Figuresreproducedwithpermissionfromref.[43].Copyright2018MDPI.
Additionally,theGaNHEMTdieandPCBwereconnectedviawirebonding,and
thiswasadvantageoustominimizethebondinglengthandmaximizethenumberof
bondstoreduceparasiticinductance.Figure7ashowsanexamplephotographofanac-
tualfabricatedKu-bandGaNHEMTIM-PAM,andFigure7bpresentsaphotoofaGaN
HEMTpowerampliermodulefeaturingextensivewirebonding.
(a)
(b)
Figure7.Examplephotosof(a)afabricatedKu-bandGaNHEMTIM-PAMand(b)aGaNHEMT
powerampliermodulewithwirebonding.Figuresreproducedorreworkedwithpermissionfrom
refs.[38,43].(a)isCopyright2018MDPIand(b)isCopyright2023IEEE.
Ontheotherhand,packaginganddieaachtechnologythateectivelydissipates
heatiscriticalforachievinglargeoutputpowersof50Wormore.Figure8a,bdisplaythe
temperaturedistributionofa20WclassGaNHEMTHPAbaredie,withonlyDCpower
appliedandwithbothDCandRFpowerapplied,usingahigh-resolutioninfrared(IR)
scope.Itisevidentthatthemaximumtemperaturenearlydoubledfrom90°Ctoover180
°CwhenRFpowerwasappliednearthecenterchannelofthetransistor,highlightingthe
necessityforeectiveheatdissipationtechniquesduringthepackagingprocess.
Figure 7. Example photos of (a) a fabricated Ku-band GaN HEMT IM-PAM and (b) a GaN HEMT
power amplifier module with wire bonding. Figures reproduced or reworked with permission from
refs. [38,43]. (a) is Copyright 2018 MDPI and (b) is Copyright 2023 IEEE.
On the other hand, packaging and die attach technology that effectively dissipates
heat is critical for achieving large output powers of 50 W or more. Figure 8a,b display the
temperature distribution of a 20 W class GaN HEMT HPA bare die, with only DC power
applied and with both DC and RF power applied, using a high-resolution infrared (IR)
scope. It is evident that the maximum temperature nearly doubled from 90
C to over
180 C
when RF power was applied near the center channel of the transistor, highlighting
the necessity for effective heat dissipation techniques during the packaging process.
Table 8compares the thermal conductivity of heat spreader materials commonly used
in die attach. Table 9summarizes the thermal conductivity of materials utilized as heat
sinks or thermal interfaces. Several studies have employed eutectic die attachments with
good thermal conductivity, mounting them on Cu-Mo-Cu flanges, which serve as excellent
heat sinks, rather than the typical copper jig [
44
]. Figure 9illustrates the heat sink structure
of a GaN HEMT die, frequently used in IM-PAMs. Reference [
49
] reports improvements
in output power and PAE by using chemical vapor deposition (CVD) diamond materials,
which possess the best thermal conductivity, as the thermal interface on top of the copper
heat sink, as shown in Figure 10.
Micromachines 2024,15, 1381 11 of 15
Micromachines 2024, 15, x FOR PEER REVIEW 11 of 15
(a) (b)
Figure 8. Temperature distribution of a 20 W class GaN HEMT HPA bare die (a) with only DC power
applied and (b) with DC power and RF power applied using a high-resolution IR scope.
Table 8 compares the thermal conductivity of heat spreader materials commonly
used in die aach. Table 9 summarizes the thermal conductivity of materials utilized as
heat sinks or thermal interfaces. Several studies have employed eutectic die aachments
with good thermal conductivity, mounting them on Cu-Mo-Cu anges, which serve as
excellent heat sinks, rather than the typical copper jig [44]. Figure 9 illustrates the heat
sink structure of a GaN HEMT die, frequently used in IM-PAMs. Reference [49] reports
improvements in output power and PAE by using chemical vapor deposition (CVD) dia-
mond materials, which possess the best thermal conductivity, as the thermal interface on
top of the copper heat sink, as shown in Figure 10.
Table 8. Comparison of thermal conductivity of heat spreader materials. (Data from [49].)
Composition Thermal Conductivity (W/mK)
PbIn 17
AuG e 44
SnPb 50
AuS n 57
SnAg 78
Ag Sintering Epoxy 100
Tab le 9. Comparison of thermal conductivity of heat sink or thermal interface materials. (Data from
[49].)
Composition Thermal Conductivity (W/mK)
Mo 140
W 170
Al 230
Cu 400
CMC
1
270–320
CVD Diamond 1000–1800
1
CMC means
copper/molybdenum/copper combination composition in equal parts.
Figure 8. Temperature distribution of a 20 W class GaN HEMT HPA bare die (a) with only DC power
applied and (b) with DC power and RF power applied using a high-resolution IR scope.
Table 8. Comparison of thermal conductivity of heat spreader materials. (Data from [49]).
Composition Thermal Conductivity (W/mK)
PbIn 17
AuGe 44
SnPb 50
AuSn 57
SnAg 78
Ag Sintering Epoxy 100
Table 9. Comparison of thermal conductivity of heat sink or thermal interface materials. (Data from
[49]).
Composition Thermal Conductivity (W/mK)
Mo 140
W 170
Al 230
Cu 400
CMC 1270–320
CVD Diamond 1000–1800
1CMC means copper/molybdenum/copper combination composition in equal parts.
Figure 9. Heat sink structure of a GaN HEMT die [44].
In this subsection, we investigated two methods for implementing Ku-band GaN
HEMT HPAs, MMICs and internally matched MIC design methods, by comparing the
reported papers. As mentioned above, MMIC HPAs can be configured in a multistage
arrangement with a compact size to achieve high gain, offering a low input power burden
and wide bandwidth. In contrast, MIC HPAs are advantageous for obtaining high output
power within a narrow bandwidth; however, due to size constraints, they are typically
limited to a single-stage configuration and require high input power. Nonetheless, advance-
ments in MMIC processes and design technologies have been significant in recent years,
suggesting that we can expect numerous studies reporting output powers exceeding 100 W
in the near future, such as in [31].
Micromachines 2024,15, 1381 12 of 15
Micromachines 2024, 15, x FOR PEER REVIEW 12 of 15
Figure 9. Heat sink structure of a GaN HEMT die [44].
Figure 10. Comparison of output power and PAE of GaN HEMT MMICs according to thermal in-
terface material and heat spreader combinations.
In this subsection, we investigated two methods for implementing Ku-band GaN
HEMT HPAs, MMICs and internally matched MIC design methods, by comparing the
reported papers. As mentioned above, MMIC HPAs can be congured in a multistage ar-
rangement with a compact size to achieve high gain, oering a low input power burden
and wide bandwidth. In contrast, MIC HPAs are advantageous for obtaining high output
power within a narrow bandwidth; however, due to size constraints, they are typically
limited to a single-stage conguration and require high input power. Nonetheless, ad-
vancements in MMIC processes and design technologies have been signicant in recent
years, suggesting that we can expect numerous studies reporting output powers exceed-
ing 100 W in the near future, such as in [31].
2.3. Other Additional Design Techniques
Ku-band GaN HEMT power ampliers have been extensively studied to achieve high
output power as a replacement for TWTAs. However, several research groups have con-
ducted and published studies aimed at improving performance beyond just output
power.
To enhance linearity, some researchers have embedded linearizers within the MMIC
or designed separate linearizer modules in front of the MIC [23,47]. In [23], a linearizer
composed of a simple diode and the inductance of a microstrip line was placed between
the buer and the power amplier, resulting in an increase of 5 dB in linear output power
with third-order intermodulation distortion (IMD3) levels below 25 dBc. In [47], a line-
arizer using two diodes, a bandpass lter (BPF), and microstrip lines was designed for the
front end of the entire system, including the MIC power amplier, improving amplitude
modulation to amplitude modulation (AMAM) and amplitude modulation to phase mod-
ulation (AMPM) by 2 dB and 5 degrees, respectively.
In addition to linearity, a Doherty power amplier was employed in [50] to increase
backo eciency for telecommunication systems, achieving drain eciencies exceeding
28% at 6 dB backo power. In [25], a control circuit for load modulation was integrated
into a balanced power amplier design, resulting in 1016 W output power and a high
power-added eciency (PAE) of 25–40% across the 6–18 GHz band, including Ku-band.
Figure 10. Comparison of output power and PAE of GaN HEMT MMICs according to thermal
interface material and heat spreader combinations.
2.3. Other Additional Design Techniques
Ku-band GaN HEMT power amplifiers have been extensively studied to achieve high
output power as a replacement for TWTAs. However, several research groups have con-
ducted and published studies aimed at improving performance beyond just
output power.
To enhance linearity, some researchers have embedded linearizers within the MMIC
or designed separate linearizer modules in front of the MIC [
23
,
47
]. In [
23
], a linearizer
composed of a simple diode and the inductance of a microstrip line was placed between
the buffer and the power amplifier, resulting in an increase of 5 dB in linear output power
with third-order intermodulation distortion (IMD3) levels below
25 dBc. In [
47
], a
linearizer using two diodes, a bandpass filter (BPF), and microstrip lines was designed
for the front end of the entire system, including the MIC power amplifier, improving
amplitude modulation to amplitude modulation (AMAM) and amplitude modulation to
phase modulation (AMPM) by 2 dB and 5 degrees, respectively.
In addition to linearity, a Doherty power amplifier was employed in [
50
] to increase
backoff efficiency for telecommunication systems, achieving drain efficiencies exceeding
28% at 6 dB backoff power. In [
25
], a control circuit for load modulation was integrated
into a balanced power amplifier design, resulting in 10–16 W output power and a high
power-added efficiency (PAE) of 25–40% across the 6–18 GHz band, including Ku-band.
With the expansion of Ku-band satellite communication and UAV communication
services, further research is anticipated to improve the linearity and efficiency of Ku-band
GaN HEMT power amplifiers while maintaining high output power.
3. Conclusions
This article reviews the current state of Ku-band GaN HEMT high-power amplifiers
for satellite communications, which are actively being developed and researched. Recent
MMIC designs focus on GaN HEMT processes with gate lengths less than 0.2
µ
m on SiC
substrates, achieving PAEs exceeding 30% and output powers nearing 100 W. Utilizing
advanced heat dissipation packaging from MIC design, Ku-band GaN HEMT amplifiers
are anticipated to expand into both GaN-on-SiC and cost-effective GaN-on-Si MMICs.
Future developments will focus on enhancing linearity, backoff efficiency, and bandwidth
to support applications in military and civilian satellite and UAV communications. Ad-
ditionally, advancements in power efficiency, thermal management, and miniaturization
will enable broader adoption across mobile and satellite platforms, with new markets like
5G/6G expected to drive commercial and defense applications. These trends underscore
the ongoing importance of research in Ku-band GaN HEMT HPA technology.
Micromachines 2024,15, 1381 13 of 15
Funding: This work was supported by the Kyonggi University Research Grant 2024.
Acknowledgments: I would like to express my gratitude to Electronic Device Solution Inc. for the
industry–university connection with this research. I would like to express my sincere gratitude to
Junghyun Kim and Hyosung Nam of Hanyang University for their invaluable assistance with my
research and experiments. I also would like to express my sincere gratitude to Jaeku Ryu, Wireless
Protocol Engineer at Amarisoft and the author of the Sharetechnote website, for his permission to cite
useful illustrations about satellite communication. I would also like to thank the authors of the many
references cited in this paper for their figures, data, and expertise.
Conflicts of Interest: The author declares no conflicts of interest.
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