Difference between GaAs and GaN power ampliﬁers.
The need for high power in the VHF, UHF, and microwave bands has led to transistors that
can easily supply tens to hundreds of watts at RF frequencies to 10 GHz and beyond.
Most of these devices are made with gallium arsenide (GaAs) or gallium nitride (GaN).
Table Of Contents
GaAs RF Power Amplifers
GaN RF Power Ampliﬁers
GaAs and GaN are compound semiconductors. Known as III-V semiconductors, they fall
into categories of elements with either three or ﬁve valence electrons. Boron, aluminum,
gallium, indium, and thallium are Category III elements. Nitrogen, phosphorus, arsenic,
antimony, and bismuth are Category V elements.
Combining a Category III element with a Category V element produces a covalent bond
with eight electrons, yielding a unique semiconductor. Such semiconductors have higher
electron mobility than silicon, so they’re more useful at higher frequencies. GaAs and GaN
have been found particularly useful for microwave power ampliﬁers.
You can use GaAs or GaN to make any type of transistor including the most popular, like
bipolar junction transistors (BJTs) and enhancement-mode MOSFETs. But other types
have emerged, such as heterojunction bipolar transistors (HBTs), metal-semiconductor
FETs (MESFETs), high electron mobility transistors (HEMTs), and laterally diffused MOS
(LDMOS). These transistors take advantage of the materials to produce the best
amplifying and power handling capability.
HBTs use the standard BJT conﬁguration but use different materials for the base and
emitter. One popular combination, a GaAs emitter and an aluminum-gallium-arsenide
(AlGaAs) base, yields very high gain at microwave frequencies out to 150 GHz.
A MESFET is essentially a JFET with a metal gate that’s used to form a Schottky junction
with the main conducting channel. The operation is depletion mode, where the device is
normally on and is turned off by a applying a negative gate voltage. MESFETs are usually
made with GaAs and have high gain at microwave frequencies.
The HEMT is a variation of the MESFET. Also called a hetrostructure FET (HFET) or
modulation-doped FET (MODFET), it is usually made with GaAs or GaN with extra layers
and a Schottky junction (Fig. 1). Depletion mode is the most common conﬁguration. The
pseudomorphic or pHEMT version improves performance by using extra layers of indium
to further speed electron movement. These devices work at frequencies up to 20 GHz or
1. The substrate in a GaN HEMT is usually sapphire or silicon carbide for best het
reduction, although silicon can be used. The 2DEG means two-dimensional electron gas,
a layer of gas made of electrons that can move in any direction but vertical.
The LDMOS transistor is simply a special version of the standard enhancement-mode
MOSFET. Designed for high power dissipation, it can achieve hundreds of watts at
frequencies to about 6 GHz. This format is used primarily with silicon devices.
GaAs RF Power Ampliﬁers
Most GaAs products are ICs, speciﬁcally monolithic microwave ICs (MMICs). These
MMICs are mostly low-signal-level gain blocks. Such ampliﬁer blocks use HBTs or
MESFETs, but some use pHEMTs. Frequency ranges up to 30 GHz are available. The
actual high-frequency cutoff (ft) or the unity gain-bandwidth for GaAs devices is in the
range of 150 GHz, although not many devices or IC ampliﬁers are available with that
reach. Such MMICs are widely used in most microwave equipment designs including
radios, satellites, radar, and electronic warfare products.
Cable TV (CATV) systems represent a major application for GaAs ampliﬁers. CATV
systems use a mix of ﬁber and coax to carry digital TV signals and high-speed Internet
service over a bandwidth from about 40 MHz to well over 1 GHz. The coax line feed
ampliﬁers used along the distribution routes of long coax runs use GaAs ampliﬁers for
their high gain and low noise characteristics.
GaAs power ampliﬁers are available with power levels up to about 5 W. That’s roughly the
upper power limit for GaAs devices, since they cannot withstand the high voltages,
currents, and heat levels of silicon or GaN. But by using multiple devices in push-pull or
parallel or combining ampliﬁer outputs in transformers or networks, power levels up to
about 20 to 40 W are possible.
The most widely used GaAs power ampliﬁers are those for cell phones. They’re made in a
variety of frequency ranges to cover the various ﬁxed cellular bands. The power level
ranges from 27.5 to 28 dBm (0.4 to 0.5 W). They’re class AB linear ampliﬁers with
maximum power-added efﬁciencies (PAEs) in the 20% to 50% range.
Note: PAE is the more common measure of RF power ampliﬁers today. Accounting for
the input power to drive the ampliﬁer, it is the ratio of the RF power output less input
power to total dc power consumed expressed as a percentage:
PAE = 100 (Pout – Pin)/Pdc
The Skyworks SKY77707/9 LTE power ampliﬁer uses a GaAs substrate (Fig. 2). However,
the transistors are HBTs made with indium-gallium-phosphide (InGaP) (Fig. 3). This is a
2. The structure of an InGaP HBT shows a GaAs substrate, upon which is built the
collector, base, and emitter layers. The resulting transistor has high gain and power at the
lower microwave frequencies below about 20 GHz.
3. The Skyworks SKY77707 GaAs/InGaP HBT LTE cell-phone power ampliﬁer is powered
with 3.4 V dc, delivers 28 dBm of power, and comes in a 3- by 3- by 0.9-mm package.
GaN RF Power Ampliﬁers
GaN transistors have been around for about 10 years. Thanks to Department of Defense
(DoD) initiatives, GaN has developed quickly to become the newest star in the microwave
power-ampliﬁer universe. Initially developed for improvised explosive device (IED)
jammers in Iraq, GaN has emerged as the technology of choice for all new microwave
and millimeter-wave electronics including radar, satellite, communications, and electronic
GaN’s high power density, or its ability to dissipate heat from a small package, makes it so
impressive. While GaAs has a basic power density of about 1.5 W/mm, GaN has a power
density ranging from 5 to 12 W/mm. It also has high electron mobility, meaning it can
amplify signals well into the upper-gigahertz ranges. Typical transistor fTs are up to 200
GHz. Furthermore, GaN can do all this at relatively high breakdown voltages levels, up to
80 V or so.
The downside to GaN is its high cost. The materials are expensive, and the processes to
make devices are costly. As volume increases, production costs will come down but will
still remain way above the process costs of bulk CMOS or even higher-cost GaAs
GaN technology today mainly focuses on microwave and millimeter-wave power
ampliﬁers. Individual ampliﬁers can achieve power levels of tens of watts. In other
parallel/push pull/Doherty conﬁgurations, power levels of hundreds to thousands of watts
are possible. Most applications are military related, with phased array radar modules and
satellite power ampliﬁers the most common. GaN power ampliﬁers are beginning to
emerge for cellular basestations as well.
Also, GaN ampliﬁers will begin to replace travelling wave tube (TWT) ampliﬁers in
satellites and radar. TWTs are capable of hundreds even thousands of watts. Primarily,
though, GaN power ampliﬁers will replace TWTs in lower-power satellites and radar.
The Array Wireless LPA200-250-10W-20-1230 GaN power ampliﬁer operates in the 2.0- to
2.5-GHz range with an output of 10 W (Fig. 4). It’s a linear ampliﬁer with an error vector
magnitude (EVM) in the 2% to 2.5% range. Applications include video surveillance links,
military unmanned aerial vehicles (UAVs) and unmanned ground vehicles (UGVs), and
4. The Array Wireless LPA200-250-10W-20-1230 linear RF power ampliﬁer produces 10 W
of output in the 2.0- to 2.5-GHz range using GaN transistors.
The GaAs product market is huge at several billion dollars per year. The market for GaN is
signiﬁcantly smaller, depending on whom you ask. It may be $150 million, but it is growing
at the expense of GaAs. Work continues on GaN production processes to reduce costs
as volumes increase. GaAs continues to dominate the microwave space with small-signal
MMICs and low-noise ampliﬁers (LNAs) as well as low-level power ampliﬁers for cell
phones. That will continue. But as GaN costs come down and GaN invades the
small-signal space, GaAs will no doubt lose market share.