High Efficiency Power Converters with Gallium
Nitride Transistors
Mark Nakmali
University of Oklahoma
Oklahoma City, Oklahoma
Mark.u.nakmali-1@ou.edu
Yutian Cui
University of Tennessee
Knoxville, Tennessee
Ycui7@vols.utk.edu
Abstract— The abilities of gallium nitride field effect transistors
are evaluated. A material comparison on the limitations and costs
of gallium nitride, silicon, and silicon carbide is explored. The
theory of operation of a gallium nitride transistor is explained and
its use and size implications in a buck power converter is
demonstrated. The efficiency of the converter circuit is then
explained as well as the means to reduce losses. Two simulations
using silicon and gallium nitride respectively are used to
demonstrate the comparison of efficiency and on-switching period.
Finally, the performance of a real gallium nitride buck converter
circuit is used to check the validity of the simulated gallium nitride
on-switching period.
Index Terms—Gallium nitride, HEMT, power converter, high
efficiency, power density, direct band gap material, III-V nitride
materials, conduction loss, switching loss
I. INTRODUCTION
It is not any new evidence, the capabilities of Gallium
Nitride (GaN). Since the first report in 1991, it has become a
topic of growing interest [1]. Because GaN is a direct band-gap
material, the energy from electron-hole recombination can turn
directly into light, which gave it its first place hold in
optoelectronic devices [1]. However, its ability for high
efficiency transistor performance was discovered when the two
dimensional electron gas was discovered. Even with more
research focus on GaN devices, it still has much room for
improvement when compared to Silicon devices.
Dr. Leon Tolbert
University of Tennessee
Knoxville, Tennessee
Tolbert@utk.edu
The graph in Figure 1 shows the breakdown voltage
compared to the specific on-resistance, as well as a scatter plot
representing where current technology is. A low on-resistance
coupled with a high breakdown voltage is desired in a switching
device, making the bottom right of this graph become the goal
for the most ideal transistors. It is important to point out that
silicon technology is already nearing its physical limit, due to its
research maturity; however, GaN still has a large margin for
improvement before reaching its limits. With the improvement
of vertically stacked GaN devices, the breakdown voltage will
become larger [3], and its already low on resistance will bring
the necessary improvements in GaN technology.
There are many other properties of GaN that set it above the
other materials. When looking at Figure 2 below, it is easy to see
that GaN has the highest qualities in electric field, energy gap,
and electron velocity. While GaN is only slightly higher than
Silicon Carbide (SiC) in energy gap and electron velocity, the
fact that GaN is cheaper than SiC should be taken into
consideration [5]. The graph also shows that the only downside
that GaN has is its poor thermal conductivity and melting point,
making it important to have proper cooling that could increase
costs. However, this can be also be decreased by growing the
GaN device on a SiC substrate and cooling the device by using
the SiC's high thermal conductivity as mentioned in [1] or it can
simply be reserved for lower power applications. After taking
into account these thermal requirements, GaN has the qualities
that give the device the high voltage and high frequency
characteristics that are desired in transistors.
Fig. 1 Comparison of the voltage breakdown versus the on-resistance including
the state of current transistors [2].
Fig. 2 Material property comparison between Si, SiC, and GaN with emphasis
on desirable switching transistor properties [4].
It is in these switching transistors that the improvements lie.
Currently, the most widely used transistors are made of silicon.
These are the most appealing for their cheap costs and
manufacturing availability. However, silicon is not necessarily
the best material to use in terms of efficiency. This is where
gallium nitride comes in.
The aim of this project is to show the efficiency of GaN as
compared to the state of art silicon MOSFET. Both materials
were placed in a synchronous buck or step down converter
circuit because of its simplicity and its wide use in power
electronics converters. That way, the results of this can be
projected onto real world converter applications in order to
demonstrate its efficiency and impact on shrinking the size of
the circuit.
II. BACKGROUND
When comparing the performances of GaN versus silicon
devices, the first thing to look at is the device specifications
between the both of them. There are many characteristics that
are desired in a transistor for the application of power converters.
The main characteristics are a high drain-to-source voltage, VDS,
a low on-resistance, RDS(on), and a small size. Table 1 is a
compilation from the data sheets of EPC-CO’s eGaN [6] and
IR’s HEXFET [7].
As is shown below in Table 1, to have a fair comparison, the
gallium nitride transistor is selected to have the same drain-tosource voltage rating , it excels in most of the other categories.
Its on-resistance and size are considerably lower than silicon’s,
allowing for a lower amount of conduction loss and space taken
on the circuit board and also lower junction capacitances due to
the smaller sizes. It also has a higher drain current rating than
silicon. One drawback on GaN is that its specified gate-to-source
voltage is very small compared to silicon. This means that extra
care must be taken when driving the gate in order to not let any
voltage overshoots break the GaN device. Nevertheless, GaN’s
advantages greatly outweigh its disadvantages when compared
to silicon.
Another aspect to consider is the price comparison of GaN
and silicon devices. On a first look, GaN is vastly more
expensive than that of silicon, costing $1900 for a two inch
diameter bulk GaN substrate compared to $25-50 for a six inch
silicon substrate [8]. It is because of this that using silicon as a
substrate in the manufacturing of GaN devices has become a
large interest. However, using a different substrate can cause
lattice defects which can affect the performance of the device.
Because of this, as [9] explains, buffer layers of aluminum
nitride or aluminum gallium nitride can be used to lessen the
strain on the GaN lattice, but this can add its own costs and
TABLE II. PRICE COMPARISON BETWEEN 60V AND 100V EPC-CO EGAN
FET VERSUS COMPETING SILICON MOSFETS [10]
complexities. Also, it is important to keep in mind that because
GaN devices are much smaller, more of them can be made on a
single wafer, lowering costs. According to EPC-CO [10], they
claim to have reduced the costs of their “eGaN” devices down
to be even cheaper than their competing silicon MOSFET
devices as shown in Table 2 above.
As shown by [11] in Figure 3 below, to fabricate a GaN
transistor, it starts with a silicon or silicon carbide substrate. A
layer of Aluminum Gallium Nitride (AlGaN) is grown on the
epitaxial layer to provide a base for the intrinsic GaN. A very
thin layer of AlGaN is then grown on the GaN, providing the
interface needed to create the two dimensional electron gas
(2DEG). Finally, the contacts and dielectric are placed on the
device. Because the 2DEG is normally conducting and it is more
desirable for it to be normally off, the configuration of the device
is set up such that the gate contact is placed inside the top AlGaN
layer such that it can be in enhancement mode [11]. There are
many other ways to accomplish this as shown in [12]; however
the main idea is to make it such that the 2DEG does not span
across the source and drain until the gate contact is charged.
GaN’s characteristically high electron velocity is caused by
the presence of the 2DEG. As [1] explains, the formation of the
2DEG is due to the boundary between the very thin AlGaN and
the intrinsic GaN material. There are two reasons why this
interface creates the 2DEG. The first is because AlGaN has a
larger bandgap than GaN, causing some of the electrons to
diffuse across the boundary. The second is due to what [1] refers
to as “polarization doping.” This is caused by both spontaneous
interactions and the strain in the AlGaN layer on top of the GaN
forming polarizations piezoelectrically. It is the sum of these two
polarizations that allow significant 2DEG densities. This 2DEG
allows there to be an abundance of free electrons even in the
normally inert GaN material.
TABLE I. GALLIUM NITRIDE AND SILICON SPECIFICATION COMPARISONS
[6] [7]
VDS [V]
RDS(on) [Ω]
ID [A]
VGS [V]
Size [mm2]
Gallium Nitride
30
0.001
60
-4<VGS<6
13.915
Silicon
30
1.3
42
-20<VGS<20
30
Fig. 3 Cross-sectional drawing of EPC-CO’s eGaN [11].
The faster switching ability of GaN also contributes to
minimizing the switching loss of the device. Looking at the
diagram above, it is caused by the transition of the device from
off to on and on to off. It is in these transitions that the voltage
and current are non-zero, making their product become a
component in the calculation of its power loss. The faster the
ability of the device to switch, the shorter the period, tsw(on) and
tsw(off), becomes, causing the area under the peaks of the power
curve to become thinner, and therefore, smaller, causing the
switching loss to become smaller.
III. METHODOLOGY
Fig. 4 Capacitor and Inductor values exponentially decay with the increase of
switching frequency [13].
With this abundance of free electrons and its high mobility,
it is able to achieve such high switching speeds. This can allow
higher frequencies which are crucial in the role of reducing the
size of the passive components in the circuit. As seen above in
Figure 4, [13] shows the exponentially decaying relationship
between capacitor and inductor values versus the increasing
frequencies. With this increase in frequency, the size of the
circuit can be allowed to be chip-scale while still maintaining
the same power specifications.
Energy efficiency particularly appeals to power electronics
in the application of converters that consume less power and
generate less heat. In the performance of the device, there is a
loss of power in the form of switching loss, conduction loss, and
gate loss [14]. Because the gate loss with GaN devices on
hundred kHz operation has a very small impact on the power
loss, it is mentioned but assumed as negligible in this project’s
calculations. However, the losses due to conduction and
switching have been taken into consideration as shown below in
Figure 5 [15].
The conduction loss plays into effect when the device is
activated and is conducting electricity. Keeping in mind that the
power lost is a product of the device’s resistance and the square
of the current through it, it is important that the on-resistance of
the device is kept as low as possible. This is also taken care of
by the 2DEG and the purity of the GaN lattice offering little
scattering, giving GaN a very low on-resistance [5]. With this
lower on resistance, the conduction loss is lowered.
The theory of operation in a buck converter circuit is
demonstrated by [16] as shown at the bottom in Figure 6 [17]. A
pulse width modulation (PWM) control signal is used to
subsequently turn on and off the high side and low side
transistors. When the high side switch is turned on, there is a
path for the supply voltage, Vin, to flow through the inductor,
thus charging it, and then to the output, Vout. The high side switch
is then turned off and the low side switch turned on, separating
the supply voltage from the output. It is during this phase that
the inductor provides current to the output. This process is then
repeated as each transistor is turned off and on.
The level of the output voltage is dependent on the duty cycle,
D, of the PWM control signal:
𝑉𝑜𝑢𝑡 = 𝐷𝑉𝑖𝑛
In order to determine the values of the other passive
components of the circuit, the following equations from [13] are
used:
𝐷(𝑉𝑖𝑛 − 𝑉𝑜𝑢𝑡 )
𝐿=
2𝐼𝑜𝑢𝑡 𝑓𝑠
and
(1 − 𝐷)
𝐶𝑜𝑢𝑡 =
,
∆𝑉
8𝑓𝑠 2 𝐿( 𝑜 )
𝑉𝑜𝑢𝑡
where L is the inductance, Coutis the capacitance, Iout is the output
current, fs is the switching frequency, and ∆Vo is the output ripple.
In terms of inductor and capacitor size, note that they become
smaller as the switching frequency increases. This is why
increasing the switching frequency will shrink the size of the
circuit as was previously demonstrated in Figure 4. Even after
finding the values of the passive components, due to losses and
packaging parasitics, the duty cycle is adjusted again during
testing to produce the desired output.
Fig. 6 A synchronous buck converter circuit [17].
Fig. 5 Representation of the switching and conduction losses shown in one
period of transistor operation. [15]
IV. RESULTS
The following, Figure 7, is a graph of one of the simulations
showing power out and power in. For all of the waveforms, the
steady state was reached before 200μs. It was only after this time
that measurements were taken to ensure that any transient value
fluctuations were not included in any of the analyses.
Fig. 7 The input power (green) and the output power (maroon).
After creating the waveforms, the average of both the Pin and
Pout are taken within the period of 240μs and 360μs. These
values were then used to find the efficiencies across all sets of
variables. Below in Figure 8 are plots of these efficiencies.
3V Input Performance
Efficiency
1
0.8
0.6
0.4
0.2
0
12.175
24.06
36.437
Output Power [W]
4V Input Performance
Efficiency
1
0.8
0.6
0.4
0.2
0
12.09
24.353
36.514
Output Power [W]
5V Input Performance
1
Efficiency
In order to compare the performance of both GaN and silicon
devices, their respective simulation models were taken from the
manufacturer’s open source codes. The GaN device is modelled
after Efficient Power Conversion’s EPC2023 enhancement
mode field effect transistor [6]. The silicon device is modelled
after International Rectifier’s IRFH8307 single n-channel
HEXFET Power MOSFET [7]. Each transistor was put into their
respective circuits using the simulation software, LTSPICE.
From the measurements, the device performances were then
analyzed for losses. Only the loss for the high side transistor was
considered. To calculate these losses, [14] provided the
equations for both switching, conduction, and gate losses. The
switching loss is calculated by
𝑉𝑑𝑠 𝐼𝑑
𝑃=(
)(𝑡𝑠𝑤(𝑟𝑖𝑠𝑒) + 𝑡𝑠𝑤(𝑓𝑎𝑙𝑙) )(𝑓𝑠𝑤 )
2
and the conduction loss is calculated by
𝑉𝑜𝑢𝑡
𝑃 = (𝐼𝑑 )2 𝑅𝑑𝑠(𝑜𝑛)
.
𝑉𝑖𝑛
The gate loss is calculated as,
𝑃 = 𝑄𝑔 𝑉𝑓𝑠𝑤 ,
where Qg is the gate charge however, as mentioned before, it is
assumed to be negligible.
After finding the amount of loss generated, the efficiency of
the circuit was then analyzed. This is first done by calculating
the input power, Pin, as
𝑃𝑖𝑛 = 𝑉𝑖𝑛 𝐼𝑠𝑜𝑢𝑟𝑐𝑒
and the output power, Pout, as
𝑃𝑜𝑢𝑡 = 𝑉𝑜𝑢𝑡 𝐼𝑜𝑢𝑡 .
The efficiency is then calculated as
𝑃𝑖𝑛
η=
.
𝑃𝑜𝑢𝑡
During the testing of the circuit, the output voltage was
designed to remain at a constant steady-state voltage of 1.2V
over all variations. The performance of the devices were tested
by varying the input voltage from 3V, 4V, 5V, and 6V, the
output current was varied by changing the value of the load
resistor to make the output power range from 12V, 24V, and
36V, and the frequency of the device was varied from 200kHz,
600kHz, and 1MHz. Finally, the switching characteristics of the
devices were explored.
In order to test the validity of the simulation’s outputs, a
physical buck converter circuit was made using gallium nitride
transistors and the switching times were compared to the
measurements of the simulation. This was checked to compare
the length of the on-switching transient period. Because the
switching losses rely so heavily on the switching transient and
they account for the greater majority of the amount of loss in a
transistor, it is a good indicator of the performance of the device.
0.8
0.6
0.4
0.2
0
12.038
24.31
Output Power [W]
36.236
6V Input Performance
Efficiency
1
0.8
0.6
0.4
0.2
0
12.169
23.996
36.529
Output Power [W]
Fig. 8 Performance of GaN (green) and silicon (blue) across all variations of
supply voltage (3-6V), output power (12-36W), and frequencies (200kHz dark,
600kHz medium, 1MHz light).
As can be seen in the Figure 8 above, there are many
different factors that play into the outcome of the efficiency. It
is clear that GaN, represented by the green lines, has a higher
efficiency than Silicon, represented by the blue lines. The
different shades of colors from dark to light represents the
performance of the device as the frequency is increased. With
the darker colors on top and the lighter colors on bottom, it
shows that efficiency is only slightly lowered with the increase
of frequency. As the input voltage increases between graphs,
efficiency decreases. Lastly, it is important to point out that with
the increase in output power, the efficiency goes down, resulting
in the negative slope seen in the graphs.
The turn on transition of the GaN and silicon simulations
were then compared as shown below in Figure 9. The beginning
of the switching period was taken as the first instance in which
the voltage increased. The end of the switching period was taken
as the maximum of the first peak. It should also be stated that
very small inductances around 100pH, were placed into the
simulations to represent the parasitic inductance that happens
with physical circuits. This is the reason why there are the
fluctuations at the end of the switching periods. In these
comparisons, only the on-switching periods were observed.
Fig. 10 The switching period of a physical synchronous buck converter
circuit as measured by an oscilloscope.
The simulated measurements in Figure 9 show that the
switching period of GaN is 2.2ns and the switching period of
silicon is 28.7ns. This means that the switching period of silicon
is about thirteen times longer than the switching period of GaN.
This is one of the main reasons why the efficiency of GaN is
better than that of silicon, especially under higher switching
frequency, as can be proved by [14].
After determining the performance of the simulations, the
performance of the GaN simulations were then compared to a
real GaN buck converter circuit. As can be seen above in Figure
10, the oscilloscope graph on the top can be compared to the
GaN simulated graph in Figure 9. According to the oscilloscope
graph, the measured switching period was 3.5ns, while the
simulated graph had a switching period of 2.2ns. This means that
the simulated graph has a 1.3ns shorter on-switching period than
that of the physical switching period. This deviation could be a
product of the imperfections of a physical circuit or it could be
that the provided simulated component from the manufacturers
was slightly exaggerated. Nonetheless, the high switching
ability of GaN is effectively demonstrated in the simulation and
the physical circuit and its advantages over silicon devices
remain valid.
V. CONCLUSION
Gallium Nitride devices are a great topic of interest because
of the many advantages that it brings to the table. Not only that,
but it still has much room to improve, giving promises to an ever
improving field of power electronics as well as many other
applications. With its high frequency capabilities, devices will
be able to become smaller and data communications can be
transferred at a much higher and more reliable rate. Though its
price may be high, as manufacturing methods are already on
their way toward improvement and soon the costs will also begin
to drop. Obviously, it will not replace the already massive silicon
market any time in the near future, but its capabilities remain
something to keep an eye on.
ACKNOWLEDGMENT
Fig. 9 Comparison of on-switching period lengths for GaN (green) and silicon
(blue)
This work was supported in part by the Engineering Research
Center Program of the National Science Foundation and the
Department of Energy under NSF Award Number EEC1041877 and the CURENT Industry Partnership
Program.
REFERENCES
[1] M. A. Khan, G. Simin, S. G. Pytel, A. Monti, E. Santi, and J. L.
Hudgins, "New Developments in Gallium Nitride and the Impact
on Power Electronics," Power Electronics Specialists
Conference, 2005. PESC '05. IEEE 36th, pp.15,26. 16-16 June
2005.
[2] M. A. Briere, “GaN provides revolutionary improvements in
power,” International Rectifier, AspenCore Inc., Internet:
http://www.electronicproducts.com/Power_Products/Power_Ma
nagement/GaN_provides_revolutionary_improvements_in_pow
er.aspx 2015 [July 19, 2015].
[3] S. Chowdhury, B. L. Swenson, M. H. Wong, U. K. Mishra,
“Current status and scope of gallium nitride-based vertical
transistors
for
high-power
electronics
application,”
Semiconductor Science and Technology, vol. 28, 2013. [On-line].
Available:
http://iopscience.iop.org/02681242/28/7/074014/pdf/0268-1242_28_7_074014.pdf [July 19,
2015].
[4] J. Millán, P. Godignon, X. Perpiñá, A. Péraz-Thomás, and J.
Rebollo, “A survey of wide bandgap power semiconductor
devices,” IEEE Transactions of Power Electronics, vol. 29, no. 5,
May 2014
[5] M. A. Briere, “GaN based power devices: cost-effective
revolutionary performance,” Power Semiconductor Materials,
[On-line]
Issue
7,
pp.
2,
Available:
http://www.irf.com/pressroom/articles/560pee0811.pdf [July 19,
2015].
[6] “EPC2023 – enhancement mode power transistor preliminary
specification sheet,” Efficient Power Conversion Corporation.
Internet:
http://epcco.com/epc/Portals/0/epc/documents/datasheets/EPC2023_preli
minary.pdf June, 2014, [July 19, 2015].
[7] “StrongIRFET™ IRFH8307TRPbF – HEXFET power
MOSFET,”
Internet:
http://www.irf.com/productinfo/datasheets/data/irfh8307pbf.pdf May 19, 2015. [July 19,
2015].
[8] “When will bulk GaN be price-competitive with silicon?” Solid
State
Technology
Internet:
http://electroiq.com/blog/2012/11/when-will-bulk-gan-be-pricecompetitive-with-silicon/ Nov. 12, 2012. [July 19, 2015].
[9] “Gallium nitride (GaN) versus silicon carbide (SiC) in the high
frequency (RF) and power switching applications,” Microsemi
PPG
Available:
http://www.digikey.com/Web%20Export/Supplier%20Content/
Microsemi_278/PDF/Microsemi_GalliumNitride_VS_SiliconCa
rbide.pdf?redirected=1 [July 19, 2015].
[10] A. Lidow and J. Strydom, “eGaN® FETs deliver the performance
of GaN at the price of silicon,” Efficient Power Conversion
Corporation.
[On-line]
Available:
http://epcco.com/epc/Portals/0/epc/documents/papers/WP017%20eGaN%
20FETs%20Deliver%20on%20Performance%20AND%20Price.
pdf 2015 [July 19, 2015].
[11] A. Lidow and J. Strydom, “Gallium nitride (GaN) technology
overview,” Efficient Power Conversion Corporation. [On-line]
Available:
http://epcco.com/epc/Portals/0/epc/documents/papers/Gallium%20Nitride
%20GaN%20Technology%20Overview.pdf 2012, [July 19,
2015].
[12] E. A. Jones, F. Wang, and B. Ozpineci, “Application-based
review of GaN HFETs,” IEEE, pp. 24-29, 2014.
[13] K. Shenai, K. Shah, H. Xing, "Performance evaluation of silicon
and gallium nitride power FETs for DC/DC power converter
applications," Aerospace and Electronics Conference
(NAECON), Proceedings of the IEEE 2010 National, pp.317,321,
14-16 July 2010.
[14] C. Cooper, “Fundamentals of buck converter efficiency,” Avnet
Electronics Marketing Americas, [On-line] Available:
http://electronicdesign.com/power/fundamentals-buckconverter-efficiency May 23, 2013, [July 19, 2015].
[15] S. Barua, “Desired characteristics of fully-controlled power
semiconductors,”
Internet:
http://protorit.blogspot.com/2013/03/fully-controlled-powersemiconductors.html 2011, [July 19, 2015].
[16] N. Mohan, T. M. Undeland, W. P. Robbins, “Power electronics,”
Hoboken, NJ: John Wiley & Sons, Inc., 2003, pp. 161-200.
[17] A. Lidow, J. Strydom, M. Rooij, and Y. Ma, “Buck converters,”
GaN Transistors for Efficient Power Conversion, ch. 5,
Available:
http://www.edn.com/design/powermanagement/4399114/Buck-Converters Oct. 22, 2012, [July 19,
2015].
[18] A. N. Alderman, “Where are the high-voltage GaN products?”
Power
Electronics,
Internet:
http://powerelectronics.com/discrete-power-semis/where-arehigh-voltage-gan-products June 1, 2010, [July 19, 2015].
[19] D. Schelle and J. Castorena, “Buck-converter design
demystified,” Power Electronics Technology, June 2006,
Available:
http://powerelectronics.com/sitefiles/powerelectronics.com/files/archive/powerelectronics.com/
mag/606PET25.pdf [July 19, 2015].