AN90021

Power GaN technology: the need for efficient power
conversion
Rev. 2.0 — 14 August 2020

application note

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Keywords

GaN FET

Abstract

Power Gallium Nitride(GaN) technology shows the greatest performance benefits against otherincumbent technologies including silicon (Si) technology. This application notes details thescalability and growth of the technology as well as the quality and reliability of Nexperia GaN
FETs. The performance of GaN FET power switches in various applications is examined.

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Power GaN technology: the need for efficient power conversion

1. Introduction
The biggest motivation and driver for semiconductor power device innovation is improved efficiency
in power conversion. Power Gallium Nitride (GaN) technology shows the greatest performance
benefits against other incumbent technologies, including silicon (Si) technology. A reduction in
power losses is the key challenge industries are facing. Pressure from society and government
legislation for reduced CO2 emission are all trending towards more efficient power conversion andelectrification.
Automotive electrification, telecom infrastructure, server storage and industrial automation using
power electronics are the biggest trends in the technology sector. GaN field-effect transistors(FETs) can enable us to achieve the best efficiency with lower system costs while making thesystem lighter, smaller and cooler. Specifically, electrification in the automotive sector can be thebiggest beneficiary of this new power GaN FET technology.
To make all-electric vehicles (xEVs) a real replacement for vehicles that have fossil-fuelled internal
combustion engines, efficient power conversion is key. There are many challenges, such as range
anxiety, harmful emissions, power losses, heat dissipation, cooling systems, system weight andpower densities. On this journey, industry is working in many ways to address these major issues toreduce range anxiety and harmful emissions. These are integral parts to make the xEV platforms acommercial and social reality.
Application areas in which there is a drive for significant improvement include:•••

Improved more efficient power conversion• AC-to-DC onboard charging• DC-to-DC power conversion• DC-to-AC inverter to drive the traction motors• Improved power density• Simpler driver and control scheme
Improved traction motors• Improved efficiency• Better torque and power• Lower losses• Higher dV/dt handling
Improved batteries, storage and battery management systems

Efficient power conversion helps significantly in many ways. An example is a 200 kW inverter with
95% efficiency versus 99% efficiency. The reduction in power loss at full load is from 10 kW to 2
kW, about one-fifth, meaning a significant decrease in the size and rating of the cooling system.
Not only is the loss 8 kW lower, (which can be effective used for traction power), but the smaller
cooling system can also help with reducing the cooling energy consumption and with size andweight. Hence, a longer range or smaller battery can be achieved.
This highlights the importance of achieving very high efficiencies. Power GaN FETs based on
GaN on Si epitaxy (epi) help significantly in this area, not only by offering higher efficiency but
also the most desirable scalability for growth to support the xEV growth ambitions. The simple Sifabrication (fab) process steps also allow the best cost roadmap for commercial viability. PowerGaN technology currently serves with 650 V products for 400 V battery voltage and can serve
up to 800 V battery systems with up to 1,200 V power GaN devices. The power range can be up
to 300 kW.

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Power GaN technology: the need for efficient power conversion

2. Power GaN FET switches
High-voltage (HV) power semiconductor switches are the fundamental building blocks of any power
conversion. Si-based insulated-gate bipolar transistors (IGBTs) are currently dominating this market
with significant maturity in the absence of any better alternatives. The improvement of Si IGBTs andcombining them with silicon carbide (SiC) diodes helps to achieve incremental higher efficiencies,
but has a limit on how much further improvement is possible. Si IGBTs are fundamentally limited
in frequency of operation, speed and poor high-temperature performance along with poor lowcurrent characteristics. Si super-junction (SJ) technology is dominating power conversion at higher
frequencies like AC-to-DC power factor correction (PFC) and DC-to-DC power conversion. To
achieve higher efficiency, these devices are fundamentally limited to size and cost due to theirinherent material limitation for high-frequency operation. These limitations can be summarised byswitching crossover losses, conduction, and reverse recovery losses.
In contrast however, wide band-gap (WBG) materials like GaN and SiC, as summarized in Table
1 are free from reverse recovery loss and can offer very low switching cross-over losses (dueto very fast turn on and off characteristics) and lower conduction losses. WBG materials witha higher critical electric field and higher mobility together give the lowest drain source on-state
resistance (RDSon) for higher voltages and a significantly better switching figure of merit. The WBG
devices beginning to enter the market show significant promise and remove many of the limitations
naturally imposed by Si IGBT and Si SJ devices. Some of the difficult switched-application
topologies where Si SJ FETs cannot be used due to diode reverse recovery can easily use power
GaN FETs and take full advantage of the reduced component counts and higher efficiency withsimpler control schemes. The faster switching speeds and higher frequencies of operation enabled
by GaN power transistors help improve signal control, higher cutoff frequencies for passive filter
designs, and lower ripple currents, allowing for smaller inductors, capacitors, and transformers.
Consequently, the compact and smaller system solution offers cost savings.
Table 1. Material properties
Material Band Gap Critical Electric Electron(eV)
Field Ecmobility µ02(MV/cm)(cm /Vs)

Thermalconductivity(W/cmK)

Saturation velocity(cm/s)

GaN

3.4

3.5

2,000

1.3

2.5 x 10

SiC

3.3

2.2

950

3.7

2 x 10

Si

1.12

0.23

1,400

1.5

1 x 10777There are two main options for current power GaN FETs: enhancement mode (E-mode) or single
die normally off devices and depletion mode (D-mode) or two-die normally off devices. The stability
and leakage currents of the E-mode gates are of concern but the two-die normally off or cascodeconfiguration currently offers peace of mind as driving these FETs is simple and robust. The Emode device drive is complex, especially for HV, high-power applications. For these applications,to avoid gate bounce and harmful shoot-through situations, it is necessary to have a high gate
threshold voltage and stable gate drive without worrying about overdrive, which currently is notachievable with existing E-mode technologies. For operations up to 1 MHz switching frequency,
cascode GaN FETs are best suited, although current HV power-conversion frequencies are around
only 300 kHz and traction inverter frequencies are still below 40 kHz. The GaN on Si two-die
normally off configuration allows significant design flexibility. Nexperia GaN FET offers a ±20 V
gate rating with an oxide/insulator gate, 4 V gate threshold voltage with 0 V turn off, and low gatecharge. Hence, simple Si drivers are suitable for use with these devices and for 0 – 8, 10, or 12
V, any gate drive can be used. In contrast, the SiC technology generally requires at least 15 V, avery-high-current driver with a negative gate drive capability to turn off the device adds costs forthe driver and increased driver and switching losses. Nexperia GaN also brings a very good antiparallel diode built-in that helps with the robust freewheeling conduction path. The cascode version
offers significant freedom to make the gate structure have the same robustness that automotive
customers are used to. This is valid for both Si FET and the GaN high electron mobility transistor(HEMT) with insulated gates.

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Power GaN technology: the need for efficient power conversion
Fig. 1 shows the cross-section for the GaN HEMT, which works with the formation of the 2D
electron gas (2DEG) due to the spontaneous and piezoelectric polarization combined at theinterface of GaN and AlxGa1-xN. GaN Epi is formed on the Si substrate via the seed layer and a
graded layer of GaN and AlGaN layers before the pure GaN layer grows. A thin layer of AlGaN
then forms the 2DEG. Electron mobility in this layer is very high, hence the name.
Gate Dielectric

Field plate/s
D

SourceGate

Dielectric

Drain

AlGaN
GaN

2DEG Channel

GS

Si Substrate

aaa-028116

aaa-032042

Fig. 1.

GaN HEMT cross section

Fig. 2.

GaN HEMT graphic symbol

Fig. 2 shows how combining a low-voltage (LV; 30 V) robust Si MOSFET with the cascodeconfiguration can eliminate all of the major concerns regarding the poor gate structures of the Emode devices and make the entire usage very simple. An LV Si-based gate structure is a very
mature technology and engineers are accustomed to using them.
The reverse recovery charge (Qrr) for the cascode power GaN is very low as shown in Fig. 3and mostly capacitive and allows the full potential of the power GaN technology. Actual reverse
recovery charge (Qrr) for the LV Si FET used here is only ~12 nC. The E-mode device channel
mobility is much lower compared to the channel mobility in a D-mode GaN HEMT channel.

Si MOSFET
Reverse turn-off, 41 mΩ
400 V, 40 A, 800 A/μs, Qrr = 2790 nC

GaN FET
Reverse turn-off, 41 mΩ
400 V, 40 A, 800 A/μs, Qrr = 144 nC

Fig. 3.

Qrr, reverse recovery charges for GaN FET vs Si MOSFET
Driving a cascode device is also very simple. Cascode device operation is shown in Fig. 4 andFig. 5 for different bias situations. Power GaN FETs can be used in a bi-directional form and also
allow simpler bi-directional power conversion.

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Power GaN technology: the need for efficient power conversion
D

D

D

G

G

G
S

S

S

Forward conduction

Reverse conduction 1(VGS < Vth)Reverse conduction 2(VGS > Vth)aaa-032059

Fig. 4.

GaN FET operation
I
ID
(A)

aaa-032043

200

VGS = 10 V

I
150

8V

100

7V

50

6V
5V

0

III
-50

VGS = 0 V

-100

VGS = 8 V

-150
-10

Fig. 5.

-8

-6

-4

-2

0

2

4

6

8
10
VDS (V)

Cascode GaN HEMFET forward and reverse output characteristics

Since power GaN transistors were introduced to the market, significant improvements inperformance, reliability, cost, and availability have taken place. More capable GaN powertransistors are becoming available to drive higher power to be compatible with the EV requirements
while being well positioned for applications in data centres, telecom infrastructures and industrial.

3. Scalability and growth
Si wafers are widely available in different sizes. GaN on Si wafers allow us to use 150 mm, 200 mm
metal organic chemical vapor deposition (MOCVD) reactors for epi growth and can be processedin mature silicon semiconductor fabs. There is a lot of Silicon fab capacity that can be utilised.
Processing is similar to standard Si processing, which is not so complex. Tremendous growthpotential is better addressed with power GaN technology on Si which is at the correct cost point.

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Power GaN technology: the need for efficient power conversion

4. Applications and performance
Whether it is AC-to-DC PFC stage, a DC-to-DC converter Fig. 6 or traction inverter Fig. 7 the basic
building block for most topologies is a half-bridge (Fig. 9). Hence when GaN FETs are comparedagainst Si FETs in a simple boost converter, then the GaN FET shows its superior performancedue to the differences in material properties (Fig. 8). All of these applications can take advantageof these benefits and hence reduce losses. The advantages of power GAN FETs compared to Si
IGBTs are clearly shown by light load performance, high temperature and high frequency operation
where the Si IGBT losses are comparatively high.
Isolated DC-DC(Bidirectional)

PFC
400 VDC BusPower 6.6 kW

GaN
FET

Si
FET

GaN
FET

GaN
FET

GaN
FET

GaN
FET
HV
Battery(280 VDC
To
420 VDC)

240 VAC
Single
Phase

GaN
FET

Si
FET

GaN
FET

GaN
FET

GaN
FET

GaN
FET

AC-DC Converter for OBC
and DC-AC Inverter

Fig. 6.

aaa-029378

AC-to-DC PFC stage and isolated DC-DC configuration
VDC Bus
GaN
FET

GaN
FET

GaN
FET

s
GaN
FET

GaN
FET

GaN
FET

TRACTION INVERTER

Fig. 7.

AN90021

application note

CABLE
PSMN

aaa-032041

Traction inverter

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Power GaN technology: the need for efficient power conversion

Q2
L1

Vin
C1

VS

VO
C2

Q1

GND

aaa-032060

Vin = 240 V, Vout = 400 V, fsw = 100 kHz
Same converter with replacement
switches

GaN
Si

Fig. 8.

Half-bridge boost converter(GaN switch vs Si CoolMOS)

Fig. 9.

Half-bridge boost converter

GaN FET losses are significantly lower due to the absence of reverse recovery losses andswitching crossover losses. It is possible to achieve near ideal turn on and off losses with dV/dt ofapproximately 200 V/ns.
GaN switches are extremely fast and routinely used in radio-frequency amplifiers at gigahertz
frequencies, although at much lower voltages but show the capability as both use the GaN HEMT
structure. Since the GaN devices are very fast and can be used in applications with high dV/dt andhigh dI/dt, care must be taken to optimize the PCB layouts. To minimize parasitic inductance layout
optimization is absolutely fundamental to Power GaN usage.
To maximize the performance of surface-mount packages that have very low inductance, highcurrent capability, then high-performance modules are essential and are being developed.
Using GaN FETs in a traction inverter requires the GaN FETs to be slowed down substantially toensure that the dV/dt does not damage the motor winding insulation.
Existing low price motor technology requires that the dV/dt is limited to around 10 V/ns, which
provides significant potential to improve the motor design and construction to enable an increase inoperating frequency up to >40 kHz leading to improved power density and efficiency.

5. Quality and reliability:
Power GaN FET technology currently showing good quality and reliability as multiple vendors
demonstrated Joint Electron Device Engineering Council (JEDEC) and Automotive Electronics
Council (AEC) Q101 quality standards. To demonstrate the reliability of power GaN technology,
these are minimums and must be fulfilled. For GaN, only the existing quality standards are
not enough as the material is new and operates differently. Dynamic RDSon or current collapse
phenomena is well known for power GaN FETs and this method for testing is introduced anddevices are verified. Material quality, trapping and appropriate de-trapping that is responsible forthe dynamic RDSon can be measured and given high-level confidence for its usage as the values
getting better and are now around 10%. We need to continuously look at the different failure modes
and take the devices to failure and understand the physics of failure.
Beyond AEC-Q101 qualifications, for validating the GaN FETs’ reliability in actual operating
conditions, several identical half-bridge circuits (with continuous current conduction mode) wereprepared using one high and one low side GAN063-650WSA. These were operated continuouslyfor 1000 hours as synchronous-boost converters with the conditions: Vin = 200 V, Vout = 480 V,
Pout = 800 W, Tj = 175 °C and frequency = 300 KHz. There is no indication of any degradationin the performance of any circuits for all samples for the whole duration of 1000 hours of test.
Following the high temperature switching tests, all devices were tested for shifts in dynamic RDSon,leakage current, and threshold voltage. All parameters were found to be stable, with any parametricshifts below the allowed levels.

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Power GaN technology: the need for efficient power conversion
The device specification has 800 V transient voltage capability guaranteed by test to eliminate any
concerns for over voltage spikes Similarly many other over stresses like voltage and temperatures
are used and different acceleration factors are defined for end of life and Failure in Time (FIT) rates
estimated for the application situations. As the volume of products shipped increases, real field
failure rates will be determined.
Power GaN technology is ready to take its place for efficient power conversion. We are beginningto see its adoption in the non-auto segments and soon it will be used in automotive applications
which can take advantage of lower losses and higher power densities. Si technology is wellestablished in the market but reaching its limit. Power GaN technology will be the norm in the future
as fear of unknown is reduced going forward.

6. References and further reading:[1] AN90005 - Understanding Power GaN FET data sheet parameters[2] AN90006 - Circuit design and PCB layout recommendations for GaN FET half-bridges

7. Revision history
Table 2. Revision history
Revision
Date
numberDescription

2.0

2020-08-14

Fig. 3 corrected, minor editorial changes.

1.0

2020-07-28

Initial version

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8. Legal information
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modifications or additions. Nexperia does not give any representations or
warranties as to the accuracy or completeness of information included hereinand shall have no liability for the consequences of use of such information.

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special or consequential damages (including - without limitation - lost
profits, lost savings, business interruption, costs related to the removalor replacement of any products or rework charges) whether or not suchdamages are based on tort (including negligence), warranty, breach ofcontract or any other legal theory.
Notwithstanding any damages that customer might incur for any reason
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for the products described herein shall be limited in accordance with theTerms and conditions of commercial sale of Nexperia.
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to information published in this document, including without limitation
specifications and product descriptions, at any time and without notice. This
document supersedes and replaces all information supplied prior to the
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warranted to be suitable for use in life support, life-critical or safety-criticalsystems or equipment, nor in applications where failure or malfunctionof an Nexperia product can reasonably be expected to result in personal
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suppliers accept no liability for inclusion and/or use of Nexperia products insuch equipment or applications and therefore such inclusion and/or use is at
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or warranty that such applications will be suitable for the specified use
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Power GaN technology: the need for efficient power conversion

List of Tables
Table 1. Material properties................................................. 3
Table 2. Revision history......................................................8

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List of Figures
Fig. 1. GaN HEMT cross section......................................... 4
Fig. 2. GaN HEMT graphic symbol...................................... 4
Fig. 3. Qrr, reverse recovery charges for GaN FET vs
Si MOSFET.......................................................................... 4
Fig. 4. GaN FET operation...................................................5
Fig. 5. Cascode GaN HEMFET forward and reverse
output characteristics........................................................... 5
Fig. 6. AC-to-DC PFC stage and isolated DC-DC
configuration......................................................................... 6
Fig. 7. Traction inverter........................................................ 6
Fig. 8. Half-bridge boost converter(GaN switch vs Si CoolMOS)............................................... 7
Fig. 9. Half-bridge boost converter.......................................7

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Contents
1. Introduction................................................................... 2
2. Power GaN FET switches............................................ 3
3. Scalability and growth................................................. 5
4. Applications and performance.................................... 6
5. Quality and reliability:.................................................. 7
6. References and further reading:................................. 8
7. Revision history............................................................8
8. Legal information......................................................... 9© Nexperia B.V. 2020. All rights reserved

For more information, please visit: http://www.nexperia.com
For sales office addresses, please send an email to: salesaddresses@nexperia.com
Date of release: 14 August 2020

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