Whitepaper

Power GaN technology:
the need for efficient
power conversion
Dr Dilder Chowdhury - Device Architect GaN

One of the key challenges for industry is to reduce power losses. Facing
growing pressure from society and increasing government legislation for
reduced CO2 emissions, many industries are investing in more efficient
power conversion and increased electrification. These include automotive
electrification, telecom infrastructure, server storage, and industrial
automation, where there is significant growth in the use of power electronics.
That in turn is leading to increased demand for efficient, innovative,
high-power FETs based on GaN technology.

Within the semiconductor industry, the biggest motivationand driver for power device innovation is improved
efficiency in power conversion. Amongst the varioustechnologies, gallium nitride (GaN) technology shows thegreatest performance benefits, when compared to silicon(Si) and silicon carbide (SiC) solutions. Specifically, GaN
field-effect transistors (FETs) deliver the best efficiency
with low system costs while making systems lighter,smaller, and cooler.
Since power GaN transistors and specifically GaN-on-Sidevices were introduced to the market, significant
improvements in performance, reliability, cost, andavailability have taken place. More capable GaN powertransistors are becoming available to drive higher power.
Compatible with electric vehicle (EV) requirements they
are also well positioned for applications in data centers,telecom infrastructures, and industrial applications.
Automotive electrification leads the drive on efficiency
Electrification in the automotive sector is likely to be thebiggest beneficiary of the new power GaN FET technology.
To make all-electric vehicles (xEVs) efficient power
conversion is key as any power loss in xEVs can impact range,
one of the primary concerns surrounding electric vehicles.
Those losses also mean cooling systems are needed todissipate the heat generated, adding to a vehicle’s systemcomplexity and crucially weight.

While the automotive industry is working to address many
issues including battery weight and power density, there isa drive for significant improvement in the following areas:› Improved and more efficient power conversion› AC/DC onboard charging› DC/DC power conversion› DC/AC inverters to drive the traction motors› improved power density› simpler drivers and control scheme› Improved traction motors› improved efficiency› better torque and power› lower losses› higher dv/dt handling capability› Improved batteries, storage, and battery-management
systemMore efficient power conversion can help significantly inmany ways. For example, improving the efficiency of a 200
kW inverter from 95% efficiency to 99% efficiency reduces
power loss in a full load from 10 kW to 2 kW, or about onefifth. Not only is the loss 8 kW lower (which can be effective
for useful traction power), but the reduced need for cooling
can also help by reducing the cooling energy consumption
and on the size and weight of the cooling system. This in turn,can lead to a longer operating range or the same range witha smaller battery.

The above demonstrates the importance of achieving
extremely high efficiencies. Power GaN FETs based on
GaN-on-Si epitaxy (epi) help significantly in this area byoffering higher efficiency, but also by offering the scalabilityto support xEV growth ambitions. The simple Si fabrication(fab) process steps also allow the best cost roadmap forcommercial viability. Power GaN technology currently
supports 400 V battery systems with 650 V devices and canserve battery systems up to 800 V with 1200 V power GaNdevices. The power range for both can be up to 300 kW.
High voltage silicon transistors reaching their limits
High-voltage (HV) power semiconductor switches are thefundamental building blocks of any power conversion. Inthe absence of better alternatives, Si-based insulated-gate
bipolar transistors (IGBTs) currently dominate the traction
inverter market, even with significant technology maturity.
Incrementally higher efficiencies have been achieved
through improvements to Si IGBTs, and by combining
them with SiC diodes, but the possibility for further
improvement is limited. Si IGBTs are also fundamentally
limited in operating frequency, speed, and have poor hightemperature performance and low-current characteristics.
At higher frequencies, Si super junction (SJ) technology
dominates the market for AC/DC power factor corrector(PFC) and DC/DC power conversion. But inherent material
limitations, including switching crossover, conduction,and reverse recovery losses, limit the ability to achieve
higher efficiencies for high-frequency operation.
Power GaN steps up as replacement
In contrast, wide bandgap (WBG) materials, like GaN andSiC, are free from reverse recovery loss and can offer very
low switching cross-over losses (due to very fast turn on
and off characteristics) and lower conduction losses. WBG
materials with a higher critical electric field and higher
mobility together give the lowest source drain on-state
resistance RDS(on) for higher voltages and a significantly
better switching figure of merit. The WBG devices beginningto enter the market show significant promise and remove
many of the limitations naturally imposed by Si IGBT andSi 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 ofthe reduced component counts and higher efficiency withsimpler control schemes. The faster switching speeds andhigher frequencies of operation enabled by GaN powertransistors help improve signal control, higher cut-off
frequencies for passive filter designs, and lower ripple
currents, allowing for smaller inductors, capacitors, andtransformers. Consequently, the compact and smaller
system solution offers cost savings.

Figure 1 shows the cross-section for a GaN high electron
mobility transistor (HEMT), which works with the formationof the 2D electron gas (2DEG) due to the spontaneous andpiezoelectric polarization combined at the interface of GaN
and aluminium gallium nitride (AlGaN) layers. Epi is formedon the Si substrate via the seed layer. Then a graded layerof GaN and AlGaN layers are added before the pure GaN
layer grows. Finally, a thin layer of AlGaN forms the 2DEG.
Electron mobility in this layer is very high, hence the name.

Field Plate/sSource

Gate

Drain
Dielectric

AIGaN

GaN
2DEG Channel

Gate Dielectric

Si Substrate
Figure 1 GaN HEMT structure

Cascode simplicity and robustness
There are two main options for current power GaN FETs:enhancement mode (E-mode) or single-die normally offdevices and depletion mode (D-mode) or two-die normallyoff devices. While there are concerns on the stability andleakage currents of the E-mode gates, driving the two-die
normally off or cascode configuration D-mode devices
currently is simple and robust. Therefore, for operations
up to 1 MHz switching frequency, cascode GaN FETs arebest suited.
The GaN-on-Si two-die normally off configuration allows
significant design flexibility. Nexperia GaN FETs offer a ± 20 V
gate rating with an oxide / insulator gate, 4 V gate thresholdvoltage with 0 V real turn off, and low gate charge. Hence,simple Si drivers are suitable for use with these devices andfor 0–8, 10, or 12 V, any gate drive can be used. In contrast,
SiC technology generally requires at least 15 V, and a veryhigh-current driver with a negative gate drive capability
to turn off the device adds costs for the driver as well as
increased driver and switching losses. Nexperia GaN devices
also feature an exceptionally good, built-in, antiparallel diodethat helps with the robust freewheeling conduction path.
The cascode version offers significant freedom to makethe gate structure design and have the same robustness
expected by automotive customers.

Figure 2 shows how combining a low-voltage (30 V) robust Si
MOSFET with the cascode configuration D-mode GaN FET
can eliminate all of the major concerns regarding the poorgate structures of the E-mode devices and make the entire
usage very simple. Low-voltage Si-based gate structures area very mature technology which engineers are accustomed
to using.

Application and performance
Whether it is the AC/DC PFC stage, a DC/DC converter(Figure 5), or traction inverter (Figure 6), the basic building
block for most topologies is a half-bridge (Figure 7).
So when GaN FETs are compared against Si FETs in asimple boost converter, the GaN FET shows its superiorperformance.

DGS

Figure 5 AC-DC PFC stage and isolated DC-DC configuration
Figure 2 Cascode schematic

The reverse recover charge (Qrr) for low-voltage Si is verylow (see Figure 3) and allows the full potential of the power
GaN technology. In addition, driving a cascode device issimple. Cascode device operation is shown for different
bias situations in Figure 4. Power GaN FETs can be used ina bidirectional form to allow simpler bidirectional power
conversion.

VDC BusGaN
FETGaN
FETGaN
FETscable
PMSM
GaN
FET

GaN
FET

GaN
FETTraction Inverter

400

40

400

40

300

30300

30

200

20200

2010100

100-10144nC

ID (A)

00-100-102790nC-200-20-200-20-300

-30

-300

-30

-40

-400

-50

-500

-400

GaN

-500
7.4

7.45

7.5

7.55

7.6

7.65

Si-40
-50

7.4

7.7

7.45

7.5

7.55

7.6

7.65

7.7

Time (nS)Time (nS)

Figure 6 Traction Inverter

50Figure 3 Qrr, Reverse recovery charges for GaN FET vs Si FETCurrent (A)0-100

1001009990988097709660955094409330922091

1090Half Bridge Boost Converter
Vin = 240V, Vout = 400V, Fsw = 100 kHz
Same converter with replacement switches

V0
Q2
L1

VIN

VS

500

1000

1500

2000

2500

3000

C2

C100

Q1
GND

3500

Output Power (W)
GaN
Si

DDGGS

S

Reverse conduction 1(Vgs<Vth)

Cascode GaN HEMT
15A

Vgs = 8V

Vgs = 4V

10A

5A

Vgs = 3V0A

DFigure 7 Half bridge boost converter (GaN switch vs Si CoolMOS

GS
Forward conduction

Ids

Voltage (V)10060

VD (V)

Loss (W)500

ID (A)

Eﬃciency (%)

60050

VD (V)

Voltage (V)

60600
500

IIII

Vgs = 0V,1V, 2V

Reverse conduction 2(Vgs>Vth)GaN FET losses are significantly lower due to the absence ofreverse recovery losses and switching crossover losses. It is
possible to achieve nearly ideal turn on and off losses withdV/dt of approximately 200 V/ns. GaN switches are extremely
fast and routinely used in the radio-frequency amplifiers
at gigahertz frequencies. While at those frequencies they
operate at much lower voltages, it does show their ability as
both use the GaN HEMT structure.

-5A

-10A

-15A
-3V

-2V

-1V0VVds

Figure 4 GaN FET operation

1V

2V

3V

Since the GaN devices are very fast and can be used inapplications with high dV/dt and high dI/dt, care must
be taken to optimize the PCB layouts. To minimize
parasitic inductances, layout optimization is pretty muchfundamental to power GaN use.

To maximize the performance of surface-mount packages
and low inductance, high-current, high-performance
modules are essential.
Currently, using GaN devices in a traction inverter means
slowing it down significantly to save the motor windings.
They are nearly limited to a dV/dt of 10 V/ns, which holds
significant potential to improve the motors and take thefrequency up to 40 kHz to improve the power density
significantly.
Quality and reliability
Power GaN FET technology currently shows good quality
and reliability as multiple vendors have demonstrated
Joint Electron Device Engineering Council (JEDEC) andAutomotive Electronics Council (AEC) Q101 quality
standards. These are the minimum standards that mustbe fulfilled to demonstrate the reliability of power GaN
technology.
However, for GaN devices, existing quality standards
are not sufficient as the material is new and operates
differently. For example, dynamic RDS(on) or current collapse
phenomena is well known for power GaN FETs. Material
quality, trapping, and appropriate detrapping that is
responsible for the dynamic Rds(on) can be measured andgiven high-level confidence for its usage as the values aregetting better and are now around 10%.
Going even further beyond AEC Q101, qualifications forvalidating the reliability of GaN FETs under actual operating
conditions, several identical half-bridge circuits (withcontinuous current conduction mode) were prepared using
one high-side and one low-side GAN063-650WSA.
These were operated continuously for 1,000 hours assynchronous-boost converters with the following conditions:
Vin = 200 V, Vout = 48 V, Pout = 80 W, Tj = 175 °C, andfrequency = 300 KHz. There was no indication of any
degradation in the performance of any circuits for all samples
for the entire 1,000 hr duration of the test. Following hightemperature switching tests, all devices were tested forshifts in dynamic Rds(on), leakage current and thresholdvoltage. All parameters were found to be stable, with any
parametric shifts below the allowed levels.

The device specification has 800 V transient voltage
capability guaranteed by test to eliminate any concerns forover voltage spikes. Similarly, other overstresses like voltage
and temperatures were used and different acceleration
factors defined for the end-of-life and Failure in Time (FIT)
rates estimated for the application situations. As the volumeof products shipped increases, real field failure rates will bedetermined.
Ready for market
Power GaN technology is ready to take its place in themarket for efficient power conversion. We are alreadybeginning to see its adoption in non-automotive segments,
and it will soon be used in automotive applications which
can take advantage of lower losses and higher power
densities. While Si technology is well established in themarket, it is reaching its limits. As its benefits become clear
and the volume of installed devices grows, Power GaNtechnology will become the norm for high efficiency
power conversion.
To find out more visit www.nexperia.com/gan-fets

About Nexperia
Nexperia is a leading expert in the high-volume
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required by every electronic design in the world.
The company’s portfolio includes diodes, bipolartransistors, ESD protection devices, MOSFETs,GaN FETs and analog & logic ICs. Headquartered in
Nijmegen, the Netherlands, Nexperia annually ships
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Date of release:
May 2021