IEEE Electrification Magazine - June 2020 - 7

efficiency versus 99% efficiency. A reduction in power loss
in a full load is from 10 to 2 kW, about one-fifth, which
means a significant decrease of the cooling system. Not
only is the loss 8 kW lower (which can be effective for
useful traction power), but the smaller cooling also can
help with reducing the cooling energy consumption and
on size and weight. 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) can 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 Si fabrication (fab) process steps also
allow the best cost roadmap for commercial viability.
Power GaN technology currently serves with 650-V
products for 400-V battery voltage and can serve up to
800-V battery systems with 1,200-V power GaN devices.
The power range can be up to 300 kW.

Power GaN FET Switches
High-voltage (HV) power semiconductor switches are the
fundamental building blocks of any power conversion. Sibased 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 and combining them with silicon carbide (SiC) diodes helps to achieve incrementally higher
efficiencies, but has a limit on how much improvement is
possible. Si IGBTs are fundamentally limited in frequency
of operations, speed, and poor high-temperature performance along with poor low-current characteristics. Si
superjunction (SJ) technology is dominating power conversion at higher frequencies like ac-dc power factor correction (PFC) and dc-dc power conversion. To achieve
higher efficiencies, these devices are fundamentally limited in size and cost due to their inherent material limitations for high-frequency operation. These limitations can
be summarized by switching crossover, conduction, and
reverse recovery losses.
In contrast, however, wide bandgap (WBG) materials
like GaN and SiC, as summarized in Table 1 are free
from reverse recovery loss and can offer very low
switching crossover 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 (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 with simpler
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.
There 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 cascade configuration currently offers peace of mind as
driving these FETs is simple and robust. The E-mode
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 not
achievable 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 gate charge. 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, a very-high-current driver
with a negative gate drive capability to turn off the de--
vice adds costs for the 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

TABLE 1. The material properties.
Material

Band Gap (eV)

Critical Electric
Field Ec (MV/cm)

Electron Mobility
µ0 (cm2/Vs)

Thermal Conductivity
(W/cmK)

Saturation
-Velocity (cm/s)

GaN

3.4

3.5

2,000

1.3

2.5 × 107

SiC

3.3

2.2

950

3.7

2 × 107

Si

1.12

0.23

1,400

1.5

1 × 107

	

IEEE Elec trific ation Magazine / J UNE 2 0 2 0

7



IEEE Electrification Magazine - June 2020

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https://www.nxtbook.com/nxtbooks/pes/electrification_september2022
https://www.nxtbook.com/nxtbooks/pes/electrification_june2022
https://www.nxtbook.com/nxtbooks/pes/electrification_march2022
https://www.nxtbook.com/nxtbooks/pes/electrification_december2021
https://www.nxtbook.com/nxtbooks/pes/electrification_september2021
https://www.nxtbook.com/nxtbooks/pes/electrification_june2021
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https://www.nxtbook.com/nxtbooks/pes/electrification_december2017
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https://www.nxtbook.com/nxtbooks/pes/electrification_september2014
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