Fujitsu Laboratories has developed a GaN power transistor for
mains PSUs, claiming it to have the highest current density
yet.
GaN is inherently a fast technology, and the material withstands
voltages better than both Si and GaAs.
This particular HEMT (high electron mobility transistor) has
been engineered to be completely off when less than 2V is applied
to the gate. "The technology blocks the flow of current from power
supplies in stand-by mode," explained the firm, claiming: "GaN
HEMTs have less than one-fifth the on-state loss of silicon
transistors and have excellent high-speed characteristics, so that
switching losses are less than 1% of those of silicon
transistors."
In 2008, the Lab developed a RF power-HEMT for basestations with
a three-layer cap structure by
sandwiching an aluminium-nitride layer between n-type GaN
layers.
This AlN layer produced a gate threshold voltage of 0.5V, "it
could not be used for power supplies, which need to apply 2 to 4V
in order to apply hundreds of volts at the drain", said Fujitsu.
"In addition, power supplies effectively need to have an on-state
current density of at least 600mA/mm.
Now the Lab has modified the 2008 structure, developing a way to
remove the cap layers and a part of the AlGaN layer, but only
immediately below the gate.
"By leaving a thin n-type AlGaN layer on the GaN carrier layer,
while suppressing damage to the GaN layer, on-state voltage can be
increased beyond +2V while preserving the total interruption in
stand-by, enabling high-speed performance when turned on,"
explained Fujitsu.
A gate was developed that uses an insulated structure with an
atomically-flat oxide film "because this suppresses gate leak
current in which travelling electrons flow to the gate electrode
when turned on, a positive voltage can be applied to the gate
electrode, resulting in high on-state current density," said
Fujitsu.
Current density is 829mA/mm, double that of last year's
design.
"Among transistors that can achieve an on-state voltage of at
least 2V and completely interrupt current when off, Fujitsu's new
transistor features the world's highest on-state current density,
making it the first GaN HEMT that has the characteristics required
for power supply use," claimed the firm.
The firm aims to be manufacturing PSUs using the transistor by
approximately 2011.
Optional caption for optional figure Fujitsu4
Maximum current density and threshold voltage benchmarks for
this transistor compared by Fujitsu with others with a
breakdown-voltage of 100 V or higher, and a gate threshold voltage
above 0V.
In 2008, Fujitsu revealed a similar transistor, but aimed at RF
amplifiers in phone basestations.
HEMTs were invented in 1979 by Dr Takashi Mimura at
Fujitsu Laboratories.