<s>
A	O
high-electron-mobility	B-Algorithm
transistor	I-Algorithm
(	O
HEMT	B-Algorithm
)	O
,	O
also	O
known	O
as	O
heterostructure	O
FET	B-Application
(	O
HFET	B-Algorithm
)	O
or	O
modulation-doped	O
FET	B-Application
(	O
MODFET	B-Algorithm
)	O
,	O
is	O
a	O
field-effect	O
transistor	O
incorporating	O
a	O
junction	O
between	O
two	O
materials	O
with	O
different	O
band	O
gaps	O
(	O
i.e.	O
</s>
<s>
a	O
heterojunction	O
)	O
as	O
the	O
channel	O
instead	O
of	O
a	O
doped	O
region	O
(	O
as	O
is	O
generally	O
the	O
case	O
for	O
a	O
MOSFET	B-Architecture
)	O
.	O
</s>
<s>
Devices	O
incorporating	O
more	O
indium	O
generally	O
show	O
better	O
high-frequency	O
performance	O
,	O
while	O
in	O
recent	O
years	O
,	O
gallium	O
nitride	O
HEMTs	B-Algorithm
have	O
attracted	O
attention	O
due	O
to	O
their	O
high-power	O
performance	O
.	O
</s>
<s>
Like	O
other	O
FETs	B-Application
,	O
HEMTs	B-Algorithm
are	O
used	O
in	O
integrated	O
circuits	O
as	O
digital	O
on-off	O
switches	O
.	O
</s>
<s>
FETs	B-Application
can	O
also	O
be	O
used	O
as	O
amplifiers	O
for	O
large	O
amounts	O
of	O
current	O
using	O
a	O
small	O
voltage	O
as	O
a	O
control	O
signal	O
.	O
</s>
<s>
Both	O
of	O
these	O
uses	O
are	O
made	O
possible	O
by	O
the	O
FET	B-Application
’s	O
unique	O
current	O
–	O
voltage	O
characteristics	O
.	O
</s>
<s>
HEMT	B-Algorithm
transistors	O
are	O
able	O
to	O
operate	O
at	O
higher	O
frequencies	O
than	O
ordinary	O
transistors	O
,	O
up	O
to	O
millimeter	O
wave	O
frequencies	O
,	O
and	O
are	O
used	O
in	O
high-frequency	O
products	O
such	O
as	O
cell	O
phones	O
,	O
satellite	O
television	O
receivers	O
,	O
voltage	O
converters	O
,	O
and	O
radar	B-Application
equipment	O
.	O
</s>
<s>
Advantages	O
of	O
HEMTs	B-Algorithm
are	O
that	O
they	O
have	O
high	O
gain	O
,	O
this	O
makes	O
them	O
useful	O
as	O
amplifiers	O
;	O
high	O
switching	O
speeds	O
,	O
which	O
are	O
achieved	O
because	O
the	O
main	O
charge	O
carriers	O
in	O
MODFETs	B-Algorithm
are	O
majority	O
carriers	O
,	O
and	O
minority	O
carriers	O
are	O
not	O
significantly	O
involved	O
;	O
and	O
extremely	O
low	O
noise	O
values	O
because	O
the	O
current	O
variation	O
in	O
these	O
devices	O
is	O
low	O
compared	O
to	O
other	O
.	O
</s>
<s>
The	O
invention	O
of	O
the	O
high-electron-mobility	B-Algorithm
transistor	I-Algorithm
(	O
HEMT	B-Algorithm
)	O
is	O
usually	O
attributed	O
to	O
physicist	O
Takashi	O
Mimura	O
( 三村	O
高志	O
)	O
,	O
while	O
working	O
at	O
Fujitsu	O
in	O
Japan	O
.	O
</s>
<s>
The	O
basis	O
for	O
the	O
HEMT	B-Algorithm
was	O
the	O
GaAs	O
(	O
gallium	O
arsenide	O
)	O
MOSFET	B-Architecture
(	O
metal	B-Architecture
–	I-Architecture
oxide	I-Architecture
–	I-Architecture
semiconductor	I-Architecture
field-effect	I-Architecture
transistor	I-Architecture
)	O
,	O
which	O
Mimura	O
had	O
been	O
researching	O
as	O
an	O
alternative	O
to	O
the	O
standard	O
silicon	O
(	O
Si	O
)	O
MOSFET	B-Architecture
since	O
1977	O
.	O
</s>
<s>
He	O
conceived	O
the	O
HEMT	B-Algorithm
in	O
Spring	O
1979	O
,	O
when	O
he	O
read	O
about	O
a	O
modulated-doped	O
heterojunction	O
superlattice	B-General_Concept
developed	O
at	O
Bell	O
Labs	O
in	O
the	O
United	O
States	O
,	O
by	O
Ray	O
Dingle	O
,	O
Arthur	O
Gossard	O
and	O
Horst	O
Störmer	O
who	O
filed	O
a	O
patent	O
in	O
April	O
1978	O
.	O
</s>
<s>
Mimura	O
filed	O
a	O
patent	O
disclosure	O
for	O
a	O
HEMT	B-Algorithm
in	O
August	O
1979	O
,	O
and	O
then	O
a	O
patent	O
later	O
that	O
year	O
.	O
</s>
<s>
The	O
first	O
demonstration	O
of	O
a	O
HEMT	B-Algorithm
device	O
,	O
the	O
D-HEMT	O
,	O
was	O
presented	O
by	O
Mimura	O
and	O
Satoshi	O
Hiyamizu	O
in	O
May	O
1980	O
,	O
and	O
then	O
they	O
later	O
demonstrated	O
the	O
first	O
E-HEMT	O
in	O
August	O
1980	O
.	O
</s>
<s>
The	O
first	O
demonstration	O
of	O
an	O
"	O
inverted	O
"	O
HEMT	B-Algorithm
was	O
presented	O
by	O
Delagebeaudeuf	O
and	O
Nuyen	O
in	O
August	O
1980	O
.	O
</s>
<s>
One	O
of	O
the	O
earliest	O
mentions	O
of	O
a	O
GaN-based	O
HEMT	B-Algorithm
is	O
in	O
the	O
1993	O
Applied	O
Physics	O
Letters	O
article	O
,	O
by	O
Khan	O
et	O
al	O
.	O
</s>
<s>
Ye	O
and	O
B	O
.	O
Yang	O
et	O
al	O
demonstrated	O
a	O
GaN	O
(	O
gallium	O
nitride	O
)	O
metal	B-Architecture
–	I-Architecture
oxide	I-Architecture
–	I-Architecture
semiconductor	I-Architecture
HEMT	B-Algorithm
(	O
MOS-HEMT	O
)	O
.	O
</s>
<s>
It	O
used	O
atomic	O
layer	O
deposition	O
(	O
ALD	O
)	O
aluminum	O
oxide	O
(	O
Al2O3	O
)	O
film	O
both	O
as	O
a	O
gate	O
dielectric	O
and	O
for	O
surface	B-Application
passivation	I-Application
.	O
</s>
<s>
HEMTs	B-Algorithm
are	O
heterojunctions	O
.	O
</s>
<s>
The	O
HEMTs	B-Algorithm
 '	O
exceptional	O
carrier	O
mobility	O
and	O
switching	O
speed	O
come	O
from	O
the	O
following	O
conditions	O
:	O
The	O
wide	O
band	O
element	O
is	O
doped	O
with	O
donor	O
atoms	O
;	O
thus	O
it	O
has	O
excess	O
electrons	O
in	O
its	O
conduction	O
band	O
.	O
</s>
<s>
An	O
important	O
aspect	O
of	O
HEMTs	B-Algorithm
is	O
that	O
the	O
band	O
discontinuities	O
across	O
the	O
conduction	O
and	O
valence	O
bands	O
can	O
be	O
modified	O
separately	O
.	O
</s>
<s>
As	O
HEMTs	B-Algorithm
require	O
electrons	O
to	O
be	O
the	O
main	O
carriers	O
,	O
a	O
graded	O
doping	O
can	O
be	O
applied	O
in	O
one	O
of	O
the	O
materials	O
,	O
thus	O
making	O
the	O
conduction	O
band	O
discontinuity	O
smaller	O
and	O
keeping	O
the	O
valence	O
band	O
discontinuity	O
the	O
same	O
.	O
</s>
<s>
The	O
term	O
"	O
modulation	B-Algorithm
doping	I-Algorithm
"	O
refers	O
to	O
the	O
fact	O
that	O
the	O
dopants	O
are	O
spatially	O
in	O
a	O
different	O
region	O
from	O
the	O
current	O
carrying	O
electrons	O
.	O
</s>
<s>
HEMTs	B-Algorithm
avoid	O
this	O
through	O
the	O
use	O
of	O
high	O
mobility	O
electrons	O
generated	O
using	O
the	O
heterojunction	O
of	O
a	O
highly	O
doped	O
wide-bandgap	O
n-type	O
donor-supply	O
layer	O
(	O
AlGaAs	O
in	O
our	O
example	O
)	O
and	O
a	O
non-doped	O
narrow-bandgap	O
channel	O
layer	O
with	O
no	O
dopant	O
impurities	O
(	O
GaAs	O
in	O
this	O
case	O
)	O
.	O
</s>
<s>
The	O
n-type	O
AlGaAs	O
layer	O
of	O
the	O
HEMT	B-Algorithm
is	O
depleted	O
completely	O
through	O
two	O
depletion	O
mechanisms	O
:	O
</s>
<s>
MODFETs	B-Algorithm
can	O
be	O
manufactured	O
by	O
epitaxial	O
growth	O
of	O
a	O
strained	O
SiGe	O
layer	O
.	O
</s>
<s>
The	O
end	O
result	O
is	O
a	O
FET	B-Application
with	O
ultra-high	O
switching	O
speeds	O
and	O
low	O
noise	O
.	O
</s>
<s>
InP	O
and	O
GaN	O
are	O
starting	O
to	O
replace	O
SiGe	O
as	O
the	O
base	O
material	O
in	O
MODFETs	B-Algorithm
because	O
of	O
their	O
better	O
noise	O
and	O
power	O
ratios	O
.	O
</s>
<s>
A	O
HEMT	B-Algorithm
where	O
this	O
rule	O
is	O
violated	O
is	O
called	O
a	O
pHEMT	B-Algorithm
or	O
pseudomorphic	O
HEMT	B-Algorithm
.	O
</s>
<s>
This	O
is	O
done	O
in	O
the	O
mHEMT	B-Algorithm
or	O
metamorphic	O
HEMT	B-Algorithm
,	O
an	O
advancement	O
of	O
the	O
pHEMT	B-Algorithm
.	O
</s>
<s>
HEMTs	B-Algorithm
made	O
of	O
semiconductor	O
hetero-interfaces	O
lacking	O
interfacial	O
net	O
polarization	O
charge	O
,	O
such	O
as	O
AlGaAs/GaAs	O
,	O
require	O
positive	O
gate	O
voltage	O
or	O
appropriate	O
donor-doping	O
in	O
the	O
AlGaAs	O
barrier	O
to	O
attract	O
the	O
electrons	O
towards	O
the	O
gate	O
,	O
which	O
forms	O
the	O
2D	O
electron	O
gas	O
and	O
enables	O
conduction	O
of	O
electron	O
currents	O
.	O
</s>
<s>
This	O
behaviour	O
is	O
similar	O
to	O
that	O
of	O
commonly	O
used	O
field-effect	O
transistors	O
in	O
the	O
enhancement	O
mode	O
,	O
and	O
such	O
a	O
device	O
is	O
called	O
enhancement	O
HEMT	B-Algorithm
,	O
or	O
eHEMT	O
.	O
</s>
<s>
When	O
a	O
HEMT	B-Algorithm
is	O
built	O
from	O
AlGaN/GaN	O
,	O
higher	O
power	O
density	O
and	O
breakdown	O
voltage	O
can	O
be	O
achieved	O
.	O
</s>
<s>
Such	O
a	O
transistor	O
is	O
normally	O
on	O
,	O
and	O
will	O
turn	O
off	O
only	O
if	O
the	O
gate	O
is	O
negatively	O
biased	O
-	O
thus	O
this	O
kind	O
of	O
HEMT	B-Algorithm
is	O
known	O
as	O
depletion	O
HEMT	B-Algorithm
,	O
or	O
dHEMT	O
.	O
</s>
<s>
In	O
contrast	O
to	O
a	O
modulation-doped	O
HEMT	B-Algorithm
,	O
an	O
induced	O
high	B-Algorithm
electron	I-Algorithm
mobility	I-Algorithm
transistor	I-Algorithm
provides	O
the	O
flexibility	O
to	O
tune	O
different	O
electron	O
densities	O
with	O
a	O
top	O
gate	O
,	O
since	O
the	O
charge	O
carriers	O
are	O
"	O
induced	O
"	O
to	O
the	O
2DEG	O
plane	O
rather	O
than	O
created	O
by	O
dopants	O
.	O
</s>
<s>
Applications	O
(	O
eg	O
for	O
AlGaAs	O
on	O
GaAs	O
)	O
are	O
similar	O
to	O
those	O
of	O
MESFETs	O
–	O
microwave	O
and	O
millimeter	O
wave	O
communications	O
,	O
imaging	O
,	O
radar	B-Application
,	O
and	O
radio	O
astronomy	O
–	O
any	O
application	O
where	O
high	O
gain	O
and	O
low	O
noise	O
at	O
high	O
frequencies	O
are	O
required	O
.	O
</s>
<s>
HEMTs	B-Algorithm
have	O
shown	O
current	O
gain	O
to	O
frequencies	O
greater	O
than	O
600GHz	O
and	O
power	O
gain	O
to	O
frequencies	O
greater	O
than	O
1	O
THz	O
.	O
</s>
<s>
(	O
Heterojunction	B-Algorithm
bipolar	I-Algorithm
transistors	I-Algorithm
were	O
demonstrated	O
at	O
current	O
gain	O
frequencies	O
over	O
600GHz	O
in	O
April	O
2005	O
.	O
)	O
</s>
<s>
Numerous	O
companies	O
worldwide	O
develop	O
and	O
manufacture	O
HEMT-based	O
devices	O
.	O
</s>
<s>
These	O
can	O
be	O
discrete	O
transistors	O
but	O
are	O
more	O
usually	O
in	O
the	O
form	O
of	O
a	O
'	O
monolithic	B-Algorithm
microwave	I-Algorithm
integrated	I-Algorithm
circuit	I-Algorithm
 '	O
(	O
MMIC	B-Algorithm
)	O
.	O
</s>
<s>
HEMTs	B-Algorithm
are	O
found	O
in	O
many	O
types	O
of	O
equipment	O
ranging	O
from	O
cellphones	O
and	O
DBS	O
receivers	O
to	O
electronic	O
warfare	O
systems	O
such	O
as	O
radar	B-Application
and	O
for	O
radio	O
astronomy	O
.	O
</s>
<s>
Furthermore	O
,	O
gallium	O
nitride	O
HEMTs	B-Algorithm
on	O
silicon	O
substrates	O
are	O
used	O
as	O
power	O
switching	O
transistors	O
for	O
voltage	O
converter	O
applications	O
.	O
</s>
<s>
Compared	O
to	O
silicon	O
power	O
transistors	O
gallium	O
nitride	O
HEMTs	B-Algorithm
feature	O
low	O
on-state	O
resistances	O
,	O
and	O
low	O
due	O
to	O
the	O
wide	O
bandgap	O
properties	O
.	O
</s>
<s>
Gallium	O
nitride	O
power	O
HEMTs	B-Algorithm
are	O
commercially	O
available	O
up	O
to	O
voltages	O
of	O
200	O
V-600	O
V	O
.	O
</s>
