1*D1JURXSIMFEDK-2010

Ho

k k aid

Current controllability and stability ofmulti-mesa-channel AlGaN/GaN HEMTs
Tamotsu Hashizume and Kota Ohi
Research Center for Integrated Quantum Electronics (RCIQE)
Hokkaido University
JST-CRESTo U n iv e r s i t y

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Research issues for GaN inverter
There are many issues to be solved in GaN transistors
1) Surface control
2) Characterization and control of deep levels
3) IS interface and MIS (MOS) gate structures
4) Reliability characterization
5) Design, fabrication and characterization of
optimum device structure for GaN HEMT

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Ho

k k aido U n iv e r s i t y

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Mesa-gate AlGaN/GaN HEMT with a single channel H

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okk

a i d o U n iv e r s i t y

To improve gate controllability and field uniformity in channel,
the mesa-gate structures have been investigated.
SEM image

soursedsource

W

gate

gate
LG

drain
AlGaN
GaN

LG
0.4μm

normal fieldnano-channel

Wtop
drain

1µm

mesa-gate
lateral field

AlGaN
2DEG

lateral field● ECR-assisted dry etching● etching depth: 50 nm

GaN
Tamura, Hashizume, Appl. Phys. Express, 2008

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Mesa-gate AlGaN/ GaN HEMT with a single channel H

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okk

drain I-V

transfergood gate control

systematic VTH shifta i d o U n iv e r s i t y

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Potential distribution -calculationHo

Wchannel > 100 nm
AlGaN
gate
GaN

200 nmk k aido U n iv e r s i t y

Wchannel < 100 nm
2DEG

AlGaN

60 nm

d = 50 nm

VG = -2.5 V

GaN
0 eV

side-gate effect is weak,
due to high 2DEG density

gate control is similar toa planar structure

VG = -2.5 V
4.0 eVthe potential modulation fromthe side gate and through theundoped GaN layer becomesremarkablesurrounding gate effect

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Multi-mesa-channel AlGaN/GaN HEMTHo

k k aido U n iv e r s i t ySurrounding-field effect in the mesa-gate with Wchannel less than 100 nm

Multi-channel AlGaN/GaN HEMTs
parallel channels

Multi-mesa channel
periodic trench just under gatechannelgate

Ni/Au gate

sourcetrenchsource

drain
AlGaN
GaN

a problem islarge access resistance

AlGaN
GaN
AlGaN gate
GaN

trenchdrain
60 ~ 400 nm
trench
2DEG

50 nm

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Fabrication process and SEM observation
Wtotal = 60 µm
trenchJURXS

Hok k aido U n iv e r s i t y

ECR-plasma assisted RIBE

d = 50 nm

CH4/H2/Ar/N2 = 5/15/3/3 sccm
microwave power: 250 W
etching rate: 10 nm/min.@RT

AlGaN
GaN
trenchgate

period400 nmsourcesourceLG
drain

LGD= 10 μm(power switching
application)

AlGaN
GaN

periodical
trench
drain

gate

80 nm
channel width

Jpn. J. Appl. Phys. 48, 081002 (2009)

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Comparison of I-V characteristics between
conventional and MMC HEMTs

GaN

conventional planar
Drain current m(A)D

I (mA)

-1V

15
10
-3V

5

2

4
V

DS

6
(V)

8

MMC (Wtop= 80nm)

25

VGS= +1V0

k k aid

AlGaN

20 W= 60 μm

0

Ho

80 nm

LG= 0.5 μm
G/D spacing: 10 μm

25

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10

Weff= 32µm
VG= 0 V

20
15

-1 V

10

-2 V

5
0

-3 V
0

2

4

6

8

Drain voltage (V)10o U n iv e r s i t y

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Gate leakage characteristicsHok k aido U n iv e r s i t ymesa-gate
AlGaN
planar HEMT

2DEG
GaN

MMC HEMTThe leakage through the mesa
region is negligible in the MMC
HEMT

No significant difference ofgate leakage currents
between two HEMTs

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Transfer characteristicsof planar
MMC
Transfer characteristics
in linear region(VDSand=0.5V) H HEMTs
 IDS-VG characteristics

okk gm-VG characteri

2.5

2.0
1.5
1.0
0.5

LGD = 10µm
planarWtop=60µm

W =60µµm
Weff2.0

VDS=0.5V

MMC
Wtop
MMC
=70nm Wtop
=50nm

0-6 -5 -4 -3 -2 -1 0
gate voltage (V)gm (mS)drain current (mA)

3.0
2.5

a i d o U n iv e r s i t y

1.5
1.0
0.5

0
1
-6 -5 -4 -3gate v
 Systematic shift in threshold voltage was observed with n
As Wtop decreases, a systematic shift in threshold voltage is observed.
MMC HEMT is effective as the control approach of thresho
MMC structure is attractive for the control of threshold voltage.
 In spite of fact that the W
eff of MMC HEMTs are much narr
Jpn. J. Appl. Phys. 48, 081002 (2009)
planar HEMT, the MMC HEMT exhibited a current and g alm

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VTH control in MMC HEMT

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Excellent agreement between
the experimental and calculated
VTH values
Surrounding-field effect isremarkable when Wtop of themesa channel less than 100 nm

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Increase of gm in MMC HEMT
5

gm (mS)

4

VDS=0.5V

MMC
Weff=32μm

k k aido U n iv e r s i t y

Pronounced gm increaseprobably due tosurrounding-field effect3
planar2 W=60μm

AlGaN

1
0
-6

Ho

60 nm

d = 50 nm
GaN-5-4 -3 -2 -1
Gate Voltage (V)

0

0 eV

VG = -2.5 V
4.0 eVTemperaturedependence of subthreshold characteristics *D1
0-6 -5 -4 -3 -2 -1 0
1

13

 Temperature dependence
Sub-threshold characteristics
H Temperaturedependence
Gate Voltage(V)of subthreshold(a)characteristics

Drain Current (A)IDS, IG (A)10-4

MMC ( Wtop= 50nm )10ID-5ID

VDS=0.2V
planar
S=102
mV/dec

10-6

10-7

10-8

I
10-9G

10-10

Subthreshold value (mV/dec)

10-3

10-2

MMC

Wtop=70nm

S=80

100K

ID

Wtop=50nm
200K

300K

S=76

IG

-11
10300K
IG
VDS=0.2V
10-12
10-13
-6 -2.0-5-4-3-1.0 -2-1
Gate
Voltage(V)
Gate
Voltage(V)okk

of S-value

00

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a i d o U n iv e r s i t y

120
mkT
S = 2.3 ⋅q

100

planar
.3
1
m=

80
60

MMC

40

1
m=

20
0

0

100

200

300

temperature (K)(b) dependence of subthreshold current characteristics
 Clear temperature• smallcharacteristics
sub-thresholdinslope
Temperaturedependence
ure 2 (a) Transferalinearregion(VDS8) of S slope isofTemperaturedependence
to theory.
•highon/offratio(
10
S slopeis close to theory.5 V) and (b) logarithmic plots of drain current andclosegate

kage current as a function of gate voltage forImprovement
the planar of gate controllabilityd MMC HEMTs.
Excellent gate controllability

9Jpn. J. Appl. Phys. 48, 081002 (2009)

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Breakdowncharacteristics of planar and MMC HEMTs
JURXSBreakdowncharacteristicsof planarand MMC
HEMTs
Breakdowncharacteristicsof planarand MMCHEMTs
H Breakdown characteristics!LGD=1,10µm"

okk

a i d o U n iv e r s i t y

1010

LGD=1µµm
10-0
LGD=1µµm
10-1 MMC
10-2 -2
MMC
10 VG=-5V
10-3 -3 VG=-5V
10

10-0-1

LGD=10µµm
LGD=10µ
µm
MMC
10-1
-2
MMC
10-2
VG=-5V10
VG=-5V
10-3-310-1
drain current (A/mm)

drain current (A/mm)

 Breakdown
characteristics!LGD=1,10µm"-0-010-410-4
10-5

10-5

10-610-6
10-710-7
0 0

10

breakdown
breakdown

4040

10

10-6-610

breakdown
breakdownplanarplanar
VGV=-7V
G=-7V

10
10-7-760
00 50
2002002502503003006080
50 100100150150
drain voltage
voltage (V)
drain
(V)

Drain voltage (V)reported
data
VB=80~150V
reporteddata: V: =80~150V
B

(L

-4

10-4
10

10-5-5

planarplanar
VGV=-7V
G=-7V
2020

10

=3,5µm , without FP )Drain voltage (V)reporteddata: VB:=120~400V
reporteddata
V =120~400V
B(L=10µm , without FP )GD
( LGD=3,5µm, without FPshowed)(GDLGD=10µm under, without FP )Bothdevicessimilar breakdown behavior

Bothdevices showedsimilar breakdown behavior under off-state operation.
off-state
operation

Both devices showed similar breakdown behavior under off-state operation.

No significant degradation in the breakdown
No significant
degradationin thebreakdowncharacteristics
of MMCHEMT.

characteristics of MMC HEMT.

11

11

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Current stability of MMC HEMT-thermal effect-

Jpn. J. Appl. Phys. 48, 081002 (2009)

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Current decrease after off-state stress in planar HEMT Hoplanar HEMT

drain current (mA)50

initialk k aid

仮想ゲート
virtualgate model
AlGaN

40

JURXS

G

RDac

D

VGS= 0V
GaN

2DEG

30
20

after off-state stress

10

VGS= -8V
VD= 30Vt = 60 s

0
0

2

4

6

8

10

12

during the stresselectrons are injected tothe surface states
negatively charged states
reduce the 2DEG density

drain voltage (V)increase of drain access
resistance RDaccesso U n iv e r s i t y

(b)

IDS

RON_after ! RON_before

Drain Current (mA)

Drain Current (mA)

17

Drain Cdrain stress bias.VFrom
the I-V slope in the
DS_stress=10V
RON. F
5 shown in the inset, we calculated
28 refreshment*D1tional planar HEMT,
20V RON drastically
JURXS incr
IDS-VDS measurement
29
Current(Afterstabilityof MMC HEMT against
drain stressoff-stressbias. From the calculation of t
stress)Ho
30V30
k k aid
G=+1V
distribution along the @V
AlGaN
surface,er sitypo Univthe(a)31
0 centration of electric field was confirmed a
1
0
2
3
15
32
on
the
drain
side.
The
higherdrain
RON ration
beforeandafter stress s
Drain
Voltage(V)
planar HEMT
field concentration, resulting in the prono
33
off-statestress(b) . In comparison, for thecrease with VDS_stressbefore stress 34
10
8 we observed less change in RON even afte
35
VDS_stress=10V
drain
stress to the HEMT. This shows that t
RON_before
36
ture has resistanceto the off-stress-induc!"#!
37
6 lapse.
5$%"#$$
3820V
OneRON_afterof the possible mechanisms for t
30V @V39
surface charging effect arising!&$%!
G=-3.6V
4 lapse is theplanar0
trapping at$%"#$$
surface statesof AlGaN [14]. A
40
15
VDS the initial access resistance4(a),we defined
VG_stress
41
MMC HEMT (Wtop=65nm)
drain sides as RS0 and MMC
RD0, respectively
2
42
=-8V
before stressstress can induce the electron injection into
43
10
face from the gate edges. As a result, the44near the gate edges negatively charges up
40of AlGaN.
50
0 10at surface
20 30
45 =10V
VDS_stresstrapping
statesSuch aV(V)5
ing causes aDS_stresslocal depletion of 2DEG densi
46
20V
and source regions,47(c) resulting in the increase
30Vtance (RS_stress and RD_stress), as shown in Fig@V
48G=+1V
Figure 3 (a) Flowchart
off-stress-induced
currentc
Using fora simpleequivalent circuit
show
0
1
0
249
3
-VDS characteristics(c)changemeasurement, (b)IDSestimatedchangeinandtheaccessres
Drain Voltage 50(V)
Phys.weStatusSolidithe(c),in press(2012)
planar HEMTfrom stressthe fitting
resistance as a functionof the drainbias.of the IDS-VD
VDS_stress = 10 ~ 50V
27

off-state stress. Therefore, the operation of the MMC[6] T. Hashizume, S
37
HEMT is rather insensitive to change in access resistance,
18
Lett. *D1
83, 2952 (2038
JURXS Y.
leading to resistance to off-stress-induced current collapse.
Okamoto,Advantageof high-impedance channel in MMC HEMT [7] Y.
39
moto, T. Inoue, M
H
Table 1 Change in access resistance40
yando kkM.
aidoKuzuhara
U n iv e r s i t
41(2004).
Before stress (R0) After stress (R0 + Rthstress)th42[8] M. Tajima and
The!9th!International!Conference!on!Nitride!Semiconductors,!10 ~ 15 July,!SECC,!Glasgow,!UK
43
RS_access4~5!
60 ~ 70 !
061001 (2011).
44[9] T. Tamura, J. Ko
RD_access40 ~ 50 !
140 ~ 180 !
45
Phys. Express 1, 0
! MMC!HEMT!(W=65nm)
top46[10] K. Ohi and T. H
(2009).
47" Rch estimated!from!experimentS
SourseSurface[11] J. W. Chung, J.
48+ R1"mcharging
h i 49 RS0 +!R
RS_accessS_stress
IEEE Electron De
V
:!0!~
+1V
!!!!!!!!!!!!!! !!!!!!!!!!!!!
Gthth[12] M. Fieger, M. E
ference!on!Nitride!Semiconductors,!10… ~ 15 July,!SECC,!Glasgow,!UK
50G …
G
Rchannel :!4 ~ 6k! Noculak, H. Kalis
65nm
51 !!!!!!!!!!!!!! !!!!!!!!!!!!! Rch
can, J. Cryst. Gro
52 R +!R
10"m
RD_accessD0
D_stress[13] H. F. Sun and C
53! RD_stress
Gate : 100!~ 150!!
(2007).
54
Drain
D[14] R. Ventury, N. Q
Surface charging
Wtop
65nm
t =65nm
55
Trans. Electron D" Rch estimated!from!experiment
56
Figurelayout
6 Plan-view
the gate electrode and surface• Gate electrodeis theillustrationof
same" MMC!HEMT
57
charging
regions
for
the
MMC
HEMT.
with that for the planar HEMT.

Effect!of!access!resistance!on!I"V!behavior!of!MMC!HEMT

ce!on!I"V!behavior!of!MMC!HEMTRch

VG :!0!~ +1V
RchannelS_stress:!4 ,~RD_stress
6k!
The!similar!R

Copyright line will be provided by the publisher

Rchannel ! RD_stressestimated!for!the!planar!HEMT
p

Operation of the MMC HEMT is rather insensitive to change in access resistance,!operation
RD_stressto :off-stress-induced
150!!
of100!~
the MMCHEMT
is collapse.
rather insensitive toleadingTheto resistancecurrent

65nm
p=65nmchangeg in Raccess due to the high"impedancegpchannel,, leadingg" 435637!"

19" 89:;<=:,>?@*D1

Advantage of high-impedance channel in MMC HEMTKnee!" ! )*+,-.

# IDS-VDSGH

Drain Current (mA)30

planar10

LGD=1!m

20

VKnee/BC

DE

10

0

0

2
4
6
8
Drain Voltage (V)

6

0

MM

o U n iv e r s i t y

LGD=1!m
VKnee/BC

drain

DF

Raccess~ T10"

LGD=10!m2
10

k k aidsource4

LGD=10!mHo

W=60!m

MMC

8

JURXS

0

2
4
6
8
Drain Voltage (V)10p

RON is strongly dependent on
High-impedance channel
LGDNO%PQVknee/BCRFS3IJKL)*+,LGD (drain access resistance)
brought weak dependency

IDS_AfterST/IDS_BeforeST (20*D1!"#$%#&./012MMC HEMT3
JURXS
MMC
Summary)*+,-4planar5&6789/:

90

Hoplanar80

k k aido U n iv e r s i t y

CD • MMC;<&'(
To improve current controllability, MMCs with channel widths less
70 Knee)I3)*=>?#3@AB+than 100
nm were fabricated in an AlGaN/GaN
HEMT, by forming a
JKLMN
VG=1V a gate electrode.
periodic60trench structure under! `abcdeb#fghi
0

2
4
6
8
10
Drain Voltage(V)• Control of thresholdvoltage

SourceRchannel• High R
current
drivability
channel!R
accessLO(PQ2Gate

G-DHRST#UV3WX&YZ2• Good subthreshold characteristics[3@A\]^&_/:• Good current stability

Raccess
Drain

9

Unique and promising characteristics for inverter application• surrounding field effect and high-impedance channel
Very recently, similar device structures were applied to AlGaN/GaN
HEMTs [EDL, 33, 360(2012), EDL, 33, 354(2012)].