Oxidation Control of GaAs/AlGaAs/InGaAs pHEMT
for High Efficiency and Low Voltage Wireless Applications
Can Zheng, Robert Coffie, Dario Buttari, James Champlain, and Umesh Mishra
Electrical and Computer Engineering Department
University of California at Santa Barbara, USA
GaAs on Insulator (GOI)
Advantages of GaAs On Insulator (GOI):
• Elimination of substrate leakage current• Reduced output conductance• Excellent charge control• High efficiency
Source DrainGate
GaAs channel
Insulating buffer
Source DrainGate
GaAs buffer
GaAs channel
Insulating buffer can be obtained by the lateral wet oxidation of Al0.98GaAs layer
Conventional FET GOI FET
150 Å In0.20GaAs Channel
n+ Al 0.22 GaAsAlAs Defect Diffusion Barrier
graded Al xGaAs (x=0.98 -0.22)
Al0.98GaAs Oxidation Layer
(100) GaAs Substrate
LT GaAs
GaAs Smoothing Layer
GaAs Smoothing Layer
Al 0.22 GaAsn+ GaAs Contact Layer
n+ Al 0.22 GaAsAl 0.22 GaAs
Al 0.22 GaAs
Oxide Buffer
pHEMT
Advantages of GaAs On Insulator (GOI):
• Elimination of substrate leakage current• Reduced output conductance• Excellent charge control• High efficiency
Advantages of pHEMT:
• Higher mobility of InGaAs channel• Increased sheet carrier concentration due to the large conduction band discontinuity between InGaAs/AlGaAs
• Superior power and efficiency performance• low noise
GOI pHEMT
MBE Grown GOI pHEMT Structure
DC I-V Characteristics of Fully Oxidized GOI & Control pHEMTs
GOI and Control pHEMTs: 0.7 µm long, 150 µm wide
Vgs = 0.5 to – 2.5V, step = 0.5 VVgs = 0.5 to – 2.5V, step = 0.5 V
• IDSS (GOI) ~ 436 mA/mm, IDSS (Control) ~ 586 mA/mm.
• Decreased current level in fully oxidized GOI sample is due to back depletion from high defect density oxide-semiconductor interface and charge compensation.
0
100
200
300
400
500
600
700
800
0 1 2 3 4 5
I DS
(mA
/mm
)
VDS
(V)
0
100
200
300
400
500
600
700
800
0 1 2 3 4 5
I DS
(mA
/mm
)
VDS
(V)
Control GOI
The Effect of Traps: Back Depletion
• 1D Poisson simulation indicates more charge loss with increased oxide-semiconductor interface defect density, causing up to 20% charge loss for GOI MESFETs grown on LT AlGaAs or GaAs buffer.
• MESFETs grown on LT GaAs buffer shows charge loss up to 30 ~ 40% after the oxidation.
•Additional charge loss can be contributed to charge compensation during the oxidation process.
ntrap
ntrap increases
wdep,owdep,o +∆wdep
Oxide OxideLT GaAs LT GaAs
ntrap increases
wdep,owdep,o +∆wdep
Oxide OxideLT GaAs LT GaAs
ntrap increases
wdep,owdep,o +∆wdep
Oxide OxideLT GaAs LT GaAs
0.75
0.8
0.85
0.9
0.95
1
0 5 1012 1 1013 1.5 1013 2 1013
LT GaAs BufferLT AlGaAs Buffer
Nor
mal
ized
Cha
nnel
Cha
rge
(nsh
/ n 0
)
Oxide Top Interface Trap Density (cm-2)
Minimize Charge Loss by Growing LT AlGaAs Buffer
Charge Loss of fully oxidized GOI MESFET
• GOI MESFET directly grown on GaAs substrate: 80%.
• GOI MESFET on LT Grown GaAs buffer ( Tg = 270 ºC): 35 ~ 40%.
• GOI MESFET on LT Al0.3Ga0.7As buffer (Tg = 270 ~ 400 ºC): 6 ~ 17 %
The growth and anneal temperature of the LT
Al0.3Ga0.7As buffer is critical to obtain the
minimized charge loss.
0
5
10
15
20
260 280 300 320 340 360 380 400 420
LT AlGaAs Growth Temperature (°C)
Cha
rge
Loss
( %
)
gm Characteristics ofFully Oxidized GOI & Control pHEMTs
• gm_(GOI) ~ 366 mS/mm, gm (Control) ~ 280 mS /mm
• GOI sample has sharper turn-on and higher gm near pinch-off
GOI and Control pHEMTs: gate 0.7 µm long, 150 µm wide, Vds = 2.5 V
0
100
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800
0
50
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400
-3 -2.5 -2 -1.5 -1 -0.5 0 0.5
Vgs(volts)
I d(m
A/m
m)
gm(m
S/mm
)
Control
0
100
200
300
400
500
600
700
800
0
50
100
150
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400
-3 -2.5 -2 -1.5 -1 -0.5 0 0.5
Vgs(volts)
I d(m
A/m
m)
gm(m
S/mm
)
GOI
• gm peaking was observed for GOI pHEMT due to impact ionization
• Control sample showed constant gm curves s
m gv vg Cl d l
ε= = i
G
gm peaking in Fully Oxidized GOI pHEMT
AlGaAs
Al2O3SI GaAs Substrate
n+ GaAs n+ GaAs
• Traps are generated during the oxidation process at the oxide semiconductor interface and in the adjacent layers. • Impact ionization of the traps underneath the gate by hot electrons can take place at substantially lower drain bias compared to the electron-hole pair generation by band to band ionization.• Electrons removed from the deep levels shift the threshold voltage to the negative direction, therefore causing the increase in current.
Gain peaking is attributed to an increase in drain current induced by trap-related threshold voltage shift.
InGaAsAlGaAs
DS
-0.08
-0.07
-0.06
-0.05
-0.04
-0.03
-0.02
0
100
200
300
400
-3 -2.5 -2 -1.5 -1 -0.5 0 0.5
I g (m
A/m
m) gm (m
S/mm
)
Vgs
(V)
Interface traps
Threshold Voltage Shift in GOI pHEMT
0
20
40
60
80
-1.5 -1 -0.5 0 0.5
ID (mA/mm)_1ID (mA/mm)_2
I D (m
A/m
m)
VGS
(V)
VDS
= 0.2 V
• The threshold voltage of a GOI pHEMT was measured in the dark at a low VDS of 0.2V.
• The 80mV threshold voltage shift toward the positive voltage direction was observed after up to 5V VDSapplied to the GOI pHEMT.
• The threshold voltage shift can be contributed to the negatively charged trap states underneath the gate.
Threshold Voltage of GOI pHEMTbefore and after high bias applied.
Transconductance Characteristics in SOI MOSFET’s
Front gate and back-gate characteristics in NMOS SOI transistors for both high-does SIMOX wafers or BSOI wafers
•The rapid kink phenomenon has been contributed to the presence of energetically –localized trap states at SOI/BOX interface.
Takeo Ushiki, Koji Kotani, et al. IEEE Transactions on Electron Devices, Vol. 47, 2000, P360
1. Controlled Wet Oxidation of AlGaAs
4. Gate recess etch and gate metallization
Partially Oxidized pHEMT Process Flow
3. Mesa Isolation and S/D Ohmic Contact
2. Anneal (600°C/3min)
pHEMT
oxidation depth
SiNx Cap
Oxide AlGaAs
G
pHEMT
oxidation depth
SiNx Cap
Oxide AlGaAs
S D
pHEMT
oxidation depth
Oxide AlGaAs
S D
pHEMT
oxidation depth
Oxide AlGaAs
Partially Oxidized pHEMTs
gate
source drain
S DGs dg
s dg
oxidation trench
Fully oxidized devices• Truly insulating buffer• charge loss• gm peaking
Partially oxidized devices• insulating buffer underneath the source• reduced charge loss• no gm peaking
Subthreshold Leakage Current
10-2
10-1
100
101
102
103
-3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5
I DS
(mA
/mm
)
VGS
(V)
Oxide
source draingate
channel
A
Oxide
source draingate
channel
B
A
B
Partially oxidized pHEMT can effectively block the subthreshold leakage current compared with the un-oxidized control sample, especially when the high field drain side of the gate edge is blocked by the oxide.
Control
Oxidation Distance from Source Edge
source draingate
channel
0.5
0.6
0.7
0.8
0.9
1
-3 -2 -1 0 1 2 3 4
S G D
Oxidation Distance from Source Edge (µm)
I DSS
_ox
/ ID
SS_u
n (%
)
IDSS_unoxidized = 667 mA/mm
Partially Oxidized & Control pHEMTs DC Characteristics
250
300
350
400
450
-3 -2 -1 0 1 2 3 4
Gm
_max
(mS/
mm
)
Oxidation Distance from Source Edge (µm)
S G D
Gm_max_unoxidized = 341 mS/mm
GOI and Control pHEMTs: gate 0.7 µm long, 150 µm wide
• Hall measurement shows charge density of 3.54 x 1012/cm2and mobility of 6467cm2/(v.sec)
• Oxidation conditions : 400ºC, 30 minutes, oxidation depth 5 µm
• Charge loss up to 30% for fully oxidized sample was observed
• High gm peaking was observed for samples with the oxide extending beyond the drain edge of the gate
-20
0
20
40
60
80
100
-3 -2 -1 0 1 2 3 4
Dra
in C
urre
nt D
ispe
rsio
n (%
)
Oxidation Depth (µµµµm)
S G D
Unoxidized
Oxidized
Unoxidized
Oxidized
-20
0
20
40
60
80
100
-3 -2 -1 0 1 2 3 4
Dra
in C
urre
nt D
ispe
rsio
n (%
)
Oxidation Depth (µµµµm)
S G D
LT GaAs buffer (001235_2) LT AlGaAs buffer (010882_2)
Dispersion in Partially Oxidized pHEMTs
Serious dispersion was observed for the pHEMTs with oxide extending beyond the gate region. The pHEMTs with an LT AlGaAs buffer showed lower dispersion compared to the pHEMTs with an LT GaAs buffer when oxide is around the source region.
Submicron GOI & Control pHEMTs RF Characteristics
GOI and Control pHEMTs: gate 0.7 µm long, 150 µm wide
• ATN loadpull measurement: 8 GHz, Class AB bias, 3.5 V VDS
• Lower gain is observed for the partially oxidized sample due to the reduction in channel charge after the oxidation
• Highest power added efficiency was observed for samples with oxide front terminated around source edge due to decreased subthreshold leakage current
• RF measurement is limited by the available matching states of our Loadpull system
Gai
n (d
B)
Unoxidized
Partially-oxidized
14
15
16
17
18
-3 -2 -1 0 1 2 3 4
S G D
Oxidation Distance from Source Edge (µm)
PAE
( %
)
30
35
40
45
50
55
60
-3 -2 -1 0 1 2 3 4
Unoxidized
Partially-oxidized
Oxidation Distance from Source Edge (µm)
S G D
Conclusions
• Oxidized Al0.98Ga0.02As offers an insulating buffer for MESFETs and pHEMTs, increasing the efficiency of the device.
• Partially oxidized devices retain the benefits of the oxide buffer, while not suffering as greatly from the charge loss problem.
• Partially oxidized pHEMTs with oxidation stopped around the source region showed improved power added efficiency at low power supply.
• Transconductance peaking due to impact ionization of the traps underneath the gate was observed when oxide extended beyond the gate.
• Serious dispersion was observed for the pHEMTs with oxide extending beyond the gate region. The pHEMTs with an LT AlGaAs buffer showed lower dispersion compared to the pHEMTs with an LT GaAs buffer when oxide is around the source region.