Non-Alloyed Ohmic Contact in HEMTs
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Introduction
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Ohmic contacts for HEMTs
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Non-alloyed ohmic contact technology
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Summary
2004. 6. 22.
Microwave devices
2003-21577
노 훈 희
Millimeter-wave Device & Circuit Lab. (MDCL)
Introduction - ohmic contacts
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To allow electrical current to flow into or out of the
semiconductor
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Linear I-V characteristic
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Stable over time & temperature
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Little parasitic resistance
Importance of Ohmic contact
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The contact resistance in a GaAs FET may be half of the total
(parasitic) source resistance
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Noise figure is particularly sensitive to such resistance
Millimeter-wave Device & Circuit Lab. (MDCL)
Ohmic contacts
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Alloyed Ohmic contact – widely used
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Non-alloyed Ohmic contact
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AuGe/Ni, AuGe/Au, AuGe/Ni/Au, Au/Ge/Ni/Ag, Au/Ge/Ni
Au/Pt/Ti, Pd/Ge, Pd/Sn, Pd/Sn/Au,
Regrown Ohmic contact
Millimeter-wave Device & Circuit Lab. (MDCL)
Non-alloyed Ohmic contact Technology
The Au/Ge/Ni system has been widely used as the ohmic
contact th the devices, but it has some drawbacks,
- roughness
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- non uniformity
- poor thermal stability
Advantage of non-alloyed ohmic contact
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Extremely short ohmic length with low parasitic RS
 Sharply defined ohmic edges can be obtained in the device
channel
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Direct connection between the source/drain and gate with the same
metal
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Provides very good surface morphology
 Good gate-level lithographic definition in integrated circuits
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RC shows excellent uniformity and wafer to wafer reproducibility
Millimeter-wave Device & Circuit Lab. (MDCL)
Mechanism of Nonalloyed Ohmic contact 1.4E18cm3
3.3x10-7Ω∙cm2
+
1.5x10-7Ω∙cm2
II
~10-6Ω∙cm2
Millimeter-wave Device & Circuit Lab. (MDCL)
IEEE Trans.Electron Devices, 1989
Mechanism of Nonalloyed Ohmic contact
RS as a function of the spacing
between the source and gate LSG
IEEE Trans.Electron Devices, 1989
RS as a function of Loh
with LSG=2um
Millimeter-wave Device & Circuit Lab. (MDCL)
Inserted n+ doped InAlAs layer
RC depends most significantly on two factors :
0.067Ωmm
- The doping level in all of the n+-doped layers
- The thickness of the carrier supply layer
IEEE Trans.Electron Devices, 1996
Millimeter-wave Device & Circuit Lab. (MDCL)
Nonalloyed PdGe ohmic contact
The gate with Ti/Pt/Au metals were not
degraded at temperatures lower than 325C
If the ohmic contact forming at temperatures
lower than 300C is developed...
Source/drain electrodes could be formed
after the fabrication of the fine gate electrode
It makes the self aligned gate (SAG) process
possible in the fabrication of HEMT
This promises more precise alignment of a fine
gate between source and drain electrodes
Applied physics letters, 1999
Millimeter-wave Device & Circuit Lab. (MDCL)
Nonalloyed PdGe ohmic contact
1.2x10-7Ω∙cm2
Applied physics letters, 1999
Millimeter-wave Device & Circuit Lab. (MDCL)
Nonalloyed Thermally stable Pd/Sn and Pd/Sn/Au Ohmic contacts
One of the most important criteria
for an ohmic contact is its thermal stability
Pd/Sn
Pd/Sn/Au
410C
Pd/Ge
IEEE Trans. Electron devices,2001
Millimeter-wave Device & Circuit Lab. (MDCL)
Application of Nano HEMT
Issue 1 : Fine gate level Litho.
- Good metal morphology
LGS
S
G
D
→ Good Mix & Match process
( optical & e-beam litho.)
RS
Intrinsic
RD
Issue 2 : Small RS → Small LGS, Small RC
- Self align gate process
→ Good gate length uniformity
Millimeter-wave Device & Circuit Lab. (MDCL)
Summary


A short ohmic length with low parasitic RS is a great
advantage of nonalloyed ohmic contacts in high-density
VLSI
The nonalloyed ohmic contact had been prescribed from
actual use in a HEMT structure because of necessarily high
doping concentrations, in the range of 1019cm-3
→ electron turnnelling
Millimeter-wave Device & Circuit Lab. (MDCL)
Reference
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Shigeru Kuroda, "A New Fabrication Technology for AlGaAs/GaAs HEMT
LSI's Using InGaAs Nonalloyed Ohmic Contacts" IEEE TRANSACTION ON
EL, 1989
Kevin J. Chen, "High-Performance InP-Based Enhancement-Mode HEMT's
Using Non-Alloyed Ohmic Contacts and Pt-Based Buried-Gate
Technologies" IEEE TRANSACTION ON ED, 1996
Kevin J. Chen, "High-Performance Enhancement-Mode InAlAs/InGaAs
HEMT's Using Non-Alloyed Ohmic Contacts and Pt-Based Buried-Gate
Technologies" IPRM,1995
D.A.J.Moran, "Self-aligned 0.12um T-gate InGaAs/InAlAs HEMT technology
utilising a non-annealed ohmic contact strategy" ESSDR 2003
Jung-Woo Oh “Application of nonalloyed PdGe ohmic contact to selfaligned gate AlGaAs/InGaAs pseudomorphic high-electron-mobility
transistor” Applied Physics letters, 1999
C.K. Peng " Extremely low non-alloyed and alloyed contact resistance
using an InAs cap layer on InGaAs by molecular-beam epitaxy"
S. Kuroda,"HEMT with nonalloyed ohmic contact using n+InGaAs cap
layer" IEEE Electron Device Lett. 1987
P. O'Connor, "Gold-germanium-based ohmic contacts to the twodimensional electron gas at selectively doped semiconductor
heterointerfaces" IEEE Trans. Electron. Devices 1987
M.S.Islanm “Novel Nonalloyed Thermally Stable Pd/Sn and Pd/Sn/Au
Ohmic Contacts for the Fabrication of GaAs MESFETs” IEEE trans.
Electron Devices.2001
Millimeter-wave Device & Circuit Lab. (MDCL)