Millimeter-wave Device & Circuit Lab. (MDCL) Non-Alloyed Ohmic Contact in HEMTs Microwave devices 노 훈 희  Introduction  Ohmic contacts for HEMTs  Non-alloyed ohmic contact technology  Summary

Millimeter-wave Device & Circuit Lab. (MDCL) Introduction - ohmic contacts  To allow electrical current to flow into or out of the semiconductor  Linear I-V characteristic  Stable over time & temperature  Little parasitic resistance  Importance of Ohmic contact  The contact resistance in a GaAs FET may be half of the total (parasitic) source resistance  Noise figure is particularly sensitive to such resistance

Millimeter-wave Device & Circuit Lab. (MDCL) Ohmic contacts  Alloyed Ohmic contact – widely used  AuGe/Ni, AuGe/Au, AuGe/Ni/Au, Au/Ge/Ni/Ag, Au/Ge/Ni  Non-alloyed Ohmic contact  Au/Pt/Ti, Pd/Ge, Pd/Sn, Pd/Sn/Au,  Regrown Ohmic contact

Millimeter-wave Device & Circuit Lab. (MDCL) Non-alloyed Ohmic contact Technology  Advantage of non-alloyed ohmic contact  Extremely short ohmic length with low parasitic R S  Sharply defined ohmic edges can be obtained in the device channel  Direct connection between the source/drain and gate with the same metal  Provides very good surface morphology  Good gate-level lithographic definition in integrated circuits  R C shows excellent uniformity and wafer to wafer reproducibility The Au/Ge/Ni system has been widely used as the ohmic contact th the devices, but it has some drawbacks, - roughness - non uniformity - poor thermal stability

Millimeter-wave Device & Circuit Lab. (MDCL) Mechanism of Nonalloyed Ohmic contact - 1.5x10 -7 Ω∙cm 2 3.3x10 -7 Ω∙cm 2 1.4E18cm 3 ~10 -6 Ω∙cm 2 + II IEEE Trans.Electron Devices, 1989

Millimeter-wave Device & Circuit Lab. (MDCL) R S as a function of the spacing between the source and gate L SG R S as a function of L oh with L SG =2um Mechanism of Nonalloyed Ohmic contact IEEE Trans.Electron Devices, 1989

Millimeter-wave Device & Circuit Lab. (MDCL) Inserted n + doped InAlAs layer R C depends most significantly on two factors : - The doping level in all of the n + -doped layers - The thickness of the carrier supply layer 0.067Ω  mm IEEE Trans.Electron Devices, 1996

Millimeter-wave Device & Circuit Lab. (MDCL) Nonalloyed PdGe ohmic contact 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 The gate with Ti/Pt/Au metals were not degraded at temperatures lower than 325  C 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 -7 Ω∙cm 2 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 410  C Pd/Sn Pd/Sn/Au Pd/Ge IEEE Trans. Electron devices,2001

Millimeter-wave Device & Circuit Lab. (MDCL) Application of Nano HEMT SD G L GS RSRS RDRD Intrinsic Issue 2 : Small R S → Small L GS, Small R C - Self align gate process → Good gate length uniformity Issue 1 : Fine gate level Litho. - Good metal morphology → Good Mix & Match process ( optical & e-beam litho.)

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 cm -3 → electron turnnelling

Millimeter-wave Device & Circuit Lab. (MDCL) Reference  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 self- aligned 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  P. O'Connor, "Gold-germanium-based ohmic contacts to the two- dimensional 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