High Temperature Stable Contacts for GaN HEMTs and LEDs
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1108-A01-04
High Temperature Stable Contacts for GaN HEMTs and LEDs S.J.Pearton1, L.F.Voss1*, R.Khanna1**, Wantae Lim1, L.Stafford1***, F. Ren 2, A.Dabiran3 and A.Osinsky3 1 Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA 2 Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA 3 SVT Associates, Eden Prairie, MN 55344, USA * current address, Lawrence Livermore National Lab, Livermore, CA ** current address, Oerlikon, St. Petersburg, FL 33716 *** current address, University of Montreal, Montreal, Canada ABSTRACT There is continued interest in developing more stable contacts to a variety of GaN-based devices. In this paper we give two examples of devices that show improved thermal stability when boride, nitride or Ir diffusion barriers are employed in Ohmic contact stacks. AlGaN/GaN High Electron Mobility Transistors (HEMTs) were fabricated with Ti/Al/X /Ti/Au source/drain Ohmic (where X is TiB2, ZrN, TiN, TaN or Ir) contacts and subjected to long-term annealing at 350°C. For GaN layers with an electron concentration of ~3×1017 cm-3, the minimum specific contact resistance achieved is 6×10-5 Ω cm2 for Ti/Al/TiN/Ti/Au after annealing at 800°C. The specific contact resistance was found to strongly depend on the doping level, suggesting that tunneling is the dominant mechanism of current flow. By comparison with companion devices with conventional Ti/Al/Ni/Au Ohmic contacts, the HEMTs with boride-based Ohmic metal showed superior stability of both source-drain current and transconductance after 25 days aging at 350°C. The gate current for standard HEMTs increases during aging and the standard Ohmic contacts eventually fail by shorting to the gate contact. Similarly, InGaN/GaN multiple quantum well light-emitting diodes (MQW-LEDs) were fabricated with either Ni/Au/TiB2/Ti/Au or Ni/Au/Ir/Au p-Ohmic contacts. Both of these contacts showed superior long-term thermal stability compared to LEDs with conventional Ni/Au contacts.
INTRODUCTION At present there is great interest in wide-bandgap GaN and related materials because of their exciting applications in visible and ultraviolet (UV) lasers and light-emitting diodes (LEDs) for display and data storage, high electron mobility transistors (HEMTs) for high-temperature and high-power electronics, and solar-blind UV detectors. One of the remaining obstacles for commercialization of GaN HEMTs power amplifiers is the development of more reliable and thermally stable Ohmic contacts. GaN high electron mobility transistor (HEMT) power amplifiers have now entered the commercialization stage for use in wireless communications and military applications. There are also possible applications in improved automotive radar and power electronics for hybrid electric vehicles and in advanced satellite communication systems. GaN HEMTs can provide the high-power, high-efficiency, high-linearity RF power transistors required in base stations for mobile data network services. One of the major issues with
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