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Advances in Current Flow Control in Nitride Heterostructures for Hot Electron Transistors

This document outlines the advancements in potential barriers and current flow in nitride devices, particularly focusing on Schottky barriers formed on p-GaN. It covers the fabrication status of Hot Electron Transistors (HETs) and the electrical characteristics of GaN/AlGaN barrier structures. Key discussions include the influence of dislocations and defects on tunneling phenomena, conduction band energy simulations, and the effectiveness of specific barrier thicknesses on overall device performance. The results indicate improvements in mobility and tunneling efficiency, setting the stage for enhanced transistor functionality.

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Advances in Current Flow Control in Nitride Heterostructures for Hot Electron Transistors

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  1. Barrier Current Flowin Nitride Heterostructures Peter Asbeck, S.S.Lau, Ed Yu Lin Jia, Dongjiang Qiao, L.S.Yu UCSD asbeck@ece.ucsd.edu February 12, 2002

  2. Outline • Potential barriers in nitride devices • Structure and current flow • Schottky barriers on p-GaN • Status of HET fabrication

  3. Hot Electron Transistor (HET) 50A B AlGaN: xAl=0.15 C E Depth InGaN: xIn=0.10 50A 1.5 E 1 B B Energy (eV) HET 0.5 n-GaN n-AlGaN/GaN 0 -0.5 0 1000 2000 3000 C Depth (Angstrom) Advantages High mobility base No Mg ionization problems Potentially fast

  4. 1.5E+18 1.5 Schottky 1 n-GaN 1.E+18 Ohmic AlGaN 0.5 n- GaN Energy (eV) Concentration (cm-3) 5.E+17 0 n+ GaN Buffer layer -0.5 0.E+00 3000 n SiC substrate 0 1000 2000 0 0.1 0.2 0.3 Depth (A) AlGaN layer Depth (um) GaN/AlGaN/GaN Barrier Materials by R. Davis Group Simulated conduction band energy DV DV Sample Expected Measured xAl 13 % d=100A 1.20eV 1.43eV xAl 13% d=50A 0.60eV 0.95eV

  5. GaN / AlGaN /GaN Barrier I-V Curves Vs Temperature 100A AlGaN Barrier 50A AlGaN Barrier exp(V/Eoo) Eoo ~ 38 meV (independent of temperature) Theoretical ~ 5meV Eoo ~ 48 meV (independent of temperature)

  6. GaN / AlGaN /GaN Barrier I-V Curves Vs Temperature Reverse Characteristics 100A AlGaN Barrier Modified Fowler-Nordheim Plot Fit with Eoo=40meV Theoretical Eoo= 5 meV

  7. GaN Schottky Barriers: Reverse Current Ni on n GaN 3e17cm-3 Fowler-Nordheim Tunneling Through Depletion Region Expect Eoo=5 meV Fit with Eoo= 50 meV

  8. Schottky Barrier on n- GaN Forward current L.S.Yu, S.S.Lau et al, UCSD (1998) T=360K T=220K • Log slope largely T invariant • => not thermionic emission • Very good fit with tunneling formalism • except Eoo= 19.5meV fitting • Eoo= 3.1meV theory => Defect assisted tunneling

  9. Dislocation Effects Line Charge effect: Electrostatic effects reduce potential barrier, allowing tunneling to occur more readily For reduction of barrier for electrons, require positively charged dislocation n-GaN Easier tunneling Dislocation line charge > 0 Dislocation line charge < 0

  10. Trap-Assisted Tunneling • Explains SILC (stress-induced leakage current) • in Si nonvolatile memory • Explains leakage currents in LT or IT GaAs For point defects separated by ~50A to allow tunneling, need ~ 5e18cm-3 Dislocations can provide states within gap correlated spatially for convenient tunneling

  11. ohmic contact Schottky contact Schottky barrier of Ni on p-GaN Mg doped MOCVD grown Sapphire substrate P~1e17cm-3 Expect Eoo=16 meV Fit with Eoo= 56 meV

  12. Cm C G Gm rs C-V results of Ni/p-GaN Schottky contact Corrections for Rs and Gp Needed to obtain C B=2.68 eV - 2.87 eV

  13. Profile of acceptor concentration • Na ~1019/cm3 within 200Å from the sample surface • tapers off to ~ 1018/cm3 • 10 to 100 times higher than p ~1017/cm3 (determined from Hall measurement)

  14. HET - Fabrication Approach Regrown emitter structure Si3N4 n AlGaN n AlGaN n+ GaN n+ GaN n AlGaN n AlGaN SiC SiC E E B B B n+ GaN n AlGaN C C n+ GaN n+ GaN n AlGaN n AlGaN SiC SiC => Base contacts can be formed after alloying of emitter and collector contacts => No need to etch through GaN to reach base

  15. HET Fabrication Status n+GaN 8e18 500A i-GaN 940 A SiN i-GaN 60A n+ AlGaN 5e18 60A Al:25% n+GaN 8e18 100A i-AlGaN 2000A Al:15% n+AlGaN 5e18 6000A Al:15% SiC substrate JD634 AlGaN Barrier HET Initial growth NCSU SiN deposition &patterning UCSD Regrowth NCSU Final processing - UCSD

  16. Plans • Refine HET fabrication • Continue barrier current investigation • Continue p contact studies

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