Deeply Recessed AlGaNGaN HEMTs on SiC Substrate

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Deeply Recessed AlGaN/GaN HEMTson SiC Substrate
L. Shen, C. Poblenz, S. Rajan, A Chini, M. H.
Wong, T. Palacios, A. Chakraborty, S. Heikman,
S. Keller, J. Speck and U. K. Mishra
Department of Electrical and Computer Engineering
Department of Materials
University of California, Santa Barbara
Also funded by Raytheon(DARPA)
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Outline

• Introduction
• Difficulty of thick graded AlGaN growth on SiC
• Different growth methods
• MOCVD partially graded AlGaN cap
• MBE fully graded AlGaN cap
• Summary

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Epitaxial design

• Graded AlGaN cap
• low gate leakage
• high stress
• limited thickness

• GaN cap
• high gate leakage
• low stress
• no thickness limitation

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GaN-capped HEMT with SiO2 layer

• Thin SiO2 layer reduce the gate leakage current
  (1 2 orders) and increase the breakdown voltage
  (gt90V).

• 12W/mm with peak PAE of 41.8 at 10GHz with drain
  bias of 45V without passivation.

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AlGaN cracking SiC vs. Sapphire
Mismatch in Thermal Expansion between substrate
and (Al)GaN cause cracking upon cooldown. The
effect is worse on 4H SiC.
Duringgrowth
Aftercooldown
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Thick graded AlGaN cap on SiC

• Thinner graded AlGaN layer (100nm) relieve high
  stress
• 150nm UID Al0.05Ga0.95N on top
• 2DEG density 8.51012/cm2

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DC Performance

• Small amount of dispersion under large load line
• Gate leakage current larger than that of the
  fully graded AlGaN cap sample on sapphire
• Breakdown voltage larger than 100V

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Power Performance

• On SiC substrate. No passivation and insulator.
• 6.3W/mm with a peak PAE of 77 at 4GHz VD30V,
  ID80mA/mm.
• 12.8W/mm with a peak PAE of 65 at 4GHz VD50V,
  ID80mA/mm.

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Power Performance

• On SiC substrate. No passivation and insulator.
• 14.1W/mm with a peak PAE of 53 at 4GHz VD60V,
  ID50mA/mm.
• 15.2W/mm with a peak PAE of 45 at 4GHz VD70V,
  ID50mA/mm.

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Stability
-- by Y.F. Wu _at_CREE SBTC

• 150C, 24-hour RF stress at 2dB gain compression
  point
• Output power, gain and drain current constant
• Gate leakage reduced slightly, due to the
  annealing affect

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Stability
-- by Y.F. Wu _at_CREE SBTC
24-hour RF stress (2dB compression) at 150C
before
after

• No degradation of power performance
• Slight improvement due to annealing affect caused
  by heating

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MBE vs. MOCVD
Growth temperature
MBE
MOCVD
Low growth temperature
High growth temperature
High thermal mismatch
Low thermal mismatch
Thick graded AlGaN
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Epitaxial Design

• Thick cap layers downgrade to 0.05, 0.1 and 0.15
• Simulated 2DEG density 1.21013/cm2 better for
  dispersion suppression

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Growth Issues
Al composition of the AlGaN at the surface
0.05
0.10
0.15

• Growth temperature 720C Carbon-free buffer
• Good surface morphology
• Al0.15Ga0.85N surface termination is close to the
  limitation of cracking

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DC and pulsed I-V

• Small amount of dispersion under large load line
• Similar Gate leakages for different Al
  composition surface termination

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Power Performance

• Al0.15Ga0.85N surface termination. On SiC
  substrate. No passivation and insulator.
• 6.3W/mm with a peak PAE of 75 at 4GHz VD30V,
  ID80mA/mm.
• 13.7W/mm with a peak PAE of 68 at 4GHz VD50V,
  ID80mA/mm.

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Power Performance
Breakdown

• Similar power performance for three different Al
  composition surface termination samples
• The degradation of power performance of
  Al0.05Ga0.95N surface termination sample is due
  to the large gate leakage and breakdown

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Summary

• Partially graded AlGaN cap HEMTs were implemented
  successfully by MOCVD on SiC substrates. Output
  power density of 15.2W/mm with a peak PAE of 45
  was obtained at drain bias of 70V at 4GHz without
  any passivation and insulator.
• Fully graded AlGaN cap HEMTs were implemented
  successfully by MBE on SiC substrates due to its
  lower thermal stress. Different Al composition
  surface termination were investigated. Output
  power density of 13.7W/mm with a peak PAE of 68
  was obtained at drain bias of 50V at 4GHz without
  any passivation and insulator.