Progress on Deeply Recessed AlGaNGaN HEMTs

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Progress on Deeply Recessed AlGaN/GaN HEMTs
L. Shen, S. Heikman, M. H. Wong, S. Rajan, T.
Palacios, A. Chakraborty, L. McCarthy, S. Keller
and U. K. Mishra
Department of Electrical and Computer Engineering
University of California, Santa Barbara
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Outline

• Introduction
• Epitaxial Field plate on GaN/AlGaN/GaN HEMTs
• Growth of thick graded AlGaN layer on SiC
  substrate
• Summary

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GaN/AlGaN/GaN HEMT

• Graded layer is doped by Si to remove holes.
• Thin AlN layer increases charge and removes the
  alloy disorder scattering, therefore improving
  the mobility.

• Thick GaN cap layer keeps the surface far away
  from the channel, thus decreasing the dispersion.

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Thick graded AlGaN cap

• A thick graded AlGaN cap layer doped by Si was
  introduced to reduce the gate leakage current and
  increase the breakdown voltage.

• 8.5W/mm with peak PAE of 57 was obtained at
  4GHz.

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Epitaxial field plate

• The introduction of the field plate greatly
  improved the performance of GaN-based HEMT by
  increasing the breakdown voltage and decreasing
  dispersion. Both are due to the reduced electric
  field.
• The epitaxial field plate combines the concepts
  of the thick epitaxial cap layer and field plate,
  to reduce peak electric field.

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Electric field distribution

• The device without field plate has one strong
  electric field peak at the corner of the gate on
  the drain side while the one with field plate has
  two peaks with lower amplitudes.

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Electric field distribution

• The peak of the electric field at the corner of
  the gate in the device reduces when field plate
  length LFP increases. The reduction rate also
  decreases with the increased LFP.

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DC and pulsed I-V performance

• On sapphire substrate.
• DC and pulsed I-V performances remain same when
  the length of the field plate changes.
• No dispersion is observed at pulse width of
  200ns.

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Gate leakage and breakdown
Gate-drain VBR
IG_at_VGD50V

• Gate-drain leakage current decreases with a short
  field plate, then starts to increase gradually
  with increased field plate.
• Gate-drain breakdown voltage increases when field
  plate is short (lt0.4um), then drops.

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Small signal performance

• The ft decreases with the increased field plate
  length LFP, due to the larger gate-drain
  capacitance.
• The initial increase of the fMAX can be explained
  by the reduction of the gate resistance by the
  large field plate. Then it decreases.

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Power performance

• Power performance remains similar when the length
  of the field plate increases. This is probably
  because the reduced electric field peak is close
  to the corner to the gate and far away from the
  surface. Moreover, the effect of the surface to
  the channel is not as obvious as the conventional
  AlGaN/GaN HEMT.

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Power performance

• The HEMTs on sapphire substrate with 0.2-0.8mm
  field plate can be biased at 40V while the
  devices without and with 1mm field plate cant.
  The drop of the PAE with the increased field
  plate may be due to the reduction of the gain.

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Power performance

• HEMT with 0.2mm field plate on sapphire
  substrate.
• Device was biased at VD40V, ID50mA/mm
• 8.4W/mm with a PAE of 64 was obtained at 4GHz.

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The growth of the thick graded AlGaN layer on SiC
substrate

• Necessity good thermal conductivity of SiC
  substrate leads to better power performance
• Difficulty tensile stress in the thick graded
  AlGaN layer grown on SiC substrate is higher than
  that on sapphire substrate, resulting in the
  cracking.

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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

• The thickness of graded region is reduced to
  100nm, and then a 150nm UID Al0.05Ga0.95N is
  grown on top of it. The new design relieves the
  high stress by the reduction of the graded layer
  thickness while retaining a good breakdown
  voltage by the Al0.05Ga0.95N cap layer. A 2DEG
  density of 8.51012/cm2 is achieved.

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DC and pulsed I-V performance

• Small amount of the current collapse is observed
  at pulse width of 200ns, due to the limited
  pinch-off voltage of the drain access region.
• Gate-drain breakdown voltage of more than 90V was
  achieved.

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Power performance

• On SiC substrate. No passivation and insulator.
• 6W/mm with a peak PAE of 65 was obtained at 4GHz
  VD30V, ID50mA/mm.
• 11.6W/mm with a peak PAE of 59 was obtained at
  4GHz VD50V, ID50mA/mm.

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Power performance

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

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Power performance

• The slower increase of the output power density
  and the drop of PAE at higher drain bias is due
  to both dispersion and bad matching.

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Summary

• Epitaxial field plate on GaN capped AlGaN/GaN
  HEMTs was studied systematically. The
  introduction of a short field plate reduced
  leakage current and increased breakdown voltage.
  But it had no obvious impact on power
  performance.
• Thick graded AlGaN layer was grown on SiC
  substrate successfully by the reduction of the
  stress. Both high charge density and breakdown
  voltage were achieved at the same time. Output
  power density of 15.2W/mm with a peak PAE of 45
  was obtained at 4GHz without any passivation and
  insulator.
• Future work
• Optimization of the growth on SiC subtrate e.g.
  AlN layer for strain control.