High speed (207 GHz ft), Low Thermal Resistance, High Current Density Metamorphic InP/InGaAs/InP DHBTs grown on a GaAs Substrate

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High speed (207 GHz ft), Low Thermal Resistance,
High Current Density Metamorphic InP/InGaAs/InP
DHBTs grown on a GaAs Substrate

• Y.M. Kim, M. Dahlstrom, S. Lee, Y. Wei, M.J.W.
  Rodwell, A.C. Gossard
• Department of Electrical Engineering, Materials
  Department,
• University of California, Santa Barbara

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Technical Objective
Growth of InGaAs/InAlAs/InP HBTs on GaAs
substrates .with low leakage and high
yield .for low-cost high-volume manufacturing
of InP HBT integrated circuits on 6" diameter
substrates Gives basic data for growth which is
free of lattice constant limit
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Why InP-based HBTs ?
better device bandwidth than GaAs or Si bipolar
transistors microwave ADCs, DACs, digital
frequency synthesis better Emaxvsat than GaAs
millimeter-wave power
Why metamorphic HBTs ?--economic argument
low cost, high volume processing wafer size is
critical GaAs substrates, processes 6" diameter
now large InP substrates high cost, high
breakage, only 4" available today breakage much
worse with 6" wafers grow InP-based HBTs on
GaAs substrates for cost and manufacturability
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Metamorphic HBTs
InGaAs/InP or InGaAs/InAlAs HBT on a GaAs
substrate Lattice mismatch between substrate and
epitaxial device layersThick intervening buffer
layer to capture most defects
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Why might M-HBTs be harder than M-HEMTs ?
Much thicker depletion regions
base-collector (2kÅ) vs. gate-channel junctions
(200 Å)1,000--10,000 times more active device
area defect density, thermal resistance
more serious concerns
HBT
HEMT
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What are the potential problems ?
Defects collapse in DC gain recombination
in e/b junction surface recombination
recombination in base generation in
collector Thick (ternary) buffer layer
poor thermal conductivity
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RHEED of metamorphic layer
AlGaAsSb
InAlAs

• Show the streak lines
• Indicate good surface
• morphology

InP
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Morphology of metamorphic layer
AlGaAsSb
InAlAs
InP
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AFM image of metamorphic layer
AlGaAsSb
InAlAs
InP
Metamorphic buffer Surface roughness (nm)
AlGaAsSb 4.0
InAlAs 11.7
InP 9.5
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Thermal Conductivity Measurement
Pt wire
Metamorphic layer
GaAs subst.

• Pattern a 1x100 µm Pt line 50 nm thick
• Measure the resistance with varying input power
• As the input power increases, the Pt wire gets
  hot and the resistance increases.
• Resistance change is determined by the thermal
  conductivity of underlying layer.
• Extract thermal conductivity of film from finite
  element simulation.

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Results and Junction Temperature Calculation
1000 µm
Metamorphic buffer Thermal conductivity (W/mC)
AlGaAsSb 8.4
InAlAs 10.5
InP 16.1
GaAs bulk 44
InP bulk 69
1000 µm
HBT 8 µm x 0.5 µm
Metamorphic layer 1.5 µm
GaAs 350 µm

• 30 HBTs with 45 µm device separation
• Solve the 3D Laplace eq. to determine junction
  temp. as function of thermal conductivity
• power density 200 kW/cm2

InP buffer has best thermal conductivity though
it is smaller than bulk value.
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Thermal Conductivity vs. HBT Temp.

• Power density
• 200 kW/cm2
• 0.5 ?m x 8 ?m emitter device
• 30 HBTs with 45 ?m device seperation

AlGaAsSb (128C)
InAlAs (112C)
InP (89C)
Without metamorphic (65C)
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Power density vs. HBT Temp.

• High power density is required for future
  device.
• Need high thermal conductivity buffer layer

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Expected Reliability of HBT
Metamorphic buffer Life time relative to AlGaAsSb HBT
AlGaAsSb 1
InAlAs 6.3
InP 119
InP
InAlAs
AlGaAsSb

• Long life time shows that InP buffer is
  essential in metamorphic HBT from thermal point
  of view.

Ref) K.Kiziloglu et al. IPRM, 294 (2000)
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Mesa structure for RF measurement
 
InP emitter
emitter

• Advantage of mesa structure
• Adequate for metamorphic HBT due to the
  excellent heat flow
• High speed operation

In0.53Ga0.47As base
base
InP collector
collector
In0.53Ga0.47As subcollector
Metamorphic buffer (InP, InAlAs,AlGaAsSb)
GaAs substrate
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Structure of metamorphic M-DHBT
 
 

• 500? thick and 8e17/cm3 n-doped emitter1 layer
  was grown for low Cje
• 400 ? base with 50 meV bandgap grading
• 100 ? setback layer was introduced
• 2000 ? collector
• 1.5 µm InP metamorphic layer was grown at 470oC
  on GaAs wafer

 
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InP/InGaAs/InP Metamorphic DHBTon GaAs substrate
Growth 400 Å base, 2000 Å collector
GaAs substrate InP metamorphic buffer
layer (high thermal conductivity) Processing
conventional mesa HBT narrow 2 um base
mesa, 0.4 um emitter Results 207 GHz ft, 140
GHz fmax, 6 Volt BVCEO, b76
 
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Gummel curves
 
Large area (60?m x 60?m) Small area (0.4?m x
0.75?m)

• Small area device shows larger leakage current
  than large area device.
• The leakage current source is not the growth
  defect.
• pad to pad leakage turned out to be the source.
• There may be surface leakage through the side
  wall.
• More study is being tried

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InP/InGaAs/InP Metamorphic DHBTon GaAs substrate
 
VCE 1.5V
J 3.2e5 A/cm2
VCE 1.5V
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Summary
 

• Several materials were tried for metamorphic
• buffer layer
• InP was chosen because of high thermal
• conductivity
• Highest speed for MHBT was acquired
• More study is needed for reducing leakage current