III-V HETEROJUNCTION BIPOLAR TRANSISTORS

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III-V HETEROJUNCTION BIPOLAR TRANSISTORS

• Yan Yan
• Department of Electrical Engineering
• University of Notre Dame, Notre Dame, IN
  46556-5637
• April 27, 2004

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III-IV BIPOLAR TRANSISTOR TECHNOLOGY

• Characteristic Parameters
• Current gain
• Cutoff frequency / Speed
• Parasitic capacitances
• Methods to improve performances
• Device design
• Choice of material systems

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GaInAs/InP Buried Metal HBT- REDUCTION OF
BASE-COLLECTOR CAPACITANCE

• Buried Tungsten wires of the same area as the
  emitter metal was used to reduce CBCext
• SBCT of BM-HBT was estimated to be 22 that of
  conventional HBT, CBC 30 of conventional HBT
• fT 86 GHz, fMAX gt 135 GHz of device with an
  emitter area of 0.5 x 2.5 µm2
• fT 82 GHz, fMAX gt 200 GHz of device with an
  emitter area of 0.3 x 1.5 µm2

Schematic view of fabricated BM-HBT
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Layer structure for the buried metal - HBT
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I V Characteristics

• Current gain about 70 at the
• collector voltage of 4V
• S-parameters were measured
• from 50 MHz to 30 GHz using
• an HP8722 network analyzer
• Extrapolations of fT and fMAX were
• carried out from the -20dB/decade
• regions of current gain (h212)
• and Masons unilateral gain (U),
• respectively
• the values of fT and fMAX reached
• peak points (fT 33.5 GHz, fMAX
• 47.3 GHz) at IC 4mA and VC
• 6V

Common-emitter collector I-V characteristics of
BM-HBT with an emitter area of 2x10 µm2
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SEM image of BM-HBT
SEM image of the fabricated BM-HBT with
an Emitter width of 0.3 µm after formation of the
Dummy mesa. Good alignment between the Wires and
the emitter is observed.
A SEM view of a cross section cut by a focuse Ion
beam. Measured collector thickness was 290 nm.
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AlGaAs-GaInP Composite Emitter in GaInP/GaAs
HBT- Improved Emitter Transit Time

• Emitter Transit Time

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Composite Graded Emitter vs. Conventional Emitter

• Self-aligned HBTs are grown by CBE
• (chemical beam epitaxy)
• Increase in fT from 44 GHz to 62 GHz
• CBE 3 times lower for composite emitter
• HBT without significant IC variation
• Common limitation in high speed
• performance of HBTs large CBE (limited
• mobile carrier transport thus charges
• accumulation in the emitter)
• Composite graded AlGaAs layer forms
• an electron launcher at the interface
• with the GaInP layer, which injects the
• electrons at a higher kinetic energy
• toward the remaining part of the emitter,
• It leads to lower free carrier
• concentration (Qe) and smaller CBE

Energy band diagram of GaInP/GaAs (a)Composite
emitter (b)conventional design HBT
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Comparison of eletric field and electron density

• Compositonally graded AlGaAs emitter
• HBTs have much stronger electric
• fields present in the emitter
• The electron density is dramatically
• decreased due to the presence of a
• drift velocity component in this region
• of the emitter
• GaInP conventional emitter HBTs do
• not have a built-in electric field within
• the emitter region, and the electron
• density in this case is increased due
• to slow transport of carriers and thus
• carrier accumulation

Comparison of (a)electric field and (b)electron
density Profiles for GaInP conventional and
AlGaAs0GaInP composite emitter design HBTs.
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Comparison of electron velocity

• The figure focuses on the velocity
• characteristics responsible for the
• improved frequency characteristics
• In the case of the composite emitter
• design, the electron velocity is high
• due to the drift velocity component
• in the special emitter region
• On the other hand, the electron
• velocity of the conventional emitter
• design is slower since diffusion
• carrier transport is dominant in the
• emitter region, which consists only
• of GaInP
• An estimate of ?E using simulation
• to evaluate ?Qe/?Jc showed values
• of 0.13ps and 0.57ps, respectively.

Comparison of electron velocity profiles for
GaInP conventional and AlGaAs-GaInP emitter
design HBTs in the composite emitter region
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Comparison of CBE and fT

• CBE for a composite emitter HBT
• was found to be at least 3 times
• lower than the conventional emitter
• HBT under high IC operation
• CBE for a composite emitter HBT
• presents a weak Jc dependence
• fT was improved from 44 GHz to
• 62 GHz by using the composite
• emitter in the HBT

Comparison of CBE and fT characteristics for
GaInP/ GaAs HBT (a)composite emitter and
(b)conventional emitter
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InGaP/GaAs HBT with WSi/Ti Electrode and Buried
SiO2 in the Extrinsic Collector- decrease of
emitter size and CBC

• The width of the base contact is reduced
• by using a self-aligning process
• The buried SiO2 reduces the parasitic
• capacitance because the dielectric constant
• of SiO2 is about 1/3 of that of GaAs
• WSi/Ti is used as the base electrode instead
• of conventional gold-based electrode. Both
• WSi and Ti can be deposited by sputtering
• with good step coverage and selectively
• patternede on GaAs and SiO2 by RIE. A thin
• Ti film inserted between WSi and GaAs
• reduces the contact resistance, and made it
• possible to reduce the width of the base
• contact without the large increase in the
• base resistance
• The emitter size effect on current gain was
• suppressed by using InGaP as the emitter

Schematic cross-section of device
structure (a)Conventional HBT and (b)small-scale
HBT with a WSi/Ti base electrode and buried SiO2
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Device Performance

• The DC current gain of 20 is obtained for an HBT
  with SE of 0.3 x 1.6 µm2 due to the suppression
  of emitter size effect by using InGaP as the
  emitter material
• An HBT with SE of 0.6 x 4.6 µm2 exhibited fT of
  138 GHz and fmax of 275 GHz at IC of 4 mA
• An HBT with SE of 0.3 x 1.6 µm2 exhibited fT of
  96 GHz and fmax of 197 GHz at IC of 1 mA

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Motivation for work in InAs bipolar transistors

• Historic trend Increased the amount of Indium
  in the base of a HBT
• Higher electron mobility saturation velocity
• ? shorter base transit time
• Improved base resistance/base contact resistance
• Faster device
• Advantages compared to the traditional III-Vs
• Lower electron effective mass (0.022 m0)
• Higher electron mobility (up to 33000 cm2 V-1
  sec-1 at room temperature)
• Higher peak velocity

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Cracking study of AlxIn1-xAs on InAs
AlxIn1-xAs grown on an InAs substrate is tensile
stained, and there exists a critical thickness
for the epilayer to form cracks to relieve strain

• Two spicific regions of interest
• were studied, one with low Al
• concentration (7-9) for the
• HBT devices, and one with
• higher Al concentration (40-
• 50) for other devices.
• For x9, maximum thickness
• is around 450 Å
• Samples were examined by
• Normarski contrast interference
• Microscopy to determine whether
• they were cracked or crack free

AlxIn1-xAs epilayers on InAs substrates as a
function of Al composition
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InAs HBT device structure and I-V
Room temperature common emitter J-V
characteristics of an InAs HBT
Device structure of InAs BJT and HBT devices
ßmax for BJT is 50, ßmax for HBT is 100
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SUMMARY

• Base collector capacitance reduced by as high as
  70 in BM-HBT
• Base emitter capacitance reduced to one-third by
  composite InGaP/GaAs emitter
• Parasitic capacitance reduced by 50 in
  InGaP/GaAs HBT using WSi base electrode and
  buried SiO2 layer
• First results for InAs bipolar transistors (ß
  100)

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Reference

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Reference

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