Download advances in iii-v compound semiconductor based heterojunction

III-V HETEROJUNCTION BIPOLAR
TRANSISTORS
Yan Yan
Department of Electrical Engineering
University of Notre Dame, Notre Dame, IN 46556-5637
April 27, 2004
III-IV BIPOLAR TRANSISTOR
TECHNOLOGY
Characteristic Parameters
 Current gain
 Cutoff frequency / Speed
 Parasitic capacitances
Methods to improve performances
 Device design
 Choice of material systems
GaInAs/InP Buried Metal HBT
- REDUCTION OF BASE-COLLECTOR CAPACITANCE
Schematic view of fabricated BM-HBT
• 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 > 135 GHz of
device with an emitter area of
0.5 x 2.5 μm2
• fT = 82 GHz, fMAX > 200 GHz of
device with an emitter area of
0.3 x 1.5 μm2
Layer structure for the buried metal - HBT
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 (|h21|2)
and Mason’s 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
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.
AlGaAs-GaInP Composite Emitter in
GaInP/GaAs HBT
- Improved Emitter Transit Time
Emitter Transit Time
E
kT
kT
kT Qe
CBE  CBC  

(
 C BC )
qJ C
qJ C VBE
qJ C
transconduc tan ce
C BE Base - emitter capacitanc e
C BC Base - collector capacitanc e
Qe Electron charge in the emitter region
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
Energy band diagram of GaInP/GaAs (a)Composite concentration (Q ) and smaller C
e
BE
emitter (b)conventional design HBT
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.
Comparison of electron velocity
Comparison of electron velocity profiles for GaInP
conventional and AlGaAs-GaInP emitter design
HBTs in the composite emitter region
• 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 ‫ז‬E using simulation
to evaluate ∆Qe/∆Jc showed values
of 0.13ps and 0.57ps, respectively.
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
InGaP/GaAs HBT with WSi/Ti Electrode and
Buried SiO2 in the Extrinsic Collector
- decrease of emitter size and CBC
Schematic cross-section of device structure:
(a)Conventional HBT and (b)small-scale HBT
with a WSi/Ti base electrode and buried SiO2
• 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
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
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
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 (4050%) for other devices.
• For x=9%, 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
InAs HBT device structure and I-V
Device structure of InAs BJT and HBT devices
βmax for BJT is 50, βmax for HBT is 100
Room temperature common emitter J-V
characteristics of an InAs HBT
SUMMARY
Base collector capacitance reduced by as
high as 70% in BM-HBT
Base emitter capacitance reduced to onethird 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 (β
~ 100)
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