Silicon-Germanium
Heterojunction Bipolar Transistors
--An idea whose time has come
Ankit Goyal, IIT Roorkee
Tutor: Prof. S. Kal, IIT Kharagpur
Presentation Overview
y History, need of SiGe Technology
y Physics behind HBTs
y Bandgap Engineering
y SiGe Strained Layer Epitaxy
y SiGe HBT Fabrication: Selective-Epitaxial Growth
y Technology aspects
y Some applications of Si-Ge HBTs
y Future Trends and conclusions
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History of SiGe Technology (1/2)
y The concept of combining silicon (Si) and germanium (Ge)
into an alloy for use in transistor engineering is an old one,
and was probably envisioned by Shockley in 1950.
y However, because of difficulties in growing lattice-matched
SiGe alloy on Si, this concept is reduced to practical reality
only in the last 20 years.
y In 1957, Kroemer patented the first heterojunction Si bipolar
transistor(Si HBT).
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History of SiGe Technology (2/2)
y SiGe HBT technology was originally developed at IBM for
the high-end computing market, that effort, however, failed
to CMOS, primarily because of its high power consumption.
y In the early 1990s, IBM refocused its SiGe program towards
the rapidly developing communications market.
Interestingly, for RF communications circuits, SiGe HBT
consumes much less power than CMOS to achieve the same
level of performance.
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Need for Si-Ge?
y Due to booming market for computer and wireless
communication systems, there is a need of a single transistor
technology simultaneously capable of delivering:
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Low Power
High Linearity
Low Noise
High speed of operation for RF, analog, memory and digital circuits
Low cost
“One technology fits all”
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Why Si?
y Si is wonderfully abundant and can be easily purified.
y Si crystals can be grown in amazingly large, virtually defect
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free single crystals. (Large wafer size Æ more ICs Æ low cost)
Si can be controllably doped with both n-type and p-type.
The energy bandgap of Si is of moderate magnitude
(1.12eV at 300K)
Non Toxic and highly stable
Excellent thermal (allowing for efficient removal of dissipated
heat) and mechanical properties
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Is Si an Ideal Semiconductor? (1/2)
y The carrier mobility for both electrons and holes in Si is
comparatively small, and the maximum velocity that these
carriers can attain under high electric fields is limited to
about 1x107 cm/sec under normal conditions.
y Since the speed of a device ultimately depends on how fast
the carriers can be transported through the device under
sustainable operating conditions, Si can thus be regarded as a
somewhat “slow” semiconductor.
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Is Si an Ideal Semiconductor? (2/2)
y Is it possible to improve the performance of Si transistors
enough to be competitive for high frequency applications,
while preserving the enormous yield, cost and manufacturing
advantages associated with conventional Si fabrication?
y Answer is Yes, by practicing bandgap engineering in the
Si material system.
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Physics Behind SiGe HBTs (1/4)
y The current amplification of bipolar junction transistor(BJT)
is given by:
y In more physical terms it is written as:
y If a large β is desired, the numerator should be as large as
possible and denominator as small as possible, i.e.
NE >> NB and/or
y Making WB small
y
y This puts rather strong constraints on the device and a good
trade-off between parameters is necessary.
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Physics Behind SiGe HBTs (2/4)
y For current amplification, a low NB and small WB is
desirable, but at the same time the doping and width of the
base must be large:
To avoid punch-through
y To have low base resistance
y Base width is kept low so that the delay caused by diffusion of
the minority carriers through the base is kept low
y
y In case of heterojunction bipolar transistors(HBT), β
increases drastically with increasing bandgap difference.
y This is because intrinsic carrier concentration ni is strongly
dependent on the bandgap.
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Physics Behind SiGe HBTs (3/4)
y ni is given as:
where
y The current amplification factor for the HBT can be defined
as:
where
is the difference in bandgap between the
emitter and base.
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Physics Behind SiGe HBTs (4/4)
y As the second term in last expression can amount to several
orders of magnitude it is no longer necessary to keep the
constraints put on the BJT, i.e.
Most importantly, the base doping can be increased to values
above the emitter doping and still maintain an adequate gain.
y Base width can be lowered without risking neither punchthrough nor a too high base resistance.
y The current density can also be increased which means that an
HBT can be made smaller compared to a BJT.
y
y The unity current gain frequency(ft) increases and maximum
frequency of oscillation(fmax) increases.
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Bandgap Engineering: Introducing Ge into Si
y SiGe has a bandgap smaller than Si and hence makes bandgap
engineering possible.
y When incorporated into the base of a bipolar transistor SiGe
gives a reduction in the potential barrier to electrons in the
emitter. The result is enhanced collector current and hence
enhanced gain.
less potential barrier Æ increased collector current Æ gain
y This enhanced gain can be traded for increased base doping
and decreased basewidth, and hence improved highfrequency performance.
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Band diagram of the E/B junction of a SiGe HBT
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Bandgap Engineering Continued…
y The Ge profile is often graded across the base to give a
bandgap that decreases from emitter to collector. This gives
a quasi-drift field, which aids carrier transport across the
base, reduces the base transit time and enhances the value of
fT.
Graded Ge in base Æ decrease in bandgap from E to C Æ
quasi-drift field Æ aids transport across base Æ reduces base
transit time Æ increase in value of fT.
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Bandgap diagram showing reduction of conduction band
resulting from graded doping of germanium across the
base region of the SiGe HBT in comparison to a
conventional silicon-only bipolar Junction transistor
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Bandgap Engg.: Initial Difficulties
While the basic idea of using SiGe alloys to bandgap-engineer Si
devices dates to the 1950s, the synthesis of defect free SiGe films
was not successfully produced until the mid-1980s. Why?
y While Si and Ge can be combined to produce a chemically stable
alloy, their lattice constants differ by roughly 4.2% and thus SiGe
alloys grown on Si substrates are compressively strained.
y These SiGe strained layers are subject to a fundamental stability
criterion limiting their thickness for a given Ge concentration.
y The compressive strain associated with SiGe alloys produces an
additional bandgap shrinkage, and the net result is a bandgap
reduction of approximately 75meV for each 10% of Ge
introduced.
y
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Stability of SiGe strained layers
The lattice mismatch between pure Si (a = 5.431A) and pure Ge
(a = 5.658) is 4.17% at 300K, and increases only slightly with
increasing temperature.
y When SiGe epitaxy is grown onto a thick Si substrate host, this
inherent lattice mismatch between the SiGe film and the
underlying Si substrate can be accommodated in two ways.
y First, the lattice of the deposited SiGe alloy distorts in such a way
that it adopts the underlying Si lattice constant, resulting in
perfect crystallinity across the growth surface. This scenario is
known as “pseudomorphic” growth.
y Because of additional strain energy contained in the SiGe film it
embodies a higher energy state than for an unrestrained film.
y
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Stability of SiGe strained layers Continued..
Second and alternatively, the SiGe film relaxes via misfit
dislocation formation, resulting in a break in crystallanity across
the growth interface.
y When the strain energy in the film exceeds the activation energy
required for misfit formation and movement, the film will relax,
releasing the stored strain energy.
y SiGe growth process will occur as follows:
y
y Si substrate being thick remains essentially unchanged.
y The growth of the SiGe film will begin pseudomorphically, adopting
the underlying Si lattice constant.
y When a critical thickness is reached, the strain energy becomes too
large to maintain local equilibrium and SiGe film will relax to its
natural lattice constant, with the excess strain energy being released
via misfit formation.
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Schematic 2-D representation of both
strained and relaxed SiGe on a Si Substrate.
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Theoretical (solid) and experimental (dotted) curves
relating misfit strain and SiGe layer thickness, showing
regions of unstable SiGe films and region of
unconditionally stable films – Matthews and Blakeslee
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Stability of SiGe strained layers Contnd..
y A concept for strain adjustment has been suggested by adding
carbon into SiGe material system.
y As the lattice parameter of carbon(3.546 A) is much smaller
than that of Si and Ge, C may be used as a substitution
impurity in the SiGe to decrease the lattice mismatch.
y According to Vegard’s law, for about 12% Ge in Si and 1%
C in silicon, the mismatch is equal and opposite; and a strain
symmetrized structure with average zero strain may be
obtained.
y The incorporation of C, however, present difficult challenges
due to lattice mismatch, low solubility and SiC formation.
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SiGe HBT Fabrication: Selective-Epitaxial Growth
y Selective epitaxy is the growth of a single-crystal layer in a
window, with complete suppression of growth elsewhere.
y An overhanging p+ polysilicon extrinsic base is created in an
emitter window prior to base epitaxy:
Growth of an oxide layer
y The deposition and p+ doping of a polysilicon layer
y The deposition of a nitride layer
y The exposed vertical face of the polysilicon is covered by
nitride deposition.
y
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Selective Epitaxial Growth Continued…
The SiGe base is grown by selective epitaxy, which gives singlecrystal SiGe on the exposed collector but no deposition on the
nitride surface layer.
y It is necessary to suppress deposition of polycrystalline material on
the nitride spacer and the silicon dioxide surface layer, and it can
be achieved in number of different ways.
y The most popular method involves the use of chlorine(adding
HCL or Cl2 to growth gases).
y Chlorine increases the surface mobility of silicon and germanium
atoms, so that atoms deposited on the oxide or nitride layer are
able to diffuse across the surface to the window where the growth
is occurring.
y
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Selective Epitaxial Growth Continued…
y Polycrystalline SiGe is deposited on the overhanging p+
polysilicon to create a graft base.
y Once the graft base and selective SiGe base have made
contact, a p-type Si emitter cap is selectively grown to fill
the emitter window.
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Schematic cross-sectional view of the main
region of the self-aligned SEG SiGe HBT.
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Technology Aspects
Some of the early pay-off in using the Si/SiGe HBT was its
ability to perform at very high speeds: e.g. 65 GHz
maximum oscillation frequency in IBM’s earliest
production technology (BiCMOS 5HP).
y Since device switching at these speeds is not necessary for
the bulk of wireless circuits operating at frequencies from
900 MHz to 2.4 GHz, the usefulness of the SiGe HBT
comes at being able to trade this excess speed for
improvement in other device figures of merit, most notably
operation at lower power levels.
y
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The BiCMOS 7HP SiGe can run at maximum oscillation frequencies up
to 120 GHz, a value not used in a substantial number of high
performance applications. By reducing operating currents, however
one can trade excess speed for substantially reduced power
consumption.
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Technology Comparison in the frequency
range of 1-10 GHz
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Applications
The explosion of interest in SiGe heterojunction bipolar
technology is being driven in the first instance by the wireless
communications market.
y Wireless systems are revolutionizing both the communications
and computer industries, and providing a driving force for the
merging of these two industries into a single information industry.
y Most wireless applications tend to be in the 1–10 GHz frequency
range. Products include cordless phones, mobile phones, wireless
local area networks, TV, satellite communications and automotive
navigation and toll systems.
y A vast range of rf and mixed-signal circuits are possible with this
technology, such as low noise amplifiers, power amplifiers,
mixers, voltage controlled oscillators, synthesisers, and high speed
analogue to digital and digital to analogue converters.
y
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Applications continued…
y A second application area where SiGe HBTs are finding
application is in optical fibre communication systems
operating at 10, 20 and 40 Gb/s.
y Silicon bipolar integrated circuits have already been
reported for 10 Gb/s optical communication systems and
research is underway on both Si bipolar and SiGe
heterojunction bipolar circuits for 20 and 40 Gb/s systems.
y A variety of circuits have been realized, including dividers,
multiplexers, demultiplexers, preamplifiers and decision
circuits.
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Future trends and conclusions
y Due to SiGe’s proven ability to achieve power added
efficiencies reaching 70%, the use of SiGe HBTs for power
amplification is a very rich area of design activity.
y At present, the fastest SiGe HBTs have greater than 210 GHz
cutoff frequency (fT) and greater than 285 GHz maximum
oscillation frequency!
y Digital gates built with these HBTs show a gate delay of only
4.3 ps, with just a milliamp of electrical current.
y In the future, multiple versions of HBTs optimized for
wireless, wired, or storage application will be offered.
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Future trends and conclusions
This is quite remarkable, as put by Dr. Bernard Meyerson,
“Just as aircraft were once believed incapable of breaking an
imaginary ‘sound barrier’, silicon-based transistors were
once thought incapable of breaking a 200 GHz speed barrier”
200 GHz SiGe HBTs are a reality! … 300 GHz is on the way!
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References
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Silicon-germanium HBTs for 40 Gb/s and beyond…
III-Vs Review, Volume 14, Issue 6, August 2001, Pages 36-38
David C Ahlgren, Greg Freeman, Basanth Jagannathan and Seshadri Subbanna
Materials and technology issues for SiGe heterojunction bipolar transistors
Materials Science in Semiconductor Processing, Volume 4, Issue 6, December 2001,
Pages 521-527
Peter Ashburn
High speed SiGe heterobipolar transistors
Journal of Crystal Growth, Volume 157, Issues 1-4, 2 December 1995, Pages 207-214
Andreas Schüppen and Harry Dietrich
High-speed SiGe HBTs and their applications
Applied Surface Science, Volume 224, Issues 1-4, 15 March 2004, Pages 306-311
Katsuyoshi Washio
J. Cressler, G. Niu, “Silicon-Germanium Heterojunction Bipolar. Transistors,” Boston:
Artech House. 2003.
Applications of Silicon-Germanium Heterostructure Devices
CK Maiti, GA Armstrong - 2001
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THANK YOU!!!
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