cmos vlsi seminar pdf
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--An idea whose time has come
Ankit
Goyal, IIT Roorkee
Tutor: Prof. S. Kal, IIT Kharagpur
Silicon-Germanium
Heterojunction
Bipolar Transistors
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Presentation Overview
History, need of SiGe Technology
Physics behind HBTs
Bandgap Engineering
SiGe Strained Layer Epitaxy SiGe HBT Fabrication: Selective-Epitaxial Growth
Technology aspects
Some applications of Si-Ge HBTs
Future Trends and conclusions
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History of SiGe
Technology (1/2) 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.
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.
In 1957, Kroemer patented the first heterojunction Si bipolar
transistor(Si HBT).
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History of SiGe
Technology (2/2)
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.
In the early 1990s, IBM refocused its SiGe program towards
the rapidly developing communications market.
Interestingly, for RF communications circuits, SiGe HBTconsumes much less power than CMOS to achieve the same
level of performance.
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Bipolar Transistor4
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Need for Si-Ge? Due to booming market for computer and wireless
communication systems, there is a need of a single transistor
technology simultaneously capable of delivering:
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?
Si is wonderfully abundant and can be easily purified.
Si crystals can be grown in amazingly large, virtually defect
free single crystals. (Large wafer size more ICs 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)
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.
Since the speed of a device ultimately depends on how fast
the carriers can be transported through the device undersustainable operating conditions, Si can thus be regarded as a
somewhat slow semiconductor.
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Is Si an Ideal Semiconductor? (2/2)
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?
Answer is Yes, by practicing bandgap engineering in the
Si material system.
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Physics Behind SiGe
HBTs
(1/4) The current amplification of bipolar junction transistor(BJT)
is given by:
In more physical terms it is written as:
If a large is desired, the numerator should be as large aspossible and denominator as small as possible, i.e.
NE >> NB and/or
Making WB small
This puts rather strong constraints on the device and a good
trade-off between parameters is necessary.Silicon-Germanium Heterojunction
Bipolar Transistor9
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Physics Behind SiGe
HBTs
(2/4) 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
To have low base resistance
Base width is kept low so that the delay caused by diffusion of
the minority carriers through the base is kept low
In case of heterojunction bipolar transistors(HBT), increases drastically with increasing bandgap difference.
This is because intrinsic carrier concentration ni is strongly
dependent on the bandgap.
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Physics Behind SiGe
HBTs
(3/4)
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Bipolar Transistor11
ni is given as:
where
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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Bandgap
Engineering: Introducing Ge
into Si SiGe has a bandgap smaller than Si and hence makes bandgap
engineering possible.
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 increased collector current gain
This enhanced gain can be traded for increased base dopingand decreased basewidth, and hence improved high-
frequency performance.
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Band diagram of the E/B junction of a SiGe
HBT
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Bandgap
Engineering Continued 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 thebase, reduces the base transit time and enhances the value of
fT.
Graded Ge
in base decrease in bandgap from E to C
quasi-drift field aids transport across base reduces base
transit time 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 aconventional silicon-only bipolar Junction transistor
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Bipolar Transistor16
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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? 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. These SiGe strained layers are subject to a fundamental stability
criterion limiting their thickness for a given Ge concentration.
The compressive strain associated with SiGe alloys produces anadditional bandgap shrinkage, and the net result is a bandgap
reduction of approximately 75meV for each 10% of Ge
introduced.17 Silicon-Germanium Heterojunction
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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. When SiGe epitaxy is grown onto a thick Si substrate host, this
inherent lattice mismatch between the SiGe film and theunderlying Si substrate can be accommodated in two ways.
First, the lattice of the deposited SiGe alloy distorts in such a waythat it adopts the underlying Si lattice constant, resulting inperfect crystallinity across the growth surface. This scenario is
known as pseudomorphic growth. Because of additional strain energy contained in the SiGe film it
embodies a higher energy state than for an unrestrained film.
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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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SiGe
HBT Fabrication: Selective-Epitaxial
Growth
Selective epitaxy is the growth of a single-crystal layer in a
window, with complete suppression of growth elsewhere.
An overhanging p+ polysilicon extrinsic base is created in anemitter window prior to base epitaxy:
Growth of an oxide layer
The deposition and p+ doping of a polysilicon layer The deposition of a nitride layer
The exposed vertical face of the polysilicon is covered by
nitride deposition.
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Selective Epitaxial
Growth Continued
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Bipolar Transistor24
The SiGe base is grown by selective epitaxy, which gives single-crystal SiGe on the exposed collector but no deposition on the
nitride surface layer. It is necessary to suppress deposition of polycrystalline material on
the nitride spacer and the silicon dioxide surface layer, and it canbe achieved in number of different ways.
The most popular method involves the use of chlorine(addingHCL or Cl2 to growth gases).
Chlorine increases the surface mobility of silicon and germanium
atoms, so that atoms deposited on the oxide or nitride layer areable to diffuse across the surface to the window where the growthis occurring.
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Selective Epitaxial
Growth Continued
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Bipolar Transistor25
Polycrystalline SiGe is deposited on the overhanging p+
polysilicon to create a graft base.
Once the graft base and selective SiGe base have madecontact, 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 IBMs earliestproduction technology (BiCMOS 5HP).
Since device switching at these speeds is not necessary for
the bulk of wireless circuits operating at frequencies from900 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 notablyoperation at lower power levels.
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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 wirelesscommunications market.
Wireless systems are revolutionizing both the communicationsand computer industries, and providing a driving force for themerging of these two industries into a single information industry.
Most wireless applications tend to be in the 110 GHz frequencyrange. Products include cordless phones, mobile phones, wirelesslocal area networks, TV, satellite communications and automotivenavigation and toll systems.
A vast range of rf and mixed-signal circuits are possible with thistechnology, such as low noise amplifiers, power amplifiers,mixers, voltage controlled oscillators, synthesisers, and high speedanalogue to digital and digital to analogue converters.
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Applicationscontinued
A second application area where SiGe HBTs are finding
application is in optical fibre communication systems
operating at 10, 20 and 40 Gb/s. Silicon bipolar integrated circuits have already been
reported for 10 Gb/s optical communication systems and
research is underway on both Si bipolar and SiGeheterojunction bipolar circuits for 20 and 40 Gb/s systems.
A variety of circuits have been realized, including dividers,
multiplexers, demultiplexers, preamplifiers and decision
circuits.
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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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Bipolar Transistor33
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References Silicon-germanium HBTs for 40 Gb/s and beyondIII-Vs Review, Volume 14, Issue 6, August 2001, Pages 36-38David C Ahlgren, Greg Freeman, Basanth Jagannathan and Seshadri Subbanna
Materials and technology issues for SiGe heterojunction bipolar transistorsMaterials Science in Semiconductor Processing, Volume 4, Issue 6, December 2001,Pages 521-527Peter Ashburn
High speed SiGe heterobipolar transistorsJournal of Crystal Growth, Volume 157, Issues 1-4, 2 December 1995, Pages 207-214
Andreas Schppen and Harry Dietrich High-speed SiGe HBTs and their applications
Applied Surface Science, Volume 224, Issues 1-4, 15 March 2004, Pages 306-311Katsuyoshi Washio
J. Cressler, G. Niu, Silicon-Germanium Heterojunction Bipolar. Transistors, Boston:Artech House. 2003.
Applications of Silicon-Germanium Heterostructure DevicesCK Maiti, GA Armstrong - 2001
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THANK YOU!!!
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