Cmos Vlsi Seminar PDF

8/10/2019 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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Saturday, December 15, 2007

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

2 Silicon-Germanium Heterojunction

Bipolar Transistor


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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).


Bipolar Transistor


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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.

Silicon-Germanium Heterojunction

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


Bipolar Transistor


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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.


Bipolar Transistor


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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.


Bipolar Transistor


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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.


Bipolar Transistor10


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Physics Behind SiGe

HBTs

(3/4)



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.


Bipolar Transistor


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Band diagram of the E/B junction of a SiGe

HBT


Bipolar Transistor


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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.


Bipolar Transistor


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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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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

Bipolar Transistor


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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.


Bipolar Transistor


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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.


Bipolar Transistor


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Selective Epitaxial

Growth Continued



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



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.


Bipolar Transistor


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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.


Bipolar Transistor


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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.


Bipolar Transistor


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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 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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