Beyond CMOS

Mr.Narayana Swamy.RAssoc.Prof,Dept of ECE

T.John Institute of Technology, Bangalore

Evolutionary advances beyond CMOS

• Evolutionary approaches-Silicon on Insulator, Dual gate FETs and SiGe structures.

• SOI– Consists of thin SiFET structure grown on a buried SiO2 layer which is in turn grown on a

Si substrate.– Complete electrical isolation,– Eliminating latch up between adjacent devices (electrical behavior (electrical

interaction can occur between neighboring tubs and/or the substrate) of one device affects that of a neighboring device),

– Rad-hard or radiation hardness-Highly immune to radiation induced failure, such as soft errors (change in logic state of a device due to the accumulation of charge resulting from carrier generation following the transit of a high energy particle or ionizing radiation through the device),

– Immune to short channel effects than standard CMOS,

• Two different manner SOI devices are operated (depending on thickness of Si layer and its doping concentration)

• Partially • Fully depleted

– Partially depleted SOI devices• Made with thicker Si active layers• Disadvantage is body charging

– Fully Depleted SOI devices• Thin active layer and the doping concentration sufficiently low that it is depleted of

free carriers.

• Dual gate FET– Short channel effects are reduced – Reduce the drain-induced barrier lowering and minimize threshold sensitivity to the

channel– Relative Scaling advantage of about a factor of 2 over conventional CMOS devices– Disadvantage presence of two si-sio2 interfaces from two gates increases the surface

roughness scattering. As a result the carrier mobility is reduced thus lowering the frequency of operation of the device.

– Two different types of dual gate MOSFET structures -Symmetric and asymmetric gated devices• Surface channels turn on at same gate voltages -Same work function• Surface channels turn on at different gate voltages - different work function

– Three different ways in which dual gate MOSFET can be made• With top and bottom gates formed parallel to the plane of the substrate. In this configuration the source and

drain regions straddle the two gates and the current flow in the channel runs parallel to the substrate.• Vertical dual gate FETs the gates are perpendicular to the substrate and the current runs parallel to the

substrate- FinFET• Si and Si-Ge together to form heterostructure.The enhanced electron mobility within the strained Si acts to

increase the speed of the SiGe MOSFET. Therefore, strained Si-SiGe MOSFETs have an intrinsically higher transconductance and thus higher frequency of operation than conventional MOSFETs.

WHAT IS SHORT CHANNEL

• Channel length ~= depletion width of source and drain

• Short channel MOS has good processing speed, requires low operating potential and increases transistor density on the chip.

• Although the performance degrades with decrease in channel length.

• It faces some serious issues like DIBL, surface scattering, velocity saturation, impact ionisation, hot electron effect

DRAIN INDUCED BARRIER LOWERING(DIBL)

SURFACE SCATTERING

• In long channel Ex>>Ey, but in short channel Ex is not negligible.

• Ex and Ey field makes electron to travel in zig-zag path, reducing their mobility.

• As the channel length becomes shorter electric field Ey increases causing surface mobility to become field dependent.

Carbon nanotubes• Very small graphite sheets rolled into a seamless cylinder. Nano tubes are

very thin and are long macro molecules of carbon.

• Nano tubes can be either multi walled or single walled.– Multiwalled nanotubes consists of concentric cylinders wrapped around each

other while single walled nanotubes consists only of one layer.

• Nanotubes can act as metal or semiconductor depending upon the diameter of the tube and its helicity, or degree to which the carbon spirals around the tube. Large diameter – semi metal; narrow diameter- semiconductor.

• The energy band gap of a carbon nano tube depends mainly upon the diameter of the tube; the larger the diameter the smaller the energy gap.

• Mechanically they have largest strength to weight ratios of any material.• Very high thermal conductance, close to diamond making them efficient

heat conductors. Nano tubes can be used to cool very dense arrays of devices.

• Nano tube may provide an alternative interconnect material that has a high thermal conductivity, and can thus help in dissipating heat in a highly packed circuit.

• Nano tubes can support extremely high current densities without any electro migration problems.

• Nano tubes may find usefulness as inter connects. Their very small diameter and high conductivity make them attractive for wires in an ICs.

• Make emitters for compact, flat panel displays (emit electrons at lower applied voltages)

• Fabrication of nanotube FETs presently is done one at a time with great efforts, making them unattractive for high density circuits.

• Nano tube FET device

In this structure a single SWCNT was used to bridge two noble metal electrodes prefabricated by lithography on an oxidized silicon wafer. Here the SWCNT plays the role of channel and the metal electrodes act as source and drain. The heavily doped silicon wafer itself behaves as the back gate. These CNTFETs behaved as p-type FETs with an I (on)/I(off) ratio~105. This suffers from some of the limitations like high parasitic contact resistance (=1Mohm), low drive currents (a few Nanoamperes), and low transconductance. To reduce these limitations the next generation CNTFET developed which is known as top gate CNTFET.

Conventional vs tactile computing, molecular and biological computing

Conventional Computers

• Conventional computers are machines that follow a well described set of instructions to process data.

• Basically, a set of instructions is read into the machine and it works sequentially in an ordered way to execute a task.

• often referred to as Von Neumann computers or classical machines.

• major components are– memory, – processing, and – bandwidth.

Continued…..

• Can be thought of as structurally programmable machines. This means that the program controls the behavior of the machine.

• A compiler translates the input code into machine language that is expressed in terms of the states of simple switching devices and their connections.

• The machine computes symbolically and the result depends to some extent upon the human input.

Well suited to number crunching, communication, and data manipulation.

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Problems

• pattern recognition– Specifically, it is very difficult to program a classical computer to recognize

a complicated molecule or distinguish between different microorganisms.

• In the chemical and biological world pattern recognition is highly efficient and readily accomplished.– For example, the immune system in the human body. This behavior is an

example of a different type of computing, which we will refer to as tactile computing.

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Tactile Computing• These machines are not structurally programmable.• In Bio molecular computing, pattern processing is physical and

dynamic as opposed to the symbolic and passive processing in a conventional machine.

• Programming depends upon evolution by variation and selection. Such a molecular computer is called a tactile processor.

• A tactile processor can be thought of as a computer driven by enzymes; the inputs are converted into molecular shapes that the enzymes can recognize.

• Enzyme thus performs sophisticated pattern recognition since it scans the molecular objects within its environment, interacting with only those molecules that have a complementary shape

• Recognition is thus a tactile procedure.21

Examples1) An example of tactile computer-

– A variety of bacteria bred to dissolve oil spills.– the bacteria would be “programmed” by altering its DNA.– the unique and powerful information processing capabilities of life, pattern

and object recognition, self-organization and learning, and effective use of parallelism are harnessed.

2) sensing bioagents or toxins.– Molecules can be used to perform complicated pattern recognition of

dangerous toxins released into the air or water.– Even Very dilute amounts can be identified readily.– Can be used to fight terrorist bioattacks, or chemical warfare.

• The “program” is in the molecule itself; the computation occurs by the recognition of one molecule by an enzyme and their subsequent chemical reaction.

• Next we will examine tactile computing using biological systems, DNA, and molecules. 22

Biomolecular Computing

• Biological systems are a special case of molecular computing(broader context).

• Major qualities of molecules that make their potential usage in future applications attractive are that they offer simplified fabrication, low cost and relatively unlimited availability, and devices made from molecules are very much smaller and compact than existing CMOS structures.

• For example – A CMOS OR gate requires any where from 4-6 orders of magnitude more

surface area than a comparable molecular logic gate.– Molecular electronic offers extremely compact computing capability,i.e

Microprocessor on a pinhead. Coupled with their expected lower power consumption and relatively inexpensive to fabricate and mass produce.

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key advantages of molecular materials

• Size: on average they are about 4 nm. These device dimensions are about two orders of magnitude smaller than that which can be obtained using SiCMOStechnology.

• Three-dimensional structures: In contrast, in Si based technology, much fabrication effort and cost is required to produce three-dimensional geometries.

• High packing density: the combined features of small size and three-dimensionality make very high packing densities possible. It is estimated that the packing density can be increased by 6–9 orders of magnitude over CMOS.

• Bistability and nonlinearity: bistability and nonlinearity can be utilized to perform switching functions. Both of these properties are commonly available in molecules.

• Anisotropy: the electronic and optical properties of a molecule are inherent in the molecular structure instead of being fabricated by the processing technologies as in CMOS.

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Continued…..

• Upward construction: organic synthesis enables growth of microstructures from the small upward. In standard CMOS, device and circuit functionality is sculpted from a relatively large piece of material.

• Self-organization: self-organization, self-synthesis, and redundancy factors well known in organic and biological molecules that could potentially be applied to molecular electronic devices.

• Low power dissipation: Estimates are that the total power requirements for molecular switches will be about five orders of magnitude lower than for CMOS switches.

• Molecular engineering: it is possible that molecules can be tailored or engineered(can be selectively grown and made to inherently possess desired qualities to perform a task.) to perform specific tasks or have specific properties.

The above features of molecules also apply to biomolecules.

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Bio molecules• Bio molecules provide parallel environment for computation, they occur

abundantly in nature and their implementation due to self organization and upward construction is relatively simple compared with the difficult and costly fabrication of CMOS circuitry.

• As mentioned size is a key advantage of molecules.– The entire genetic code for humans is contained in the nucleus of most cells.– The DNA Code consists of over three billion nucleotide pairs and fits into a few double helices about

3.4nm in width and measuring micrometers in length.

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DNA-an overview

• The entire genetic code for humans is contained in the nucleus of most cells.

• The DNA code consists of over three billion nucleotide pairs and fits into a few double helices about 3.4 nm in width and measuring micrometers in length.

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• Researchers are developing what are called applets for biological systems.

• These applets enable the system to respond to an external event, “program” a cell to produce a desired chemical or enzyme, or enable a cell to identify a reagent.

• Some potential applications of biological applets:– In gene therapy for treating diseases such as hemophilia,

anemia, etc.– in the treatment of diabetes( A genetic applet can be used

that senses the glucose level in the blood and another applet can then direct the production and release of insulin if needed.).

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• DNA molecules are composed of four basic nucleic acids called

– adenine (A), guanine (G), cytosine (C) and thymine (T).

– A and T, and C and G naturally bond together to form pairs.

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Computation with DNA• To compute with DNA there are three basic steps.– encoding that maps the problem onto DNA strands,

– basic processing using a chemical process called hybridization that connects two complementary DNA strands into a double strand,

– and outputting the results. Programming DNA involves the usage of DNA tiles.

• These tiles consist of multiple strands of DNA knotted together.

• The ends of each tile are created such that the tile will recognize and attach to other pre-designed tiles to make self-assembled structures.

• These tiles can be used to add or multiply numbers30

Drawbacks

• These are basically related to the difficulty encountered in regulating or controlling the basic chemistry.

– Reliability means the degree of confidence in correctly solving a problem.

– Efficiency is related to the effective manipulation of the molecules used to perform the computation.

– Scalability is the successful reproduction of the desired event many times.

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Moletronics-molecular diodes and diode-diode logic

• Both conducting and insulating molecules are needed to form devices.

• Two most conducting molecular species are polyphenylenes and carbon nanotubes.

• Polyphenylenes are chain like molecules formed by linking a basic molecular unit, C6H4 , phenylene, which is a derivative of the benzene ring but with two free binding sites

• Molecules can also be used to build devices that mimic CMOS functionality.

• Molecular diodes and other quantum based devices have been designed.

• The primary components of these molecular structures are conducting groups called polyphenylenes and insulating groups called aliphatic molecules.

• By arranging these molecules in various orders similar device action to that found in semiconductors can be attained.

• These structures can be combined to make various logic gates such as NOR and NAND gates.

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Defect tolerant computing• Wherein the operation of a computer can be maintained even if part of its

hardware is defective.• Yield of the chips, defined as the percentage of perfectly operating chips vs the

total number made on the die, decreases with increasing chip complexity.• The ability to configure a chip to operate around defects and imperfections would

greatly improve the manufacturing yield and consequently the cost of the chips.• Existing computing hardware paradigm is that the chip must function in all

applications perfectly ( perfect performance).• The perfectly operating chip paradigm is not the only possibility for computing

hardware. It is conceivable that flawed chips can still be used provided that the defective components in the chip are known in advance and are thereby avoided during chip’s operation. Such an approach is called defect tolerant computing.

• Teramac 10^12 operations per second-many defective chips could be connected 12 operations per second-many defective chips could be connected to produce a powerful yet very inexpensive parallel computer.to produce a powerful yet very inexpensive parallel computer.

• Family tree like branching architecture and fat-tree architecture.Family tree like branching architecture and fat-tree architecture.• Teramac utilizes Fat-tree architecture.Teramac utilizes Fat-tree architecture.