What is CNTFET?
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| Carbon Nano Tube Field Effect Transistor |
CNTFET stands Carbon Nano Tube Field Effect Transistor. CNTFET is a FET which uses Carbon Nano Tube as the channel. They are widely used in many application because of their both metallic and semiconductor properties and because of their ability to carry high current. In new technology advancement there is a demand of integrated circuits with high speed performance, low power consumption and smaller dimension. The size of the transistor had reduced as per the Moore’s law but there are some disadvantages like shorter channel effect which leads to direct tunnelling, increase in gate leakage current. This disadvantages are overcome by CNTFET.
What is Carbon Nano Tube?
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| Carbon Nano Tube |
Carbon Nano tubes were first discovered by Sumio Iijima in the year 1991. These nano tubes consist of carbon atoms with diameters measured in nano metres which are linked in hexagonal shape. Each carbon atom is covalently bonded to other three atoms. The width of the carbon nanotubes are of 1nm and length maybe some centimetres. Though they are very small they are strong and not brittle or fragile. They can be bent and when released they go back to their original shape.
There are two types
1) Single-walled and its diameter is of 1nm and
2) Multi-walled in which many layers are interlinked and its diameter is of 100nm.
Carbon nano tubes are formed by rolling either single walled sheet of graphene or multi walled sheet of graphene. In graphene the carbon atoms are arranged in two dimensional honeycomb lattices. The Carbon nano tubes are made by three methods arc discharge, laser ablation of graphite and chemical vapour deposition (CVD).
Types of CNTFET:
Based on Geometry it is classified into
1) Back gate CNTFET
2) Top gate CNTFET
3) Wrap- around gate CNTFETs
4) Suspended CNTFETs
1) Back gate CNTFET:
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| Back Gate CNTFET |
In this method during the earlier stage of manufacturing, Silicon dioxide (SiO2) layer is fabricated above the silicon layer. Silicon dioxide is the gate oxide above which a single walled carbon nano tube (SWCNT) is bridged between two metal pre fabricated strip on the silicon dioxide layer by lithography. One metal is source and the other is drain. The CNT is randomly placed above the metal strips. The limitation of this model are high parasitic contact resistance, low drive currents and low transconductance. To overcome these limitations next generation CNTFET is developed.
2) Top gate CNTFET:
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| Top Gate CNTFET |
In this model as the name says the gate is fabricated over the Carbon Nano Tubes. The source and the drain are patterned by high resolution electron beam lithography. A layer of dielectric is fabricated above the carbon tube by evaporation or atomic layer deposition. Many top gate CNTFET can be fabricated on a same wafer on which the gates are electrically isolated. The advantage of this method is drain current is increased from nanometers to micrometers, transconductance is also increased. So this method is preferred over the previous method though the fabrication process is complex.
3) Wrap- around gate CNTFETs
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| Wrap around Gate CNTFET |
Wrap around CNTFET is also known as gate – all – around CNTFET and it was developed in the year 2008. In other methods only part of the nanotubes is gated. Here the entire nano tube is covered by gate which reduces leakage current and it improves the electrical performance. The fabrication process starts by wrapping the carbon nano tube with dielectric layer by atomic layer deposition. The wrapping is partially etched to expose the ends of the nanotube. Then the source, drain and gate contacts are deposited on the nanotubes.
4) Suspended CNTFETs
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| Suspended CNTFET |
In this method gate is suspended over a trench to reduce the contact with substrate and gate oxide. Thus the device performance is increased. But the drawback in this method is gate electric is air or vacuum. Only short CNTs are used because long tubes may touch the metal contact which may short the device. It is not available commercially. It used to research about the intrinsic properties of clean CNT.
I-V characteristics of CNTFET:
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| I-V Characteristics of CNTFET |
Initially when the applied voltage is below the threshold voltage the drain current is almost zero. When it crosses the threshold voltage the drain current increases gradually.
The current generated depends upon the Schottky barrier formed at the junction of CNT and the metal contact at source and the drain. The metals used here are silver, titanium, palladium and aluminium.
Comparison of CNTFET with MOSFET:
1) In MOSFET switching occurs by altering channel resistivity in CNTFET switching occurs by modulation of contact resistance.
2) CNTFET generates three to four times of drive current than MOSFET.
3) Transconductance of CNTFET is four times higher than the MOSFET.
4) The average carrier velocity is double in CNTFET than that is in MOSFET.
Advantages
- Low power consumption
- Electron mobility is high
- Lower threshold voltages
- Better control over channel formation
- No direct tunneling
- Gate leakage current is reduce
Disadvantages
- Cannot be operated at high temperature or at high electric current.
- Lifespan is less.
- Fabrication process is difficult.
- Cost of production is high.
Applications
- Used in solar cell
- Used mainly in VLSI
- Used in faster computer chips
- Cancer treatment
- Cardiac autonomic regulation
- For platelet activation
- For tissue regeneration
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