Carbon nanotube field-effect transistor

A carbon nanotube field-effect transistor (CNTFET) refers to a field-effect transistor that utilizes a single carbon nanotube or an array of carbon nanotubes as the channel material instead of bulk silicon in the traditional MOSFET structure. First demonstrated in 1998, there have been major developments in CNTFETs since.According to Moore's law, the dimensions of individual devices in an integrated circuit have been decreased by a factor of approximately two every two years. This scaling down of devices has been the driving force in technological advances since the late 20th century. However, as noted by ITRS 2009 edition, further scaling down has faced serious limits related to fabrication technology and device performances as the critical dimension shrunk down to sub-22 nm range. The limits involve electron tunneling through short channels and thin insulator films, the associated leakage currents, passive power dissipation, short channel effects, and variations in device structure and doping. These limits can be overcome to some extent and facilitate further scaling down of device dimensions by modifying the channel material in the traditional bulk MOSFET structure with a single carbon nanotube or an array of carbon nanotubes.The exceptional electrical properties of carbon nanotubes arise from the unique electronic structure of graphene itself that can roll up and form a hollow cylinder. The circumference of such carbon nanotube can be expressed in terms of a chiral vector:Ĉh=nâ1+mâ2which connects two crystallographically equivalent sites of the two-dimensional graphene sheet. Here n and m are integers and â1 and â2 are the unit vectors of the hexagonal honeycomb lattice. Therefore, the structure of any carbon nanotube can be described by an index with a pair of integers (n,m) that define its chiral vector.A carbon nanotube’s bandgap is directly affected by its chirality and diameter. If those properties can be controlled, CNTs would be a promising candidate for future nano-scale transistor devices. Moreover, because of the lack of boundaries in the perfect and hollow cylinder structure of CNTs, there is no boundary scattering. CNTs are also quasi-1D materials in which only forward scattering and back scattering are allowed, and elastic scattering means that free paths in carbon nanotubes are long, typically on the order of micrometers. As a result, quasi-ballistic transport can be observed in nanotubes at relatively long lengths and low fields.Because of the strong covalent carbon–carbon bonding in the sp2 configuration, carbon nanotubes are chemically inert and are able to transport large amounts of electric current. In theory, carbon nanotubes are also able to conduct heat nearly as well as diamond or sapphire, and because of their miniaturized dimensions, the CNTFET should switch reliably using much less power than a silicon-based device.There are many types of CNTFET devices; a general survey of the most common geometries are covered below.In CNT–metal contacts, the different work functions of the metal and the CNT result in a Schottky barrier at the source and drain, which are made of metals like silver, titanium, palladium and aluminum. Even though like Schottky barrier diodes, the barriers would have made this FET to transport only one type of carrier, the carrier transport through the metal-CNT interface is dominated by quantum mechanical tunneling through the Schottky barrier. CNTFETs can easily be thinned by the gate field such that tunneling through them results in a substantial current contribution. CNTFETs are ambipolar; either electrons or holes, or both electrons and holes can be injected simultaneously. This makes the thickness of the Schottky barrier a critical factor.CNTFETs show different characteristics compared to MOSFETs in their performances. In a planar gate structure, the p-CNTFET produces ~1500 A/m of the on-current per unit width at a gate overdrive of 0.6 V while p-MOSFET produces ~500 A/m at the same gate voltage. This on-current advantage comes from the high gate capacitance and improved channel transport. Since an effective gate capacitance per unit width of CNTFET is about double that of p-MOSFET, the compatibility with high- k gate dielectrics becomes a definite advantage for CNTFETs. About twice higher carrier velocity of CNTFETs than MOSFETs comes from the increased mobility and the band structure. CNTFETs, in addition, have about four times higher transconductance.The decrease of the current and burning of the CNT can occur due to the temperature raised by several hundreds of kelvins. Generally, the self-heating effect is much less severe in a semiconducting CNTFET than in a metallic one due to different heat dissipation mechanisms. A small fraction of the heat generated in the CNTFET is dissipated through the channel. The heat is non-uniformly distributed, and the highest values appear at the source and drain sides of the channel. Therefore, the temperature significantly gets lowered near the source and drain regions. For semiconducting CNT, the temperature rise has a relatively small effect on the I-V characteristics compared to silicon.Carbon nanotubes degrade in a few days when exposed to oxygen. There have been several works done on passivating the nanotubes with different polymers and increasing their lifetime.The most desirable future work involved in CNTFETs will be the transistor with higher reliability, cheap production cost, or the one with more enhanced performances. For example, such efforts could be made: adding effects external to the inner CNT transistor like the Schottky barrier between the CNT and metal contacts, multiple CNTs at a single gate, channel fringe capacitances, parasitic source/drain resistance, and series resistance due to the scattering effects.