Carbon nanotube based Field Effect Transistor
- 2. Contents Setbacks of MOSFETs Introduction to CNTFETs CNTFET working Important aspects of CNTFET Effect of diameter variation Conclusion Future Work
- 3. Setbacks of MOSFETs • A MOSFET is a • semiconductor device, • most commonly used in the field of VLSI Design, and Power electronics. • The scaling of MOSFET has been the driving force towards the technological advancement, But Continuous scaling include – short channel effects – high leakage current – excessive process variation and – reliability issues.
- 4. Impact of nanotechnology with time
- 5. Why CNTFET? • As the size becomes smaller, scaling the silicon MOSFET becomes harder. • Requirement of high performance channel material. • Mobility enhancement technology requirement. Advantage of using CNTFET 1-D ballistic transport of electrons and holes. High drive current and large transconductance. High temperature resilience and strong covalent bond.
- 6. CNT: The ideal transistor Material • Carbon nanotubes (CNTs) were discovered by Ijima in Japan in 1991. • CNTs can be thought as rolled up sheets of graphene. • Electrical properties depends on chirality or the direction of this distortion. • CNTs can be metallic or semiconducting depending on the chirality. Fig.1: Strip of grapheme sheet rolled up into tube
- 7. Continued… C= na1 +ma2 n, m = chirality parameter a1 = a ( 3, 0) a2 = a ( √3 2 , √3 2 ) Where a= 0.142 Å bond length between two carbon atoms. If n=m structure is armchair If n or m-0 structure is zigzag Nanotube diameter and chiral angles are given as 𝑑𝑡 = √3𝑎𝑐−𝑐 𝑚2 + 𝑚𝑛 + 𝑛2 𝜋 𝜃 = tan−1 √3𝑛 2𝑚 + 𝑛
- 8. CNTFET Type • Based on geometry Top Gate Bottom Gate Coaxial Gate • Based on operation Schottky barrier MOSFET
- 9. Bottom gate CNTFET • The first CNTFET was fabricated in 1998. These were simple devices fabricated by depositing single-wall CNTs (synthesized by laser ablation) from solution onto oxidized Si wafers which had been prepatterned with gold or platinum electrodes. • The devices displayed high on-state resistance of several MQ, low transconductance (-1,,,) and no current saturation, and they required high gate voltages (several volts) to turn them on.
- 10. Top gate CNTFET • The next generation of CNFET came in top-gated structure to improve the device performance. • This structure gives better out-turn than early structure. The improvement comes from the scaling of the dimension and the adoption of better device geometry as well as the device performance .
- 11. Coaxial gate CNTFET • Gate all around geometry. • More electrostatic control over channel.
- 12. Working principle • Basic principle operation of CNFET is the same as MOSFET where electrons are supplied by source terminal and drain terminal will collect these electrons . • In other words, current is actually flowing from drain to source terminal . • Gate terminal controls current intensity in the transistor channel and the transistor is in off state if no gate voltage is applied.
- 13. SBCNTFET • SB-CNFET works on the principle of direct tunneling through the Schottky barrier at the source channel junction . • The barrier width is controlled by the gate voltage and hence the transconductance of the device depends on the gate voltage. • At low gate bias, large barrier limits the current in the channel . As gate bias is increased, it reduces the barrier width, which increases quantum mechanical tunneling through the barrier , and therefore increases current flow in transistor channel. • In SBCNFET, the transistor action occurs by modulating the transmission coefficient of the device.
- 14. MOSFET like CNTFET • The structure of this device is slightly different than SB-CNFET since it used heavily doped terminals instead of metal. • This device is formed in order to overcome problems in SB-CNFET by operating like normal MOSFET. • This device operates on the principle of modulation the barrier height by gate voltage application. The drain current is controlled by number of charge that is induced in the channel by gate terminal.
- 15. How CNTFET works? • Operation of CNTFET compared with Si-MOSFET
- 16. Continued….
- 17. Id/Vds curve
- 19. MOSFET CNTFET Gc 5uF/cm2 2.4uF/cm2 Id 500uA/um 1500 uA/um Mobility 7x10^6 cm/s 3.5x10^6 cm/s Thermal velocity - 2 times of mosfet