1. Graphene Transistors
Charles Mohr

2. Graphene - History and Definition
Allotrope of carbon
First truly 2D-dimensional crystalline material to be identified
2D-hexagonal (honeycomb) lattice
First produced and identified in (2010 Nobel Prize)

3. Graphene - Properties Density: .77 mg/m2
Strength: 42N/m (Steel 2D breaking strength ~ N/m) 100 times stronger than steel
Theoretical Electrical Conductivity 𝜇=200,000 𝑐 𝑚 2 𝑉 −1 𝑠 −1
Thermal conductivity 𝑊 𝑚 −1 𝐾 −1 (10 times higher than copper)

4. Conventional FET For High speeds
Short gates
Fast carriers in the channel
Short gate problems (short channel effects)
Threshold-voltage roll-off
Drain-induced barrier lowering
Impaired drain-current saturation
These short channel effects are some of the biggest barriers to high speed FETs

5. Graphene in FETs - Pros
Short channel effects can be mitigated by a thin channel and a graphene channel is literally one atom thick
Extremely high carrier mobility at room temperature
These characteristics make frequencies in the 100s of GHz possible

6. Graphene in FETs - Cons Graphene has not bandgap!
No meaningful off state
𝐼 𝑜𝑛 𝐼 𝑜𝑓𝑓 ratio of ~10 ( 10 4 − are needed for digital logic)
Poor saturation behavior
Observed mobilities well below theoretical limits

7. Creating a Band Gap (i) large-area Graphene (ii) graphene nano-ribbons
(iii) bi-layer graphene
(iv) biased bi-layer graphene

8. Real Graphene Transistors
The first graphene transistor used a “back-gate” configuration which is good for proof of concept but a poor design for manufacturing at scale (large-area graphene)
Graphene nano-ribbon devices has shown on-off ratios of up to 106 which is suitable for digital logic. Back-gate device, requires high voltage swings
Top-gate nano-ribbon devices have been created but require high-k dialectric (HfO2) and get on- off ratios of about 70
Transistors have been operated with frequencies as high as 350 GHz (2013)

9. Conclusions Graphene has desirable properties for high speed FETs
A band gap must be introduced for effective switching. Graphene nanoribbons have proven especially suitable for this.
Manufacturing these devices remains a challenge.
Performance issues such as low on-off ratios and high voltage swings need to be improved
These devices show promise, but need a lot of work before being implemented at scale.

10. References
Schwierz, F. Graphene Transistors. Nature Nanotechnology. 5, (2010).
Klekachev, A. V. Graphene Transistors and Photodetecters. The Electrochemical Society (2013).
GRAPHENE. The Royal Swedish Academy of Sciences. (2010)