Chapter 8 - Ultrathin Transistor Devices Flashcards
What is the two main operation modes of a transistor?
Amplification of electronic signals and switching of electronic signal.
What are the two most important figures of merit for switching behavior?
- Subthreshhold swing S in subthreshold region
- Terminal transconductance in above-threshold region.
Which physical limitations exist for further miniaturization?
- Mobility of charge carriers decrease with decreasing channel length.
- Tunneling of charge carrier through barrier, increasing the off-current and decreasing the on/off-ratio.
- At one point we look at single atoms.
- Heat dissipation is an increasing issue.
- Increasing impact of quantum phenomena.
What are the two principal divisions of semiconductor electronics, and what are they used for?
1) Digital logic devices. Used in computing. Very complicated to make integrated circuits.
2) Radiofrequency devices. Used in wireless communication. Comparatively simple.
What are the requirements for digital logic devices?
- Excellent switching capabilities.
- Good on-off-ratio.
- Some of the MOSFETs are turned off, so that only I_off is flowing.
What are the requirements for high-speed devices?
- Should respond quickly to change in gate voltage (short channels, fast carriers in channel).
- High cut-off frequency.
How can 2D materials help to reduce the short channel effects?
Short channel effects are reduced for thin barrier region and thin channel.
Is graphene a suitable material for digital logic devices? Why/why not?
No. It has no bandgap, and there is therefore always a fairly high current going, and the ratio between the on and off-states are way too low. Plus even if the ratio was, it wouldn’t be power efficient.
What are some advantages of a graphene transistor?
- High room temperature mobility (for RF applications)
- Fermi level continuously tunable from VB to CB.
- Carrier density tunable over a wide range.
What is Klein tunneling?
The fact that massless Dirac fermions can tunnel through a barrier practically unhindered, as if the potential barrier wasn’t there. In graphene, carriers near the K and K’ points are Dirac fermions.
Positron state inside barrier (since it is attractive for positrons). Aligns with electrons outside, and the matching between the wavefunctions of electron and positron across the barrier results in perfect transparency.
Why is a graphene bipolar transistor hard to realize?
Due to Klein tunneling. There is no effective barrier for charge carriers, but they will pass straight through.
What are some advantages of TMDC-based MOSFETs?
- They have a finite band gap.
- Becomes direct band gap for ML.
- Inertness, no dangling bonds.
- Band-engineering possible by combination of different TMDCs.
What is a disadvantage of TMDC-based MOSFETs?
- Low room temperature mobility (due to scattering of charge carriers on optical phonons), so bad for RF applications.
