1. Optimization of Carbon Nanotube Field-Effect Transistors (FETs) Alexandra Ford NSE 203/EE 235 Class Presentation March 5, 2007

2. Two Papers I Will Be Presenting Today:

3. How to Optimize a CNFET Two papers discuss how to optimize a carbon nanotube FET to have – High ON current and conductance (max = 4e 2 /h) – High ON/OFF current ratios – Steep switching (max = 60 mV/decade at room T) – Highly scaled gate dielectrics and channels – Supression of ambipolar conduction by optimization of S/D contacts to carbon nanotubes (Pd contacts to electrostatically (1) or K (2) doped nanotube segments) and integration of high-quality thin gate insulator films (HfO 2 )

4. To Fabricate a P-Channel CNFET Ohmic Pd-tube contact 8 nm HfO 2 gate dielectric Al top-gate Two tube segments outside the top-gate region are electrostatically “doped” by a back gate and act as S/D electrodes Highest performance FET thus far (back gate constant bias V GS_BACK = -2V)

5. P-Channel CNFET Electrical Properties Top-gate, for tube diameter d=2.3 nm, channel length L=2  m: –Subthreshold swing of 80 mV/decade –Transconductance of g m =dI DS /dV DS |V DS =20  S (~5000 S/m when normalized by tube diameter) –ON current of I ON_sat =15-20  A (~3750  A/  m) –ON conductance of G ON =0.1 x 4e 2 /h g m and I ON_sat are higher than the state-of-the-art Si p-MOSFET by a factor of 5 at similar gate overdrive V DS = -0.3V -0.2V -0.1V

6. Top-gate geometry (Pd-electrostatically “doped” nanotube segment contact or DopedSD-FET) has superior subthreshold swing compared to back-gate geometry (Pd-tube contact or MetalSD-FET) – 70-80 mV/decade vs 130 mV/decade – Likely due to more efficient electrostatic gating for the high-k/top-gate stack (top-gate capacitance is near the quantum capacitance for a SWNT=4pF/cm) DopedSD-FET has supressed ambipolar conduction compared to MetalSD- FET: P-Channel CNFET Electrical Properties V DS = -0.3V -0.2V -0.1V V DS = -0.3V -0.2V -0.1V DopedSD-FETMetalSD-FET

7. What Happens When You Chemically Dope the S/D Nanotube Segments? Becomes possible to make a n-type CNFET – n-type CNFETs are more difficult to successfully fabricate because of high Schottky barrier contacts for high on-states The chemical doping can suppress the Schottky barriers at the contacts, resulting in devices with subthreshold swings of 70 mV/decade, ON/OFF ratios of 10 6, and negligible ambipolar conduction – As-made devices (back-gate geometry, before doping) are p-type FETs; doping with K (heavy electron donation) increases I ON and G ON from 5  A and 0.3 e 2 /h to 20  A and e 2 /h, suggesting high metal- semiconductor contact transparency (T MS ~ 1)

8. Before and After K Doping – Top-gate BLUE = p-FET (before doping) - electrostatically p-doped contacts by applying a back gate voltage of -15V RED = n-FET (same device after moderate doping with K) - this creates an n + -i-n + structure similar to a conventional n-MOSFET (nanotube segments outside the gate Ti/Pd metal/HfO 2 stack are exposed to K vapor doping to form the n + regions, are under gate remains intrinsic)

9. These I DS -V GS and I DS -V DS plots show that the n- FET (K-doped) and p-FET (electrostatically doped) have near-symmetrical characteristics: – n-FET and p-FET both have on-currents I ON of 8  A (for V DS = 0.5 V) – n-FET and p-FET transconductance values of 20 and 10  S, respectively – n-FET and p-FET have subthreshold swings of 70-80 mV/decade – AND note that I ON /I OFF for the n-FET is ~10 6 and there is no significant ambipolar p-channel conduction Before and After K Doping – Top-gate

10. Comparison to Si n-MOSFET Near-transparent n-type contacts can be formed by chemical doping with K, mainly as a result of Schottky barrier height reduction at the metal-tube junctions Heavy and moderately doped n-FETs have higher on-currents at all I ON /I OFF values than a 100 nm node Si n-MOSFET (gate length L=50 nm)

11. Conclusion p-CNFETs and n-CNFETS can be obtained by using Pd contacts, high-dielectric-constant HfO 2 films as gate insulators, and electrostatic (for p-FET) or chemical (for n-FET) doping