Reminder: PhD Orals Abstract for Tejas Krishnamohan
saraswat at cis.stanford.edu
Mon Oct 31 15:59:59 PST 2005
Physics and Technology of High Performance, Strained Germanium
Channel, Heterostructure MOSFETs
Thesis Advisor: Prof. Krishna Saraswat
Co-advisors: Prof. Yoshio Nishi and Prof. Jim Plummer
Date: Tuesday, November 1st 2005,
Time: 10am (Refreshments served at 9:45am),
Location: CIS-X Auditorium, Center For Integrated Systems, Stanford
University, CA 94305
Since the invention of the transistor in 1947, the rapid
progress in silicon-based information processing technology is
unprecedented. However, the road to scaling devices in accordance
with Moore's Law to sub-15nm dimension seems obscure and challenging.
Sustaining the required device performance at lower power dissipation
seems to be hitting 'fundamental' physical limits. The quickening
pace of MOSFET scaling is accelerating the introduction of novel
structures and high-mobility materials into the channel.
High mobility materials like germanium (Ge), strained-SixGe(1-x) and
strained-Si are very promising as future channel materials. However,
due to their smaller bandgap and higher dielectric constant, most
high mobility materials suffer from large Band-To-Band Tunneling
(BTBT) leakage currents and worse short channel effects, which may
ultimately limit their scalability.
We present novel, heterostructure MOSFETs, which can significantly
reduce the BTBT leakage currents while retaining their high channel
mobility, making them suitable for scaling into the sub-15nm regime.
Through Full band Monte-Carlo, Poisson-Schrodinger and detailed BTBT
simulations, we analyze the tradeoffs between carrier transport,
quantum mechanical effects, electrostatic integrity and BTBT leakage
in high mobility, Si/strained-SiGe/Si, heterostructure, PMOS DGFETs.
Our results show a dramatic (>100X) reduction in BTBT and excellent
electrostatic control of the channel, while maintaining very high
drive currents in these highly scaled heterostructure DGFETs.
Detailed experiments were performed to analyze and verify the
tradeoffs between higher mobility (smaller bandgap) channels and
lower Band-To-Band-Tunneling (BTBT) leakage in heterostructure
MOSFETs. Strained-SiGe MOSFETs with varying Ge percentage, Ge
thickness and Si cap thickness were fabricated on bulk Si and SOI
substrates. The ultra-thin (~2nm) strained-Ge channel heterostructure
MOSFETs show >4X mobility enhancements over bulk Si devices and >10X
BTBT reduction over surface channel strained SiGe devices.
Using the Landauer-Buttiker formalism for ballistic
currents and BTBT modeling (taking into acount quantum-mechanical
effects, different valleys, subbands and band structure) we discuss
the important considerations in choosing future channel materials
(e.g. strained-Si, Ge, III-V, CNTs) and structures for high-
performance/low-leakage MOS devices.
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