Reminder: PhD Orals Abstract for Tejas Krishnamohan

[Krishna Saraswat]

Physics and Technology of High Performance, Strained Germanium  
Channel, Heterostructure MOSFETs
by
Tejas Krishnamohan

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

ABSTRACT

             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.