1. Derek Wright Monday, March 7th, 2005
Tutorial 7
Derek Wright
Monday, March 7th, 2005

2. Silicon MOSFETs Introduction MOS Capacitors MOSFET Structure
MOSFET Scaling
Gate Dielectrics
Gates
Junctions and Contacts
Alternate MOSFET Structures

3. Introduction
MOSFETs are a kind of Field Effect Transistor used in digital ICs
Use a FET because gate voltage uses less current than BJT’s base current
BJT was developed first, and FET was theorized, but impractical
Couldn’t make the Field Effect work due to technology constraints at the time

4. MOS Capacitor It’s important to understand how a MOS capacitor works:
Capacitance is a limiting factor in IC performance
Mobile charges in gate
Mobile charges in channel (between drain and source)
Separated by dielectric (gate oxide)
= capacitor

5. MOS Capacitor

6. MOS Capacitor
Shows how a depletion layer forms
The blue charge is what lets current go from source to drain
Other good applets on the site

7. MOS Capacitor

8. MOS Capacitor Capacitance changes with applied voltage
Leads to complicated CMOS simulations
Can be exploited in some kinds of VCOs (MOS Varactor)

9. MOSFET Structure We use a MOS capacitor in inversion mode
The minority carriers form the “channel”
Ions are implanted on either side of the gate to act as sources of carriers
Contacts are put on the diffusions to form the source and drain
Carriers go from the source to the drain

10. MOSFET Structure

11. MOSFET Structure

12. MOSFET Structure

13. MOSFET Scaling Reducing the size of MOSFETs in ICs has many benefits:
Higher density
Higher speed
Lower Power
It also introduces many problems:
Thin gate oxides
Short channel effects
Higher leakage current

14. MOSFET Scaling

15. Gate Dielectrics
Gate thickness scales by 1/ with decreasing device dimensions
We’re fast approaching the practical limit of how thin SiO2 gates can get
Tunneling can occur causing gate leakage
Other problems like hot carriers start to become problematic

16. Gate Dielectrics We can use a thicker dielectric if it has a higher r
These “high-k” dielectrics mean that a given gate voltage will produce a higher E-field
Or, a given gate voltage will produce the same E-field with a thicker dielectric layer

17. Gate Dielectrics Problems with a thin gate:
Oxide thickness variation
Impurities from poly gate (particularly B)
Reliability and lifetime problems
High gate current
Gate leakage current (VG = 1V):
1pA/cm2 at 3.5 nm
10A/cm2 at 1.5 nm

18. Gate Dielectrics

19. Gate Dielectrics Solutions to gate problems: Add nitrogen to SiO2
Use high-k dielectrics
High-k dielectrics must meet a number of criteria
Must be thermally stable
Good electronic properties
Microstructural stability
Deposition tools and chemistry
Process compatibility

20. Gate Dielectrics

21. Gates Poly-silicon is used for gates because: Drawbacks include:
Adjustable work function through doping
Process compatibility
Drawbacks include:
It’s a semiconductor, so it forms a depletion layer which adds to the EOT (effective oxide thickness)
High resistivity
Metal is considered as the successor to poly-silicon gates

22. Gates

23. Junctions and Contacts
Other resistances must be less than 10% of the channel resistance (Rchan)
Rchan = [(W/L)  (ox/tox (VG – VT)]-1
L  Rchan (scaling)
  Rchan (new substrates)
ox  Rchan (high-k dielectrics)
tox  Rchan (high-k dielectrics and scaling)
VT  (VG – VT)   Rchan (doping)

24. Junctions and Contacts
Contacts connect the metal lines to the source/drain/gate of a MOSFET
Contact resistance becomes a problem as geometries shrink
This can be partially solved by using silicides:
Silicides are metal/silicon alloys with a low resistance

25. Junctions and Contacts
Formation of self-aligned silicides (salicides)
Metal is deposited over entire wafer
Reacts with exposed silicon
Unreacted metal is selectively etched off

26. Alternate MOSFET Structures
Silicon On Insulator (SOI) wafers eliminate capacitive coupling to the substrate
An oxide layer is buried below the transistors, eliminating coupling to the substrate
SOI:
reduces leakage
reduces capacitance
higher speed
less susceptible to soft errors

27. Alternate MOSFET Structures
New technologies for coming years:
High-k gate dielectrics
Low-k Dielectrics
Metal gate electrodes
SOI
Strained silicon
Vertical multi-gate structures

29. Thank You!
This presentation will be available on the web.