The George Washington University
School of Engineering and Applied Science
Department of Electrical and Computer Engineering
ECE122 – Lab 7
MOSFET Parameters and Scaling Effects
Ritu Bajpai
October 24, 2008
CMOS Scaling
• Devices are constantly shrinking in an
effort to increase the number of devices on
a chip.
• The state-of-the-art mass production is
moving into ~45nm.
• Clock speeds are scaled up to increase
performance.
Effects on the device
• Short-channel effects on VT: If we reduce
the channel length beyond a certain limit
the source and drain depletion regions
begin to affect the threshold voltage.
• Velocity saturation: With devices getting
smaller they are exposed to high electric
fields leading to drift velocity reaching its
upper bound.
Effects on the device
• Gate leakage current (IG):The gate
dielectric is needed to prevent charge from
passing from the gate to channel of a
MOSFET. All insulators, when sufficiently
thin, allow some electrons, and thus some
current, to pass through due to quantum
mechanical effects.
Effects on the device
• Sub threshold current (ID): This is the
current flowing through the transistor when
it is nominally off (Ioff). Ideally we want this
current to be low. But like Ion it is also
proportional to (VGS-VT).
Effects on the circuit
• Decreased supply voltage: As supply
voltage is reduced, the charge stored will
be small. With larger subthreshold leakage
current, coupling noise etc is will be a
challenge to get circuits to operate
properly.
• Increased role of wiring resistance,
inductance and capacitance.
Effects on the circuit
• Interconnect coupling: Wires are getting
thinner but not decreasing as rapidly in
height. This makes them look like tall thin
conductors which form parallel plate
capacitors.
• IR Drop: Narrow wires have a non
negligible resistance.
• Electromigration: This involves migration
of metal molecules due to high current
densities and narrow line widths leading to
a short or open in the metal line.
Spice MODELS
• The standard spice model is not sufficient to
capture all of these effects.
• There have been many upgrades to it in order
to increase it’s effectiveness.
Level 1
~5 Parameters
Level 49
~100 Parameters
Review of Spice Parameters
.model nmos nmos Level=1
+ Vto=1.0
Kp=3.0E-5
+ Phi=0.65
Lambda=0.02
+ Nsub=1.0E+15
Nss=1.0E+10
+ Tpg=1.00
Uo=700.0
+ Kf=1.0E-26
Is=1.0E-15
+ Pb=0.75
Cj=2.0E-4
+ Cjsw=1.00E-9
Mjsw=0.33
+ Cgbo=2.0E-10
Cgdo=4.00E-11
+ Rd=10.0
Rs=10.0
Gamma=0.35
Tox=0.1u
Ld=0.01u
Af=1.2
Js=1.0E-8
Mj=0.5
Fc=0.5
Cgso=4.00E-11
Rsh=30.0
The “REAL” Data
•
•
•
http://www.mosis.org/cgibin/cgiwrap/umosis/swp/params/ibm018/t67j_7wl_5lm_ma-params.txt file will
serve as the real data
Save the above file as .lib file.
Make changes so as to correspond to
the tanner .lib format (You can do this by
comparing the format of the library file
that you have been using to this new
one)
The test setup
The simulation
• A DC sweep of 100 points of VDS from 0 to 1.8V
• A Secondary sweep of 10 points of VGS from 0
to 1.8V
• Include this new file as the library file for
simulation
• Use command .print dc i(MNMOS_1,D) to print
NMOS characteristic curve
Analysis and Results
• Show the ID vs. VDS characteristic for the
Mosis transistor data.