17-11-2012, 02:23 PM
CMOS VLSI Design
CMOS VLSIDesign.ppt (Size: 918.5 KB / Downloads: 268)
Introduction
Integrated circuits: many transistors on one chip.
Very Large Scale Integration (VLSI): very many
Metal Oxide Semiconductor (MOS) transistor
Fast, cheap, low-power transistors
Complementary: mixture of n- and p-type leads to less power
Today: How to build your own simple CMOS chip
CMOS transistors
Building logic gates from transistors
Transistor layout and fabrication
Rest of the course: How to build a good CMOS chip
Dopants
Silicon is a semiconductor
Pure silicon has no free carriers and conducts poorly
Adding dopants increases the conductivity
Group V: extra electron (n-type)
Group III: missing electron, called hole (p-type)
nMOS Transistor
Four terminals: gate, source, drain, body
Gate – oxide – body stack looks like a capacitor
Gate and body are conductors
SiO2 (oxide) is a very good insulator
Called metal – oxide – semiconductor (MOS) capacitor
Even though gate is
no longer made of metal
nMOS Operation
When the gate is at a high voltage:
Positive charge on gate of MOS capacitor
Negative charge attracted to body
Inverts a channel under gate to n-type
Now current can flow through n-type silicon from source through channel to drain, transistor is ON
CMOS Fabrication
CMOS transistors are fabricated on silicon wafer
Lithography process similar to printing press
On each step, different materials are deposited or etched
Easiest to understand by viewing both top and cross-section of wafer in a simplified manufacturing process