[工学][第3章1] COMS VLSI Design.ppt

Introduction Integrated circuits: many transistors on one chip. Very Large Scale Integration (VLSI): very many Complementary Metal Oxide Semiconductor Fast, cheap, low power transistors 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 Silicon Lattice Transistors are built on a silicon substrate Silicon is a Group IV material Forms crystal lattice with bonds to four neighbors 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) p-n Junctions A junction between p-type and n-type semiconductor forms a diode. Current flows only in one direction 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 Body is commonly tied to ground (0 V) When the gate is at a low voltage: P-type body is at low voltage Source-body and drain-body diodes are OFF No current flows, transistor is OFF nMOS Operation Cont. 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 pMOS Transistor Similar, but doping and voltages reversed Body tied to high voltage (VDD) Gate low: transistor ON Gate high: transistor OFF Bubble indicates inverted behavior Power Supply Voltage GND = 0 V In 1980’s, VDD = 5V VDD has decreased in modern processes High VDD would damage modern tiny transistors Lower VDD saves power VDD = 3.3, 2.5, 1.8, 1.5, 1.2, 1.0, … Transistors as Switches We can view MOS tra