Exploring MOSFET Transistors & Logic Circuits: nMOS & pMOS Devices, Study notes of Logic

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E40M

MOS Transistors, CMOS Logic Circuits,

and Cheap, Powerful Computers

Reading

• Chapter 4 in the reader
• For more details look at
  • A&L 5.1 Digital Signals (goes in much more detail than we need)
  • A&L 6 - 6.3 MOS Devices

nMOS i-V Characteristics

• nMOS is still a device
  • Defined by its relationship between current and voltage
  • But it has 3 terminals!
    • Current only flows between the source and drain
    • No current flows into the gate terminal!

V

DS

i

DS

G

D

v S

i

Remember the resistor?

Simple Model of an nMOS Device

• We will model an nMOS device by components we know
  • Resistor
  • Switch
• NMOS Source = Gnd Gate = Gnd => Off Gate = Vdd => On
• This really simple model is suitable for applications where there’s one value of “On” voltage.

V

DS

i

DS On Off

Problem With nMOS Device

• While an nMOS device makes a great switch to Gnd
• It doesn’t work that well if we want to connect to Vdd
  • To turn transistor on
    • Gate needs to be higher than source
  • But we want the source to be at Vdd
    • Oops …

pMOS i

DS

vs. V

DS

Characteristics

• Similar to nMOS, but upside down!
  • Turns on when the gate-to-source voltage is < - 1 V
  • And the drain-to-source voltage should be negative
    • Source should be the terminal with the higher voltage!

V

DS

i

DS G S D

How Does a pMOS Transistor Actually Work?

(FYI – not part of this course)

nMOS and pMOS Devices “Complement”

Each Other – Complementary MOS or CMOS

• PMOS

Source = Vdd (+ supply) Gate = Gnd => On Gate = Vdd => Off

• NMOS Source = Gnd Gate = Gnd => Off Gate = Vdd => On Ron

MOS Logic Gates

What Does This Circuit Do?

• Is the output a logic function of the input? Consider Vin = GND In (^) Out

VDD

pMOS nMOS

Building Logic Gates from MOS Transistors

• Remember Boolean Logic?
  • AND, OR
  • NAND = Not-AND = AND followed by Inverter
    • Output is only low when A and B are true (high)
  • NOR = Not-OR = OR followed by Inverter
    • Output is low when either A or B is true (high)
• You can make them from MOS devices
  • But only the inverting gates (NOR and NAND)

Building a CMOS NAND Gate

• Output should be low if both input are high (true)
• Output should be high if either input is low (false)

If You Look At Your Computer Chip

• It is just billions of transistors
  • Creating many logic gates, and memory
• Take EE108A if you want know how we do that …

HOW THE MOS TRANSISTOR

CHANGED THE WORLD …