Transistors, Logic Gates, and Combinational Logic Flashcards
Transistor
electrical device that acts as an electrical switch; typically made from silicon
MOSFET
(metaloxide semiconductor fieldeffect transistor): a specialized fieldeffect transistor or FET; like all transistors, used for switching or amplifying signals
nMOSFET (nMOS)
channel carries negative charges (electrons)
pMOSFET (pMOS)
channel carries positive charges (holes)
CMOS
pairing nMOS and pMOS transistors together
Inverter
logic gate that implements logical negation
NAND
logic gate producing an output that is false only if all its inputs are true;
parallel structure on top and series on the bottom
NOR
result of the negation of the OR operator; can also be seen as an AND
gate with all the inputs inverted; parallel structure on the bottom and series on top
Gate delays
longest path (in gates) through a circuit
PLA
( programmable logic array): a kind of programmable logic device used to
implement combinational logic circuits
○ A tool to go from truth table to logic gates
○ Not the most efficient tool but it always works!
Half adder
logic circuit implementing binary addition
Combinational logic
using several logic gates together to create a logic circuit
that performs work for us like adders, multipliers, etc.
Multiplexer (MUX)
device that combines several analogs or digital input signals
and forwards them into a single output line
Transistor: electronic switch
Two complementary flavors: nMOSFET and pMOSFET (CMOS)
○ Controls the flow of current in a circuit
General structure for CMOS
All CMOS gates must have 2 parts:
■ PUN (pullup
network): pulls output “up” to PWR (logic 1)
● Rule: PUN can only contain PMOS transistors
■ PDN (pulldown
network): pulls output “down” to GND (logic 0)
● RULE: PDN can only contain NMOS transistors
○ Either PUN or PDN is “ON” at any given time (never both)
○ Output is taken where PUN and PDN intersect
How a transistor works
Off: electrons cannot move between the source and drain, no path exists
under the gate. The transistor aka “the switch” is “OFF.”
○ On (how it works in nMOS transistor):
■ Positive charge repels holes from under the gate
■ Attracts electrons from source/drain regions
■ Creates an “ntype”
channel under the oxide
● Why it’s called an “nMOS” transistor
■ Current can flow from drain to source; electrons enter source and
exit drain
■ Electrons cannot penetrate oxide
● Electric field forms across oxide, why it’s called “FET”
Logic minimization using Laws of Boolean Algebra
○ Identities
■ X AND 1 = X; X AND 0 = 0
■ X OR 1 = 1; X OR 0 = X
Logic minimization using Laws of Boolean Algebra
Associative laws
■ A AND (B AND C) = (A AND B) AND C
■ A OR (B OR C) = (A OR B) OR C
Logic minimization using Laws of Boolean Algebra
Distributive laws
■ A AND (B OR C) = (A AND B) OR (A AND C)
■ A OR (B AND C) = (A OR B) AND (A OR C)
Logic minimization using Laws of Boolean Algebra
De Morgan’s laws
■ A NAND B = NOT (A AND B) = (NOT A) OR (NOT B)
■ A NOR B = NOT (A OR B) = (NOT A) AND (NOT B)
Binary number calculators
Nbit incrementer: N 1bit half adders combined ○ Subtractor: negate combined with Nbit incrementer ○ Selector (MUX): ■ N select bits choose from 2 N inputs ■ adder/subtractor ○ Multiplier: combinational multiplier using adders and MUXs
