Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Digital Technology How Electrons Become Bits Thursday, September 15, 16 Alternatives • Fluidics - can use hydraulics to work valves • Analog - capacitors integrate, inductors differentiate, etc. • Optics - hard to control photons with photons (electro-optics) • DNA, Quantum Thursday, September 15, 16 Binary Digital Concept • On-Off circuit can represent base-2 (binary) digits (bit) values of 1 or 0 • Parallel circuits represent multi-digit values • Everything in the world can be digitized (broken 0 1 0 1 1 0 1 0 into discrete units with numerical values) • Computers can represent and operate on any kind of data using on-off circuits, through Boolean algebra, which uses True and False (1 and 0) Thursday, September 15, 16 Precursors • Mechanical - decimal gearing • Electromechanical • Binary coded decimal (4 bits for 0-9) • True binary • Vacuum tube/transistor repeats designs • IC transitions to mostly binary Thursday, September 15, 16 Words • Because number of parallel wires is fixed once a machine is built, the basic numerical unit of computation is also fixed (called word size) 1, 4, 8, 12, 16, 18, 24, 32, 36, 48, 60, 64, 68, 72, 80 • Other numerical units are multiples or integral partitions of word size Thursday, September 15, 16 Requisites for Binary Digital Technology • Some kind of electronically controlled switching device • Rapid switching between on and off states (no intermediate voltage level) • Easily wired together, with low resistance Thursday, September 15, 16 Not like this Materials • Conductors (electrons flow freely) • Insulators (electrons do not flow) • Semiconductors (in between) Thursday, September 15, 16 Energy Bands • Electrons occupy discrete energy levels around a nucleus • With multiple atoms, electrons have more levels, which become bands of energy level • Valence and Conduction • Conduction band has free electrons, with high mobility • Valence is just below • Full bands cannot conduct -- need holes Thursday, September 15, 16 Conductors • Valence and conduction band energies overlap • Valence band is nearly filled with electrons • Due to band overlap, electrons can easily attain conduction band level and move between atoms Conduction Valence Thursday, September 15, 16 Insulators • Valence and conduction bands widely separate in energy level (band gap) • No or few electrons in conduction band • Valence band not fully populated • High energy needed to lift electrons over band gap, so resist movement between atoms Conduction Valence Thursday, September 15, 16 Electrons and Holes • Electrons carry negative charge Thursday, September 15, 16 - - - -- Electrons and Holes • Moving an electron out of the Valence band leaves a hole Thursday, September 15, 16 - - - -- Electrons and Holes - - - -- • Holes attract electrons and act like positive charge carriers - Thursday, September 15, 16 Semiconductors • No overlap in Valence and Conduction bands but energy gap is small • Pure form acts as an insulator • Implanted impurities narrow band gap of material by providing extra electrons or holes Conduction Valence Thursday, September 15, 16 Dopants • Can be doped with materials to add electrons or holes • Boron adds holes (P-type) • Hole-carrier material • Phosphorus adds electrons (N-type) • Electron carrier material Thursday, September 15, 16 Transistors • Source, Gate, Drain • N-type body (pMOS) conducts holes when negative voltage (gate to source) applied • P-type body (nMOS) conducts electrons when positive voltage (gate to source) applied • Act as switches Thursday, September 15, 16 Cross Section View Thursday, September 15, 16 NMOS (P semiconductor, N-channel) -------- Gate -------N Source Oxide +++++++++++++ N Drain P semiconductor body - charge on gate. Hole carriers in body prevent flow of electron carriers between source and drain Thursday, September 15, 16 NMOS (P semiconductor, N-channel) +++++Gate+++++ Oxide -----------------------N Source N Drain Inversion Layer +++++++++++++ P semiconductor body + charge on gate repels + carriers in body, opening layer of - carriers so current can flow from source to drain Thursday, September 15, 16 PMOS (N semiconductor well, P-channel) +++++Gate+++++ P Source Oxide -------------------------- P Drain N semiconductor well P semiconductor body + charge on gate. Electron carriers in body prevent flow of hole carriers between source and drain Thursday, September 15, 16 PMOS (N semiconductor, well P-channel) --------Gate-------Oxide P Source ++++++++++++++ P Drain Inversion Layer N-------------------------semiconductor well P semiconductor body - charge on gate repels - carriers in body, opening layer of + carriers so current can flow from source to drain Thursday, September 15, 16 Schematically P-type N-type Complementary Metal Oxide Semiconductor: CMOS Thursday, September 15, 16 N-type Drain Gate Source Switch conducts electrons from source to drain when +voltage applied to gate, insulates with -voltage Thursday, September 15, 16 P-type Source Gate Drain Switch turns off (insulates) when +voltage applied to gate. Conducts holes from source to drain when -voltage applied Thursday, September 15, 16 What does it do? Vcc Out In Ground Thursday, September 15, 16 What does it do? Vcc Out In Ground Thursday, September 15, 16 What does it do? Vcc One Out In Ground Thursday, September 15, 16 Zero What does it do? Vcc Out Zero In Ground Thursday, September 15, 16 One Inverter Vcc Out In Note that Vcc and Ground were never connected together Thursday, September 15, 16 Ground Schematically NOT Thursday, September 15, 16 What Does it Do? Vcc In 2 Out In 1 In 2 Ground Thursday, September 15, 16 In 1=0, In 2=0 Vcc In 2 Out In 1 In 2 Ground Thursday, September 15, 16 In 1=0, In 2=1 Vcc In 2 Out In 1 In 2 Ground Thursday, September 15, 16 In 1=1, In 2=0 Vcc In 2 Out In 1 In 2 Ground Thursday, September 15, 16 In 1=1, In 2=1 Vcc In 2 Out In 1 In 2 Ground Thursday, September 15, 16 Truth Table View Thursday, September 15, 16 In1 In 2 Out 0 0 1 0 1 1 1 0 1 1 1 0 Compare to AND Thursday, September 15, 16 In1 In 2 Out AND 0 0 1 0 0 1 1 0 1 0 1 0 1 1 0 1 Compare to AND NAND Thursday, September 15, 16 In1 In 2 Out AND 0 0 1 0 0 1 1 0 1 0 1 0 1 1 0 1 Schematically NAND Thursday, September 15, 16 NAND Inverter NAND Thursday, September 15, 16 In1 In 2 Out 0 0 1 0 1 1 1 0 1 1 1 0 NAND Inverter NAND Thursday, September 15, 16 In1 In 2 Out 0 0 1 0 1 1 1 0 1 1 1 0 NAND Inverter NAND Thursday, September 15, 16 In1 In 2 Out 0 0 1 0 1 1 1 0 1 1 1 0 What Does it Do? Vcc In 2 In 1 Out In 2 Ground Thursday, September 15, 16 In 1=0, In 2=0 Vcc In 2 In 1 Out In 2 Ground Thursday, September 15, 16 In 1=0, In 2=1 Vcc In 2 In 1 Out In 2 Ground Thursday, September 15, 16 In 1=1, In 2=0 Vcc In 2 In 1 Out In 2 Ground Thursday, September 15, 16 In 1=1, In 2 = 1 Vcc In 2 In 1 Out In 2 Ground Thursday, September 15, 16 Truth Table View Thursday, September 15, 16 In1 In 2 Out 0 0 1 0 1 0 1 0 0 1 1 0 Compare with OR Thursday, September 15, 16 In1 In 2 Out OR 0 0 1 0 0 1 0 1 1 0 0 1 1 1 0 1 Compare with OR NOR Thursday, September 15, 16 In1 In 2 Out OR 0 0 1 0 0 1 0 1 1 0 0 1 1 1 0 1 Schematically NOR Thursday, September 15, 16 NOR Inverter NOR Thursday, September 15, 16 In1 In 2 Out 0 0 1 0 1 0 1 0 0 1 1 0 NOR Inverter NOR Thursday, September 15, 16 In1 In 2 Out 0 0 1 0 1 0 1 0 0 1 1 0 NOR Inverter NOR Thursday, September 15, 16 In1 In 2 Out 0 0 1 0 1 0 1 0 0 1 1 0 Universal Boolean Operators • AND, OR, NOT can be combined to express any Boolean expression • NAND can become NOT with addition of a wire • NOT (A NAND B) = A AND B • Can NAND become OR? Thursday, September 15, 16 NAND OR A B NAND NOT A NOT B NOT A NAND NOT B 0 0 1 1 1 0 0 1 1 1 0 1 1 0 1 0 1 1 1 1 0 0 0 1 Because we can build NOT with NAND, we can also build OR entirely with NAND circuits Thursday, September 15, 16 Universal Boolean Operators • AND, OR, NOT can be combined to express any Boolean expression • NAND can become NOT with addition of a wire • NOT (A NAND B) = A AND B • NOT A NAND NOT B = A OR B • Thus NAND is universal by itself Thursday, September 15, 16 Where We Are • NOR can also be shown to be universal • We have now seen how transistors can make circuits that mimic all the Boolean operators necessary for any expression • We will see that if we take True = 1 and False = 0, then Boolean logic is capable of representing binary (base-2) arithmetic Thursday, September 15, 16