transistors: building blocks of electronic computing lin zhong elec101, spring 2011

Transistors: Building blocks of electronic computing

Lin ZhongELEC101, Spring 2011

Key concepts

• Recap of PHY102• Ubiquity of resistance and capacitance• Energy and power consumption• Inverter: the simplest computing• Transistor– Charge state

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Electrons, charge, current• Electron

– Subatomic particle– ~9.109x10-28 gram

• Carbon atom ~2X10-23 grams

– Elementary charge e-

• Charge– Q– Negative vs. positive– Coulomb: 6.25x1018 e – Coulomb’s law

• Current– Flow of charge (electrons and electron holes)– I=dQ/dt

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Field, potential and voltage

• Electric field– Force exerted on electric charge

• Electric potential• Voltage: difference in potential– Ground (Gnd): 0 Volt– Supply voltage: 1.2-5 Volt for modern integrated

circuits• Current flows from high potential to low– How about electrons?

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Resistance and Ohm’s Law

• Opposition to the passage of a steady current– Conductor (e.g. metal) has negligible resistance– Insulator (e.g. plastic) has negligible passage– Semiconductor (silicon)

• Ohm’s Law– I=V/R or R=V/I

• R=ρ∙L/A

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Resistance is everywhere

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Resistor

• Axial lead resistors

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Voltage divider

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Vdd

Gnd

R2

R1

V=Vdd*R2/(R1+R2)

Capacitor

• Charge holder• Capacitance (capability to hold charge)– Charge per volt

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+ -

V

C=Q/V

Capacitance is everywhere

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Energy and power consumption

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V

Gnd

R

P=E/T =V*I =V2/R

E=V*Q =V*I*T=V*(V/R)*T =V2*T/R

Energy and power consumption

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V

Gnd

C Q=C*V+

-

Gnd

Gnd

C

E=0.5*Q*V =0.5*C*V2

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Are resistors and capacitors enough?

R=V/I C=Q/V

Computing is about “state”• Change• Communication• Storage

Think about a simple “computer”

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Binary “states” for input and output: HIGH or LOW (1 or 0)

How many different computers are there?1) In=0 Out=0; In=1 Out=02) In=0 Out=1; In=1 Out=03) In=0 Out=0; In=1 Out=14) In=0 Out=1; In=1 Out=1

Can resistors and capacitors build them?

How can we build 2)?

In Out

How can we build an inverter?

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In Out

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Switch

Switch-based inverter

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In

Out

Vdd

Gnd

Lab: LED

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What can one do with breadboard?

Breadboard prototype of the first IBM PC motherboard (1981)http://www.yourdictionary.com/computer/breadboard

Metal-Oxide-Semiconductor FET (MOSFET)

Polysilicon Aluminum

MOSFET state

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Controllable resistor

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Gate control

NMOS: resistance increases as gate voltage decreases

Gate control

PMOS: resistance increases as gate voltage increase

Drain

Source

Source

Drain

Controllable switch

• Change a state• Communicate a state• Store a state

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GateGate

Gate

NMOS:ON with HIGH gate

PMOS:ON with LOW gate

Change a state

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Vdd

Gnd

Input

Output

Resistor

LOW

HIGH

?

Change a state (Contd.)

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Vdd

Gnd

Input

Output

LOW

HIGH

Vdd

Gnd

Input

Output

ResistorLOW

HIGH

?

Resistor

Lab: NMOS and PMOS inverters

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What’s wrong?

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Vdd

Gnd

Input

Output

LOW

HIGH

Vdd

Gnd

Input

Output

ResistorLOW

HIGH

Resistor

Complementary MOS (CMOS)

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Lab: CMOS inverter

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Power consumption of Inverter

• Dynamic power– Remember capacitance is everywhere

• Static power

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Delay of Inverter

• Capacitance and resistance are everywhere

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A memory cell

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Read/write

Gnd

Select