transistors: building blocks of electronic computing lin zhong elec101, spring 2011
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Transistors: Building blocks of electronic computing
Lin ZhongELEC101, Spring 2011
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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)
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Capacitor
• Charge holder• Capacitance (capability to hold charge)– Charge per volt
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+ -
V
C=Q/V
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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
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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
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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
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How can we build an inverter?
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In Out
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Switch
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Switch-based inverter
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In
Out
Vdd
Gnd
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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
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Metal-Oxide-Semiconductor FET (MOSFET)
Polysilicon Aluminum
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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
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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
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Change a state
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Vdd
Gnd
Input
Output
Resistor
LOW
HIGH
?
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Change a state (Contd.)
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Vdd
Gnd
Input
Output
LOW
HIGH
Vdd
Gnd
Input
Output
ResistorLOW
HIGH
?
Resistor
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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
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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