Transistors & Semiconductors

Transistor

• Amplifying Switch– Current to base enables current from emitter to

collector– Base requires small amount of

current relative to emitter

Silicon

• SI = Element 14– 4 electrons in outer shell – 8 would be full

Silicon

• Silicon crystal– Stable diamond like structure– Thermal noise occasionally

bumps electrons, but settleback rapidly

Doping

• Doped silicon has impurities– N type :• Extra electrons• Phosphorus

– P type :• Short an electron• Boron

Doped Conductors

• Doped silicon:– Free roaming electrons

or holes– Phosphorus/Boron ions

N + P Sandwich

• N&P sandwich forms : Depletion layer– Electrons/holes merge– Ions present barrier to more movement

N + P Sandwich = Diode

• Current flow into N closes depletion layer• Current flow into P increases depletion layer

Transistor = Diode Sandwich

• Transistor is 3 layers:– NPN or PNP

• Default is to insulate

Transistor = Diode Sandwich

• Positive current at base removes spare negatives– Shrink PN depletion layer

• Relatively small current at base

• http://www.youtube.com/watch?feature=player_embedded&v=9CrcRabTQ0s#

MOS

• MOS – Metal Oxide– "Metal" conductor– Oxide insulator

• Charge to conductor produces conductive layer in silicon via field effect

MOSFET

• Metal Oxide Field Effect Transistor– NPN regions of silicon– Conductor used to create channel• Channel requires no current!!!

CMOS

• CMOS : Complementary MOSFETS– NPN and PNP used together• Always high or low signal

• http://www.youtube.com/watch?v=QO5FgM7MLGg&list=FL8JWMIsIgFMX3tv2BU-K4Uw

(Jump to 3:00)

Fabrication

• Circuits "printed" with photo reactive chemicals light

http://www.youtube.com/watch?v=35jWSQXku74&list=FL8JWMIsIgFMX3tv2BU-K4Uw– Minute 2