unit 12: transistors

itp 150 - prof. djassemi - cal poly

transistor definition & functions

• transistor: semiconductor device used to amplify & switch electronic signals & power

• 2 main functions: switching & amplifying

• made of: silicon & germanium

• solid-state devices: electrical activity happens in solid material w/ no moving parts

significance & applications

• commonly used as: switches, relays, amplifiers

• found in: radios, telephones, computers, & integrated circuits (ICs)

• integrated circuit (IC): a large # of transistors interconnected on one silicon chip

how it works

• when electrical signal is applied to transistor → electrical fields force holes & electrons to swap places

  • result: regions that normally insulate become conductive

• what controls electron flow in transistor: the base

• electrons flow from collector → emitter

configuration

• NPN

  • N-type emitter, P-type base, N-type collector

structure

• 3 layers: emitter, base, collector

• emitter: heavily doped

• base: very thin & lightly doped

• collector: largest layer & heavily doped

diode vs. transistor

• transistor: 2 pn junctions placed back-to-back

• key difference:

  • diode controls current in 1 path

  • transistor controls large current using small base current

schematic symbol

• terminal w/ arrow: emitter

• arrow indicates: direction of electron flow

• electron flow direction: C → E

• base controls terminal of transistor

operation of transistor

• 2 power sources in transistor circuit

  • 1st power source: activates base

  • 2nd power source: runs external device

• when small current flows into base: large current flows from C → E

• when no base current flows → transistor is OFF

switch & amplifier functions

• why is transistor an amplifier: small base current becomes much larger C-E current

• approximate gain: ~100x larger

• why is transistor a switch: base current turns large current ON/OFF

base voltage requirement

• typical voltage needed to turn NPN transistor ON: 0.7V

• why use base resistor?

  • limits voltage/current to protect transistor

direction of current: emitter-base (EB)

• direction: from emitter → base when forward biased

• forward biased EB junction allows charge carriers to enter base

direction of current: base-collector (BC)

• direction: base → collector

• collector pulls carriers from base toward emitter

transistor as a switch

• when base current is present: transistor acts as closed switch (ON)

• when base current is absent: transistor acts as open switch (OFF)

transistor as an amplifier

• small base current amplified

• output of amplification: larger C→E current

example circuits

• LED @ collector glows brighter than LED @ base:

  • transistor amplifies base current

• base resistor in circuits:

  • protects transistor & controls base current

• plant-watering circuit:

  • dry soil allows small base current → transistor turns ON → indicator activates

• touch circuit demonstration:

  • tiny current thru skin turns transistor ON