Intensity

I (W m-2)

Intensity = Power

I = PA

Area

Inverse square law

I d2 =A A

I d2

B B

Photoelectric effect

frequency

current

fo

Energy of photons

E = h fh is Planck’s constant

Intensity of photons

I = N h fN is number of photons

per second

Work function

Minimum energyto release electron from

a surface (E = h fo)

Kinetic Energy

E = h f - h foEnergy above minimum

appears as kinetic

Emission spectra

violet

redW2

W1

W0

Emission spectra

W2 – W1 = h fElectron ‘jumps’ from

excited level to lower level

Emission spectra

Bright emission lines

- more electrons

Absorption spectra

Photon of energy h f

W2

W1

Absorption spectra

W2 = W1 + h fElectron absorbs radiation and ‘jumps’ to excited level

Spontaneous emission

random process

Stimulated emission

Photon (energy h f)can cause atom to emit

photon (energy h f) in phase and same direction

Laser

Stimulating photon (hf)

E1

E0

Laser

MonochromaticCoherentIntense

Semiconductors

n-type

p-type

n-type

Conduction by

negative electrons

p-type

Conduction by

‘positive’ holes

Forward-biasedp-type

diode conducts

n-type

electrons

Reverse-biasedp-type

diode does not conduct

n-type

DiodeForward-biased diode

electron and hole recombine

Photon (heat) emitted

LEDForward-biased diode

electron and hole recombine

Photon (light) emitted

photodiodePhotovoltaic mode

supplies power

e.g. solar cell

photodiodePhotoconductive mode

(reverse bias)

light sensor

MOSFETdrain

n-region implant

n-channel enhancement MOSFET

oxide layer

gate

source

n-channel

p-type substrate

MOSFETCan switch on a load.Apply gate voltage VGS

to turn ‘on’ MOSFET

MOSFET

Dload

n-channel enhancement MOSFET

GS

Io

0 V

+ V

VGS

MOSFETCan also be used

as an

AMPLIFIER