Intensity I (W m-2)m-2)

Intensity = Power  I = P A Area

Inverse square law  I d 2 = A A  I d2d2 B B

Photoelectric effect frequency current fofo

Energy of photons E = h f h is Planck’s constant

Intensity of photons I = N h f N is number of photons per second

Work function Minimum energy to release electron from a surface (E = h fo)fo)

Kinetic Energy E = h f - h fofo Energy above minimum appears as kinetic

Emission spectra violet red W2W2 W1W1 W0W0

Emission spectra W2 W2 – W1 W1 = h f Electron ‘jumps’ from excited level to lower level

Emission spectra Bright emission lines - more electrons

Absorption spectra Photon of energy h f W2W2 W1W1

Absorption spectra W2 W2 = W1 W1 + h f Electron absorbs radiation and ‘jumps’ to excited level

Spontaneous emission random process

Stimulated emission Photon (energy h f)f) can cause atom to emit photon (energy h f) in phase and same direction

Laser Stimulating photon (hf) E1E1 E0E0

Laser Monochromatic Coherent Intense

Semiconductors n-type p-type

n-type Conduction by negative electrons

p-type Conduction by ‘positive’ holes

Forward-biased p-type diode conducts n-type electrons

Reverse-biased p-type diode does not conduct n-type

Diode Forward-biased diode electron and hole recombine Photon (heat) emitted

LED Forward-biased diode electron and hole recombine Photon (light) emitted

photodiode Photovoltaic mode supplies power e.g. solar cell

photodiode Photoconductive mode (reverse bias) light sensor

MOSFET drain n-region implant n-channel enhancement MOSFET oxide layer gate source n-channel p-type substrate

MOSFET Can switch on a load. Apply gate voltage V GS to turn ‘on’ MOSFET

D load n-channel enhancement MOSFET G S IoIo 0 V + V V GS

MOSFET Can also be used as an AMPLIFIER