Intensity I (W m-2)m-2). Intensity = Power I = P A Area.
Transcript of Intensity I (W m-2)m-2). Intensity = Power I = P A Area.
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