Transcript Semiconductor Theory and LEDs []

Gentle Intro to Semiconductors
ENGN/PHYS 207
The ubiquitous LED
Collection of diodes
Diode = electronic valve
Forward bias: Current flows!
Reverse bias: No current flow!
The Silicon Valley
Warm-up question(s)

What does it mean for a
material to be able to
“conduct” electricity?
Pure silicon lattice
(intrinsic semiconductor)
2D lattice model showing covalent bonds in Si crystal
Band Theory of Solids


Quantum Mechanics: Energy levels occupied by electrons are
discrete.
Amphitheatre analogy
Conduction Band
Band gap
Valence Band
Aspendos Theatre, Turkey
Energy Level Diagrams
Electron-Hole Pairs (EHPs)
Wait—what’s a hole?
Vacancy in the valence band
 Acts like mobile positive charge
 An electron can fall into a hole (Recombination)

Important Fact

Thermal energy to move and shake around a lattice
at (T = 300K)
kT = 1/40 eV
= 0.025 eV
Why pure silicon isn’t so hot
Large
energy
barrier!
Let’s dope up (?)
Group Number
III (extra hole)
IV
V (extra e-)
That’s dope: N and P type
N-type: Extra electrons
P-type: Extra holes
Dopant electrons and holes are mobile
Nuclei locked in the lattice, they are NOT mobile.
Why doping is actually good for us
N-type: easily donates electrons
into conduction band
P-type: easily accepts electrons into
valence band
The PN junction
Let there be Light…
I
VB
The Diode Equation
Silicon Diodes as Rectifiers
“Electronic Valve”
action
 Current can essentially
flow in one direction
only

0.7 V
Reverse bias
Forward bias
Rainbow of Fruit Flavors
Idealized LED turn-on voltages
I (mA)
1.8V
3.5V
3.6V
VF (volts)
Images:
 http://49chevy.blogs.com/fusor/2008/10/fun-with-electr.html
 http://wwwinst.eecs.berkeley.edu/~ee143/sp06/lectures/Semiconductor_tutorial.pdf
 http://www.allaboutcircuits.com/vol_3/chpt_2/3.html
 http://free-zg.t-com.hr/Julijan-Sribar/preview/vol1.html
 http://www.launc.tased.edu.au/online/sciences/Physics/photonics/bias.htm
 http://www.mtmscientific.com/diodes.html
 http://www.mpoweruk.com/semiconductors.htm
 http://free-zg.t-com.hr/Julijan-Sribar/preview/vol1.html
 http://electrons.wikidot.com/methods-for-the-determination-of-ground-statewavefunction
 http://www.electronics-tutorials.ws/diode/diode_8.html
Animations:
 http://pvcdrom.pveducation.org/SEMICON/PN.HTM
 http://www.acsu.buffalo.edu/~wie/applet/students/jiawang/pn.html
 http://www.youtube.com/watch?v=O3x7NdUuu0Q&feature=related
 http://www.youtube.com/watch?v=DbjR-2knrpo
Sources