Semiconductor Physics
Movement of Electrons/holes
Allen Chen
Department of Electrical Engineering
NSF GK-12
This program is based upon collaborative work supported by a National Science Foundation Grant No. 0841259; Colorado State University,
Thomas Chen, Principal Investigator, Michael A. de Miranda and Stuart Tobet Co-Principal Investigators. Any opinions, findings, conclusions or
recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science
Foundation.
Why should you care about
semiconductors?
• They are the material behind electronics or
integrated circuits (computer chips)
– Do you remember what is the most commonly
used semiconductor material? (think sand, glass)
• Solar cells are also made of semiconductors
Chemistry Review
• Recall valence electrons
• Recall different electron energy levels
– Electrons occupy energy levels from the lowest
energies upwards
– Some energy levels are “forbidden” due to wavelike properties of the atoms
• Covalent bonds
– each atom shares an electron with its nearest
neighbour
Energy Levels or Energy Bands
• Valence band
– the highest occupied energy level in a
semiconductor or insulator that is occupied by
electrons at T= 0K.
• Conduction band
– an energy band in a solid in which electrons move
freely and can produce a net electric current.
– It’s also the first unfilled level above the valence
band
Semiconductors
• Materials can be
categorized into
insulators, conductors
and semiconductors
• Why do they have the
properties that they
have?
Group IV materials
Si
Si
Si
Group IV atoms have 4
valence electrons.
These form perfect
covalent bonds.
This is an INSULATOR
What happens if every
some Si atoms are now
replaced with Phosphorous
or Arsenic?
Replacing Si w/ As = Doping!
Si
Si
Si
Extra electrons = n-type
semiconductor
Si
Si
Si
Si
As has 5 valence electrons.
Now, when As forms
covalent bonds with Si
atoms, there is an extra free
electron. Now this is a
SEMICONDUCTOR
We say this material is
“doped” with As.
Replacing Si w/ Boron
What if instead of As,
we use Boron?
Boron has 3 valence
electrons. So after
bonding, the lattice is
MISSING one electron.
This “missing electron”
is known as a “hole”
Missing electrons or
having holes = p-type
semiconductor
Summary
• N-type Semiconductors: extra electrons
• P-type Semiconductors: missing electrons, or
has extra “holes”
• In N-type silicon the extra electrons are free to
move and so can act as current carriers.
• In the P-type material the holes exert a
considerable force of attraction for any free
electrons and so they enable a current to
flow.
What is mobility?
• Wiki – the state of being in motion
• Or how “easily” the electrons move in the
material
• Which (electrons or holes) do you think has a
higher mobility?
Electron Motion
• Electron motion is RANDOM
• Drift Velocity describes this random motion
• Drift_velocity = mobility x Electric_field
– Units
• Velocity [meters per second]
• Electric Field [Volts per meter]
• Mobility (Can you derive the units for mobility? Use
factor label)
So how fast do electrons move in a
transistor?
Faster Electrons
• As transistors get smaller, the electric field
inside a transistor increases (you will learn
why in the Electric Field’s Unit)
• Higher Electric fields translates to faster or
higher-energy electrons
• Electrons w/ too much energy is a big problem
in nanoscale transistors