The energy band diagram for electrons and holes is a useful tool for understanding carrier transport in semiconductor devices. In this chapter, you get to play with the band diagram and to see the connection between the real space and the energy domain.
The bottom of the conduction band represents the potential
energy of electrons. In the absence of any electric field, the
potential energy of electrons does not change with position
and the banddiagram remains flat.
Follow an electron. Can you
measure the kinetic energy of the electron? Does it remain
the same between scatterings? What about before and after
scatterings?
Remember that the top of the vallance band
represents the potential energy of a hole.
Toggle the button below to follow a hole.
Electron/Hole Show Trace
Click on the box to the right to apply an electric field. You can
change the direction and intensity of the electric field by
clicking away from the center of the box.
· Can you explain why the banddiagram bends knowing that the
bottom of the conduction band represents the potential energy
of electrons?
·What can you say about the potential energy, kinetic energy
and total energy of an electron between two scattering events?
·What happens to the potential, kinetic and total energy of an
electron once it is scattered?
· Do you see any major change in the mean free path of
electrons as you change the electric field? Pay attention to the
value of the maximum electric field you can apply)
· Follow a hole and try to answer the same questions. Remember
that the band diagram represents the energy of electrons.
For holes, the energy axis points downward.
Electron/Hole Show Trace
Let’s try larger electric fields (>104V/cm)
· Do you see a major change in the mean free path of electrons
at high electric fields?
· When an electron in Si obtains a kinetic energy of about
0.063eV, it quickly emits an optical phonon and get scattered.
Can you see this effect?
· Use the graphs below to see how mean time between
scatterings, mean free path, and mobility change with electric
field.
Electron/Hole Show Trace
Scattering Time or Mean Free Path or Drift Velocity
You can now change the temperature and see how the band
diagram changes and how carrier transport is affected.
· What happens to the bandgap as you change the temperature?
· What happens to the average kinetic energy of electrons or
holes as you change the temperature
(use zero or small electric fields).
· What happens to the mean free path as you raise the
temperature?
Electron/Hole Show Trace