Teachers Guide
Semiconductors
Overview
Students are introduced to semiconductors and the conditions under which a
semiconductor will act like a conductor or insulator. The concept of holes is explained.
Students are shown how dopant impurities in a semiconductor crystal create conditions
for electron conduction. These ideas are then used to describe the operation of a p-n
junction (diode) in forward and reverse bias. The quantum-mechanical nature of
semiconductors is also explored.
Learning Objectives
Students will be able to:
 Explain how increasing temperature allows semiconductors to change from
insulators to conductors
 Describe what a hole is and how it helps us explain electrical conduction in
semiconductors
 Compare and contrast the effect of n-type and p-type dopants on semiconductor
conduction
 Define the depletion region of a p-n junction, explain how it is formed between a
p-type and n-type semiconductor, and how the depletion region allows current
flow under forward bias but not under reverse bias.
 Explain how energy levels in atoms form into valence and conduction bands in a
crystal and how n-type and p-type dopants donate electrons or holes to these
bands
Prerequisite Knowledge
Students should already have a basic understanding of:
 Electricity and electric circuits
 Conductors and insulators (See ET Activity “How Electrons Move”)
 Crystalline structures and covalent bonds
 Quantum mechanics (See ET Activity “Introduction to Quantum Mechanics”)
Background and resources
http://hyperphysics.phy-astr.gsu.edu/Hbase/solids/sselcn.html - Semiconductor Physics
for Solid State Electronics
http://www.allaboutcircuits.com/vol_3/chpt_3/1.html - An Introduction to Diodes and
Rectifiers
Activity Answer Guide
Page 1:
1. Based on your observation from the above
simulation, explain why a semiconductor
does not conduct electricity at low
temperature.
At low temperature, all electrons are bound to
atoms in covalent bonds. Thus, no electrons are
free to move, so no conduction occurs.
2. Based on your observation from the above
simulation, explain why a semiconductor can
conduct electricity when temperature is high
enough.
As the temperature increases, more electrons
gain enough energy to escape the covalent
bonds. These electrons are now free to
conduct.
Page 2:
1. Select the "Hide electrons" check box and
describe what you observe below.
The hole moves towards the negatively-charged
side of the electric field.
2. Explain why holes are considered as
positive charge carriers.
Electrons will hop towards the positively-charged
side of the electric field because they are
negatively charged. Holes shift toward the
negative charge, so they can be considered to
be positively charged.
The movement of electrons between covalent
bonds requires an empty space into which
adjacent electrons can move. These empty
spaces behave like positively-charged particles
when an electric field is applied. The concept of
a hole allows us to describe how these empty
spaces move and discuss their location and
population density.
Page 3:
1. What happens when there are free
electrons? Can you explain your
observation?
The free electrons provided by the antimony
impurities allow for a larger current when an
electric field is applied. The extra electron
provided by the impurities is not bound to any
atom in the crystal structure. It is free to
conduct.
2. What happens when there are no holes?
Can you explain your observation?
A lack of holes means that no electrons can
move between bonds, so no electrons flow. The
electrons can hop into the extra holes provided
by the boron impurities, which allows for more
electron flow. We can also view this situation as
holes flowing in the opposite direction of
electrons.
Page 4:
1. Run the model for a while and take a
snapshot picture of the P-N junction. Identify
the depletion region using the rectangle tool.
3. Select the "No hole" check box. Do you
observe any electric current? Deselect the
check box and observe again. Explain why
there is a difference.
There is no electric current without the presence
of holes. Every space into which an electron
could move is filled, so none of the electrons can
move. When the hole is present, it acts as an
empty space that an adjacent electron can move
into, and so electrons can hop from one site to
the next.
4. Based on your experiments and
observations with the above model, explain
why the concept of hole is useful.
2. Explain why electron movement is
stopped by the depletion region.
The electrons that flow into the p-type material
are attracted to the holes and held in place.
These electrons repel other electrons and thus
block the flow of electrons from the n-type
material.
3. Explain why electrons can flow
continuously under a forward bias voltage.
3. In a P-type semiconductor, the extra holes
provided by the dopant atoms can be found
in the:
(a) Valence band
4. At very low temperatures, electrons in an
intrinsic semiconductor will be found in the:
The battery in the circuit provides enough
energy to the electrons to cause the electrons to
flow past the hole in the p-type region.
(a) Valence band
4. Explain why electrons cannot flow
continuously under a reverse bias voltage.
Page 7:
The energy from the battery pushes electrons
away from the junction, and all of the electrons
collect on the far side of the n-type region. This
inhibits electron flow.
Page 6:
1. Take a snapshot picture of the energy
band structure of an N-type semiconductor
and annotate it to illustrate how electrons
would move when temperature increases.
1. Which of the following about
semiconductors are true? Select all that
apply.
A. They are sometimes insulators and
sometimes conductors.
D. Their electrons are bound to atoms but can
become loose when temperature increases.
2. Which of the following about holes are
true? Select all that apply.
B. A hole moves independently of atoms like a
free electron.
C. A hole hops from one bond site to another.
3. Check all statements that are TRUE
regarding the depletion region.
(a) A depletion region is formed wherever p-type
and n-type semiconductors are in contact.
(d) Applying forward bias to a p-n junction allows
electrons to flow across the depletion region.
2. Take a snapshot picture of the energy
band structure of a P-type semiconductor
and annotate it to illustrate how holes would
move when temperature increases.
4. An N-type semiconductor initially exists at
a very low temperature. As the temperature
is slowly increased, which of the following
events will happen first?
(b) Electrons in the energy gap due to the
dopant will enter the conduction band.
5. The power plugs in your home use an
alternating voltage to run your electronic
devices. This means that the polarity of the
voltage switches back and forth many times
a second, unlike a battery. The upper image
to the left shows a graph of how the voltage
changes.
If a P-N junction were to be connected to the
alternating voltage from your wall socket,
describe how current would flow through the
P-N junction and why it would flow that way.
Current would flow through the p-n junction
when the alternating voltage created a forward
bias condition in the p-n junction, and would not
flow when the p-n junction was in reverse bias.
The depletion region would block the flow of
current when the p-n junction was in reverse
bias, but would not block the current flow in
forward bias.
Further Extensions


Examine the current-voltage behavior
of p-n junctions by measuring the
ideality factor of diodes
Explore the current flow through a
diode visually with a light-emitting
diode