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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
spaces move and discuss their location and
population density.
Page 1:
Page 3:
1. Why does a semiconductor's ability to
conduct electricity increase when
temperature rises?
(c)
2. Explain your answer to the question to the
left based on your observation of the two
simulations on this page.
1. The electric current in an N-typ
semiconductor flows because
(a) there are extra electrons that can move from
atom to atom under a voltage.
2. Explain the results of the above simulation
using the concept of holes.
As I heat up the atoms they shake around more
and more electrons are freed up from their
nuclei. I can see that when I raise the
temperature more current flows, because more
electrons are free to move.
Holes provide places for electrons to move into.
When an electric field is applied electrons move
into a hole and leave behind a hole, so holes
appear to “flow” opposite to electrons. So, you
can think of an electric current as the movement
of “positive” holes instead of negative electrons.
Page 2:
Page 4:
1. Which of the following is NOT true about
the simulation of hole movement?
1. Run the simulation for a while until no
more electrons move into the P-type area.
Take a snapshot of the P-N junction and then
identify the depletion region using the
rectangle tool. Drag your image into the box
below.
(d)
2. Why is an electron hole considered as a
carrier of positive charge?
(d)
3. Select the "What if there is 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 simulation,
explain why the concept of a hole is useful.
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
2. Why does the electron movement stop
after the depletion region is formed?
(f)
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?
3. What causes the electrons to flow
continuously under a forward bias voltage?
(b) Electrons in the energy gap due to the
dopant will enter the conduction band.
(b)
4. What causes the electrons to stop flowing
under a reverse bias voltage?
(d)
Page 5:
No questions.
Page 6:
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.
No questions.
Page 7:
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.
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