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Teacher’s Guide
Transistors
Overview
Students learn that field effect transistors (FETs), like mechanical switches in circuits,
control electric current. However, unlike mechanical switches, FETs control electric
current by using the field effect. Students explore how the gate of a junction field effect
transistor (JFET) turns an electric current through a semiconductor on and off by
changing the voltage. Students learn how to use two transistors to build a logic AND
gate as well as a logic OR gate. In addition, students are introduced to MOSFETs and
carbon nanotube transistors.
Learning Objectives
Students will be able to:
 Compare a transistor to a mechanical switch
 Explain how electric fields obstructs the flow of electric currents
 Describe how JFETs work
 Explain how logic gates can be built from transistors
Prerequisite Knowledge
Students should already have a basic understanding of:
 Semiconductors: charge carriers, P-type and N-type semiconductors, and
depletion regions (See ET “Semiconductor” activity)
 Coulomb’s Law
 Logic operations
 Electronics
Background Resources
A review of semiconductors and doping:
http://www.hscphysics.edu.au/resource/Semiconductors%20Lesson%20outline.pdf
The PBS documentary Transistorized!, which discusses the history and operation of the
transistor:
http://www.pbs.org/transistor/
An explanation of Boolean logic gates, including how to construct a simple adder:
http://www.howstuffworks.com/boolean.htm
Approximate time for lesson completion: 60 minute
Activity Answer Guide
Page 1:
1. Describe what happens in the circuit when
you click the switch to turn the light off.
When the switch breaks the circuit, the electric
current is interrupted and the light is off.
2. Describe what happens in the circuit when
you click the switch to turn the light on.
When the switch connects the circuit, the electric
current flows and the light is on.
Page 2:
1. What happens to the flow of electrons
when you change the amount of negative
charge on the island?
2. Take a snapshot that shows a pinched
channel.
When the island is more negatively charged, the
size of the pink area in which the electrons are
forced out increases. When the island is less
negatively charged, the pink area shrinks. The
size of the area determines how much the flow
of the electrons is obstructed. Tell students that
an analogy of this is a log in a river
A log with a large cross section will resist more
water flow than a log with a small cross section.
2. What happens to the flow of electrons
after the island is moved to a different
location?
The electrons flow around the island wherever it
is placed.
3. What causes the electrons to flow around
the island (but not enter it)?
According to Coulomb's Law, there is repulsion
between the electrons and the negatively
charged island. This force prevents the free
electrons from entering the pink area.
Page 3:
1. Take a snapshot that shows a narrowed
channel.
3. Explain why a junction field effect
transistor can be used as a switch.
When the voltage is increased on the gate, the
region that forces electrons out is increased.
This reduces the area in which the current can
flow.
A junction field effect transistor can be used as a
switch because it can increase the amount of
voltage on the gate, pinch the channel shut, and
break the current flow. Alternatively, a junction
field transistor can reduce the amount of voltage
on the gate and allow the current to flow.
4. The gate-source voltage slider has
negative readings. Why?
An N-type semiconductor contains an
abundance of mobile electrons that carry
negative charges. In order to pinch the flow of
electrons, you need to apply a negative voltage
to the gate. This creates a repulsive electric field
for the electrons. Were the voltage positive in
this case, the electrons would be attracted to the
gates.
When you use a light switch to turn the light on,
the electrons flow in the circuit. When you use a
light switch to turn the light off, the electrons
stop flowing. Similar to a light switch, a gate of a
field effect transistor can be used to allow the
flow of the electrons or stop the flow of
electrons.
2. Which of the following graphs best
describes the relationship between the drain
current and the gate voltage in a junction
field effect transistor shown on Page 3?
Page 4:
(C)
1. Explain how an AND logic gate works
based on your observation.
3. In the field effect transistors you have
seen in this activity, the flow of electrons is
controlled by the voltage applied to the gate.
If the voltage of the gate is sensitive to the
presence of certain types of molecules or
ions, then one can make a sensor based on
this effect. Reflect on this idea. If possible,
come up with your own idea about how to
design a field effect biosensor.
I observed that the current can only flow when
voltage is applied to both gates (both set at 1).If
one of the gates has voltage (set to 0), the
circuit is broken.
Page 5:
1. Explain how an OR logic gate works based
on your observation.
I observed that the current could flow when one
of the gates has a voltage applied to it (set at 1).
When both gates have no voltage (set to 0), the
circuit is broken.
Page 6:
1. Why does there need to be a layer of oxide
in a MOSFET?
The oxide layer prevents the electrons in the
conduction channel from flowing out of the
transistor through the metal gate. Tell students
that the oxide in a MOSFET performs the same
function as a dielectric in a capacitor—the oxide
separates charge.
Page 7:
No questions.
Page 8:
1. Compare a gate of a field effect transistor
to a light switch.
A field effect biosensor would be monitoring the
presence of specific molecules. If such
molecules were detected, the sensor would
either open or close a transistor. The change in
the state would signal the presence of the
molecule, and an alarm would go off.
Further Extensions
Boolean logic
 Use a logic gates simulator on the
Internet, or use TTL chips and
breadboards, to assemble a simple
binary adder.
Transistors as amplifiers
 Explore how transistors can amplify
signals or act as variable resistors.
Transistor fabrication
 Explore the chemistry and quantum
theory behind photolithography.