Transcript Slide 1

Introduction to
Semiconductor Devices
Louis E. Frenzel
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Prerequisites
• To understand this presentation, you should
be able to:
– Draw the structure of an atom, including electrons,
protons, and neutrons.
– Define resistance and conductance.
– Label an electronic schematic, indicating current flow.
– Define Ohm’s and Kirchhoff’s laws.
– Describe the characteristics of DC and AC (sine wave)
voltages.
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Student Learning Outcomes
• Upon completion of viewing this presentation, you
should be able to:
– Define active and passive components.
– Name three main categories of active semiconductor devices.
– Name the two elements of a diode and state the primary
operational characteristics of a diode.
– Define bias and name the two types of bias and their effects on
diode operation.
– Explain the concept of a transistor.
– Explain how a transistor can switch or amplify.
– Define integrated circuit.
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Passive Components
• Semiconductor materials can be used to form almost
any kind of electronic component, including passive
components.
• Passive components like resistors, capacitors or
inductors are very common.
– A resistor is made with a piece of N or P-type
semiconductor material doped to the appropriate
resistance level.
– A capacitor is made with two plates of a highly
conductive semiconductor material separated by a pure
semiconductor material or another insulator like glass
silicon dioxide (SiO2).
– An inductor is made by making a spiral of highly
conductive semiconductor material.
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Active Electronic Components
• An active electronic component either amplifies
or switches.
– The most common active components are diodes
and transistors.
– Both diodes and transistors are easy to make with
semiconductors
• With semiconductor materials it is possible to
create complete circuits of active and/or
passive components wired together.
– These circuits are formed on a single chip of silicon
and are called integrated circuits (ICs).
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Diodes
• A semiconductor diode is
formed with pieces of N
and P-type material are
joined.
– The P material is called the
anode.
– The N material is called the
cathode.
– The resulting structure is
called a PN junction.
• A PN junction (or diode) is
a switch or component
through which electrons
will flow easily in one
direction but not in the
opposite direction.
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Biasing a PN Junction
• To get current to flow in PN
junction or diode, you have to
apply an external voltage called
bias.
• With this connection, current
only flows freely from cathode
across the junction to the
anode.
• You’d say the switch is closed
when electrons can flow through
the diode.
• Note: The current flow may be
so high that an external
resistance R is usually needed
to minimize the current flow to a
level a diode can withstand.
• This arrangement is called
forward bias.
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Reverse Bias
• A diode with the
external voltage
polarity reversed is
called reverse bias.
– With this connection no
current will flow.
– You’d say the switch is
open when electrons
can’t flow through the
diode.
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Diode Symbol
• The PN junction forms a
diode.
• To represent the diode in
schematic diagrams, we
use the symbol shown on
the right.
• Note the designations for
the anode and cathode.
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Diode Current Flow
• The direction of current flow (electrons) is shown by the
arrow.
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A Practical Diode
• A practical diode has two wire leads.
• Note the end with the band circling the body is the
cathode end.
Wire Leads
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Forward and Reverse Bias
• Examples of both forward and reverse biased
diodes:
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Practical Diodes
• Diodes, like other electronic components, are
not perfect.
• Diodes have an upper current limit that if
exceeded will destroy the diode.
• You can get diodes with current limits of a few
milliamperes up to hundreds or thousands of
amperes.
• Diodes also have an upper voltage limit when
reverse biased.
• This is the reverse break down voltage which
may be only a few volts or hundreds or
thousands of volts.
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Diode Voltage Drop
• Diodes also have a voltage drop
while they are conducting. This is
called forward voltage drop. It is in
the 0.5 to 0.9 volt range for silicon
diodes. A typical value is 0.7 volts.
• The forward drop in a germanium
diode is in the 0.2 to 0.4 volt range.
• Diodes also have a threshold
voltage approximately equal to the
forward voltage drop. This is the
minimum amount of forward voltage
that must be applied to get the diode
to conduct.
• For example if you apply a forward
bias of less than abut 0.7 volts to a
silicon diode, it will not conduct. As
soon as the bias voltage rises about
0.7 volts the diode will conduct.
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Transistors
• A transistor is a 3-terminal
semiconductor device that is
used to amplify or switch.
• By applying an external DC
voltage, current will flow from
terminal 1 through the device to
terminal 3.
• A resistor is used to set the
current level.
• A voltage or current applied to
terminal 2 is used to control how
much current flows from
terminals 1 to 3.
• A very small voltage or current
variation at terminal 2 can
produce a very large current
variation between terminals 1
and 3.
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How Transistors Switch
• The transistor is connected in
series with a resistor.
• With no (zero) voltage on
terminal 2, no current will flow in
the transistor. The transistor
acts like an open switch. The
output voltage is 3 volts as seen
through the resistor R.
• If a large enough voltage is
applied to terminal 2, the
transistor will conduct heavily
and act like a very low
resistance between terminals 1
and 3. It then acts like a closed
switch. The output voltage is
near zero.
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How a Transistor Amplifies
• The circuit is the same as the
switch.
• With a small voltage on terminal
2, a large variation in current
from terminals 1 to 3 occurs.
• For example, if a sine wave is
applied to the input, the current
through the transistor will be a
sine wave and it will produce a
sine wave voltage across the
resistor and the transistor.
• Since the current variation in the
transistor is very large, the out
put voltage is larger than the
smaller input voltage.
• The transistor amplifier is said to
have gain.
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How a Transistor Amplifies (continued)
• A key point to note in the
amplifier is that the transistor
does not actually make the input
voltage bigger. Instead, the
small input controls the larger
current through the device
produced by the external DC
voltage.
• The transistor just generates a
larger separate version of the
input voltage in the output.
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Types of Transistors
• There are two commonly used
types of transistors, metal oxide
semiconductor field effect
transistors (MOSFETs) and
bipolar junction transistors
(BJTs) called bipolars.
• MOSFETs are the most widely
used although BJTs are still
used in selected applications.
• There are sub categories of
each type and you will learn
how each works in a later
course.
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Integrated Circuits
• Integrated circuits (ICs)
are semiconductor devices
that are complete circuits
made up of transistors,
diodes, capacitors,
resistors and inductors.
• The complete circuit is
made on a single piece of
silicon called a chip.
• See Figure.
• Any circuit from a simple
amplifier to a quad core
microprocessor used in a
PC can be made.
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• Most electronic
equipment is made up
of a collection of ICs
and a small selection
of discrete passive
components like
resistors and
capacitors connected
together on a printed
circuit board (PCB).
• Figure shows a typical
board.
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In Summary
• The simplest semiconductor device is a diode made by forming a
junction between a P and N-type semiconductor material.
• The diode will pass current from cathode to anode but not in the other
direction.
• If the anode is made positive and the cathode negative, the diode is
said to be forward biased and current will flow. The reverse condition
is called reverse bias and no current flows.
• A three terminal (element) semiconductor device is called a transistor.
The voltage on one element controls the current between the other two
elements.
• A transistor is used to switch voltages or currents or to amplify small
signals into larger ones.
• MOSFETs and BJTs are the two major types of transistors
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