Chapter 24: Electronics - CPO Science

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Unit 7, Chapter 24

CPO Science

Foundations of Physics

Unit 7: Electricity and Magnetism

24.1 Semiconductors

24.2 Circuits with Diodes and Transistors

24.3 Digital Electronics

Chapter 24 Electricity and Magnetism

Chapter 24 Objectives

1.
Describe how a diode and transistor work in terms of
current and voltage.

2.
Explain the difference between a p
-
type and an n
-
type
semiconductor.

3.
Construct a half
-
wave rectifier circuit with a diode.

4.
Construct a transistor switch.

5.
Describe the relationship between inputs and outputs
of the four basic logic gates.

6.
Construct an adding circuit with logic gates.

Chapter 24 Vocabulary Terms

forward bias

reverse bias

bias voltage

p
-
type

n
-
type

depletion region

hole

collector

emitter

base

conductivity

p
-
n
junction

logic gate

rectifier

diode

transistor

amplifier

gain

analog

digital

AND

OR

NAND

NOR

binary

CPU

program

memory

bit

integrated circuit

24.1 Semiconductors

Key Question:

What are some useful
properties of
semiconductors?

AFTER Investigation 24.1

24.1 Diodes

A
diode

is a one
-
way valve for electric current.

Diodes are a basic building block of all electronics
and are used to control the direction of current
flowing in circuits.

24.1 Diodes

When a diode is connected in a circuit so current
flows through it, we say the diode is
forward
biased
.

When the diode is reversed so it blocks the flow of
current, the diode is
reverse biased
.

24.1 Diodes

In a forward
-
biased diode
the current stays at zero
until the voltage reaches
the
bias voltage

(Vb),
which is 0.6 V for
common silicon diodes.

You can think of the bias
voltage as the amount of
energy difference

it takes
to open the diode.

24.1 Transistors

A
transistor

allows you to
control

the current, not just
block it in one direction.

A good analogy for a transistor is a pipe with an

24.1 Transistors

A transistor has
three

terminals.

The main path for
current is between the
collector

and
emitter
.

The
base

controls how
much current flows, just
like the gate controlled
the flow of water in the
pipe.

24.1 Transistors

The current versus voltage
graph for a transistor is
more complicated than for a
simple resistor because
there are three variables.

A transistor is very
sensitive; ten
-
millionths of
an amp makes a big
difference in the resistance
between the collector and
emitter.

24.1 Conductivity and semiconductors

The relative ease at which electric current flows
through a material is known as
conductivity
.

Conductors

(like copper) have very high conductivity.

Insulators

(like rubber) have very low conductivity.

The conductivity of a
semiconductor

depends on its
conditions.

For example, at
low

temperatures and
low

voltages a
semiconductor acts like an insulator.

When the temperature and/or the voltage is increased,
the conductivity increases and the material acts more
like a conductor.

24.1 Metals as conductors

Metals

are good
conductors because a
small percentage of
electrons are free to
separate from atoms
and move
independently.

24.1 Nonmetals as conductors

In an
insulator
, the
electrons are tightly
bonded to atoms and
cannot move.

Since the electrons
cannot move, they
cannot carry current.

24.1 Semiconductors

The electrons in a
semiconductor

are also
bound to atoms, but the
bonds are relatively
weak.

The
density

of free
electrons is what
determines the
conductivity of a
semiconductor.

24.1 Semiconductors

If there are many free electrons to carry
current, the semiconductor acts more like
a
conductor
.

If there are few electrons, the
semiconductor acts like an
insulator
.

Silicon

is the most commonly used
semiconductor.

Atoms of silicon have 14 electrons.

Ten of the electrons are bound tightly
inside the atom.

Four electrons are near the outside of the
atom and only loosely bound.

24.1 Changing conductivity

Anything that changes the
number of free electrons has a
huge

effect on conductivity in a
semiconductor.

silicon
increases
the number of
electrons that can carry current.

Silicon with a phosphorus
impurity makes an
n
-
type
semiconductor
with current of
negative charge
.

24.1 Changing conductivity

When a small amount of boron

is mixed into silicon the
opposite effect happens.

When an electron is taken by a boron atom, the silicon
atom is left with a
positive charge

and current is carried
as electrons move.

Silicon with a boron impurity is a
p
-
type semiconductor
.

24.1 The p
-
n junction

A
p
-
n junction

forms where
p
-
type and
n
-
type
semiconductor materials meet.

The
depletion region

becomes an
insulating barrier

to
the flow of current because electrons and holes have
combined to make neutral silicon atoms.

24.1 The physics of diodes

The
depletion region

of a p
-
n junction is what
gives diodes, transistors, and all other
semiconductors their useful properties.

24.1 The physics of diodes

As the voltage increases, no current can flow
because it is blocked by a larger (insulating)
depletion region.

24.1 The physics of diodes

If the opposite voltage is applied, both electrons and
holes are repelled
toward

the depletion region.

As a result, the depletion region gets smaller.

Once the depletion region is gone, electrons are free to
carry current across the junction and the semiconductor
becomes a conductor.

24.1 The physics of diodes

In short, a p
-
n junction is a diode.

1.
The p
-
n

junction
blocks

the flow of current
from the
n
side to the
p
side.

2.
The p
-
n

junction
allows

current to flow from
the
p
side to the
n
side if the voltage
difference is more than 0.6 volts.

24.1 The physics of transistors

A transistor is made from two
p
-
n
junctions back to back.

An
npn

transistor

has a
p
-
type
layer sandwiched between two
n
-
type layers.

A
pnp

transistor

is the inverse.

An
n
-
type semiconductor is
between two layers of
p
-
type.

24.2 Circuits with diodes and transistors

A diode can convert alternating current electricity to
direct current.

When the AC cycle is positive, the voltage passes
through the diode because the diode is conducting and
has low resistance.

A single diode is called a
halfwave rectifier

since it
converts half the AC cycle to DC.

24.2 Circuits with Diodes and
Transistors

When 4 diodes are arranged in a circuit, the whole
AC cycle can be converted to DC and this is
called a
full
-
wave rectifier.

24.2 AC into DC

A bridge
-
rectifier circuit

uses the entire AC cycle
by inverting the negative
portions.

This version of the full
-
wave rectifier circuit is in
you have ever used.

24.2 A transistor switch

In many electronic circuits a small voltage or current is
used to
switch

a much larger voltage or current.

Transistors work very well for this application because
they behave like switches that can be turned on and off
electronically

instead of using manual or mechanical
action.

24.2 A transistor switch

When the current into the base is zero, a transistor has a
resistance of 100,000 ohms or more.

When a tiny current flows into the base, the resistance
drops to 10 ohms or less.

24.2 A transistor switch

The resistance difference
between “on” and “off” for
a
transistor switch

is
good enough for many
useful circuits such as an
indicator light bulb in a
mechanical circuit.

24.2 A transistor amplifier

One of the most important uses of a transistor is to
amplify

a signal.

In electronics, the word “amplify” means to make the
voltage or current of the input signal
larger

without
changing the shape of the signal.

24.2 A transistor amplifier

In an amplifier circuit,
the transistor is not
switched fully “on” like
it is in a switching
circuit.

operates partially on
and its resistance varies
between a few hundred
ohms, depending on the
specific transistor.

24.2 Electronic Logic

Logic circuits

are designed to compare inputs and
produce specific output when all the input
conditions are met.

Logic circuits assign voltages to the two logical
conditions of
TRUE (T)

and
FALSE (F).

For example, the circuit that starts your car only
works when a) the car is in park, b) the brake is
on, and c) the key is turned.

24.2 Electronic Logic

There is one output which
starts
the car if TRUE and
does not start the car if FALSE.

24.2 A transistor
logic circuit

The only way for the
output to be 3 V is
when all three
transistors are on,
which only happens if
all three inputs are
TRUE
.

24.2 Circuits with Diodes and
Transistors

Key Question:

What are some useful properties of transistors?

*Students read Section 24.2 BEFORE Investigation 24.2

24.3 Digital Electronics

A
signal

is anything that
carries information.

Today the word signal
usually means a voltage,
current, or light wave that
carries information.

A microphone converts the
variations in air pressure
from the sound wave into
variations in voltage in an
analog electrical signal
.

24.3 Digital Electronics

A
digital signal

can only be on or off.

A digital signal is very different from an analog signal.

24.3 Digital Electronics

Digital signals can send billions of ones and zeros
analog signals.

24.3 Digital Electronics

Digital signals are also easier to store, process,
and reproduce than analog signals.

24.3 Digital Electronics

24.3 Computers

A
computer

is an electronic
device for processing digital
information.

All computers have three key
systems:

1.
memory

2.
central processing unit, or
cpu

3.
input
-
output system or I/O

24.3 Computers

Circuits called
logic gates

are the basic building
blocks of computers and almost all digital systems.

The fundamental logic gates are called
AND
,

OR
,

NAND
,

and

NOR
.

24.3 Computers

Logic gates

are built from
many transistors in
integrated circuits
,
commonly known as

chips
.”

As their names imply,
these gates compare two
input voltages and
produce an output voltage
based on the inputs.

24.3 Computers

This logic circuit
compares two four
-
bit
electronic numbers.

The output of this circuit
will be four ones (3V on
each) only if the number
entered by the keyboard
exactly matches the
number in the computer’s
memory.

24.3 Digital Electronics

Key Question:

How do you construct
electronic logic
circuits?

*Students read Section 24.3 BEFORE Investigation 24.3