Unit 7, Chapter 24

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Unit 7, Chapter 24
CPO Science Foundations of Physics
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Unit 7 Electricity and Magnetism
Chapter 24 Electricity and Magnetism

• 24.1 Semiconductors
• 24.2 Circuits with Diodes and Transistors
• 24.3 Digital Electronics

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Chapter 24 Objectives

• Describe how a diode and transistor work in terms
  of current and voltage.
• Explain the difference between a p-type and an
  n-type semiconductor.
• Construct a half-wave rectifier circuit with a
  diode.
• Construct a transistor switch.
• Describe the relationship between inputs and
  outputs of the four basic logic gates.
• Construct an adding circuit with logic gates.

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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

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24.1 Semiconductors

• Key Question
• What are some useful properties of semiconductors?

Students read Section 24.1 AFTER Investigation
24.1
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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.

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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.

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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.

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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
  adjustable gate.

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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.

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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.

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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.

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24.1 Metals as conductors

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

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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.

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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.

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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.

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24.1 Changing conductivity

• Anything that changes the number of free
  electrons has a huge effect on conductivity in a
  semiconductor.
• Adding a phosphorus impurity to 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.

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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.

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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.

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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.

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24.1 The physics of diodes

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

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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.

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24.1 The physics of diodes

• In short, a p-n junction is a diode.
• The p-n junction blocks the flow of current from
  the n side to the p side.
• 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.

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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.

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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.

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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.

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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 nearly every AC adapter you have ever used.

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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.

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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.

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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.

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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.

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24.2 A transistor amplifier

• In an amplifier circuit, the transistor is not
  switched fully on like it is in a switching
  circuit.
• Instead, the transistor operates partially on and
  its resistance varies between a few hundred ohms
  and about 10,000 ohms, depending on the specific
  transistor.

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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.

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24.2 Electronic Logic

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

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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.

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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
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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.

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24.3 Digital Electronics

• A digital signal can only be on or off.
• A digital signal is very different from an analog
  signal.

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24.3 Digital Electronics

• Digital signals can send billions of ones and
  zeros per second, carrying more information than
  analog signals.

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24.3 Digital Electronics

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

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24.3 Digital Electronics
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24.3 Computers

• A computer is an electronic device for processing
  digital information.
• All computers have three key systems
• memory
• central processing unit, or cpu
• input-output system or I/O

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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.

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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.

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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
  computers memory.

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24.3 Digital Electronics

• Key Question
• How do you construct electronic logic circuits?

Students read Section 24.3 BEFORE Investigation
24.3
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Application Electronic Addition of Two Numbers