Unit 7, Chapter 24 CPO Science Foundations of Physics.
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Transcript of Unit 7, Chapter 24 CPO Science Foundations of Physics.
Unit 7, Chapter 24
CPO ScienceFoundations 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?
*Students read Section 24.1 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 adjustable gate.
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.
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.
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 nearly every AC adapter 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.
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.
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 per second, carrying more information than 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. memory2. central processing unit,
or cpu3. 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
Application: Electronic Addition of Two Numbers