Physics Chp t 24

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.


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.


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.


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.


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.


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


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


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.


Key Question:

How do you construct electronic logic circuits?

*Students read Section 24.3 BEFORE Investigation 24.3

Application: Electronic Addition of Two Numbers

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Physics Chp t 24