Types of semiconductors

UNIT 1

SEMICONDUCTORS

Figure 1.1. A Semiconductor Wafer.

Contents

Ø What is a Semiconductor?

Ø Types of Semiconductors.

Ø How Semiconductors Work.

Ø Doping of Semiconductors.

 

WHAT IS A SEMICONDUCTOR?

Warm-up

1. “Find someone who…” activity. One student walks around the classroom and asks questions to his fellow students. If the answer is “Yes” he writes down the student’s name on the blackboard.

Ask your fellow students:

Have you ever….? Name of a fellow student

1) ….. worked on the computer?

2) ….. used a calculator?

3) ….. watched plasma TV?

4) ….. kept food in the fridge?

5) ….. talked on the cell phone?

6) ….. heated up the food in the microwave oven?

7) ….. made a presentation on the notebook?

8) …..thought what makes all these electronic devices work?

Listening

2. Listen to the text and find the answer to the question: What makes modern electronic devices work?

We use electronic devices every day, every hour, every minute of our life. We like them, we need them, we use all the advantages they give us, they help us, they save our time, they make us feel comfortable. Modern day electronic devices and many of the functions and features that we take for granted would not exist without these materials. Each electronic device, from a simple toaster that can sense when toast is too brown to the most technologically advanced home theater system, uses these materials. What materials?

These materials are semiconductors. They have had a monumental impact on our society. Semiconductors are used extensively in electronic circuits. You find semiconductors at the heart of microprocessor chips and transistors. Anything that's computerized or uses radio waves depends on semiconductors. Most semiconductors are crystals made of certain materials, most commonly silicon.

The magic word “semiconductor” is composed of two words: “semi” which means not completely and “conductor” which means a material or an object that conducts heat, sound, light, or electricity. Everybody is familiar with "electricity". It is present everywhere; it runs many appliances. In simple terms, the current must pass through wires so that the electricity can reach all these appliances. So a conductor is nothing but a material having ability to conduct this electricity. Semiconductors conduct electricity to some extent, less than the conductors, how much do you think? Well, it depends on the type of a material or its mixture and size. A semiconductor is a material that has intermediate conductivity between a conductor and an insulator.

3. Listen to the text once more and choose among the following expressions the ones you hear:

1) Semiconductors make us feel comfortable.

2) Everybody is familiar with electricity.

3) Electronic devices are used in electronic circuits.

4) “Semi” which means not completely.

5) We use semiconductors every day.

6) Most semiconductors are crystals.

Reading

4. Let's take a closer look at semiconductors. Read the whole text and come up with the title for it.

Read the first part of the text and fill in the gaps with the words and word combinations in the box.

crystal lattice nucleus conductor electrons orbit vibrating ions copper positive ion net motion atom

 

All matter consists of atoms. Each (1)……………has electrons orbiting the nucleus. The (2)………………..contains the same amount of positive charge as the negative charge possessed by the orbiting electrons. The ability of any material to conduct electricity depends primarily on the behaviour of the (3)……………..in the outer orbits. Therefore, it is necessary to review briefly some aspects of solid-state physics.

In a metallic conductor such as (4)……………., the atoms are arranged in a regular array called a crystal lattice. The electrons in the outer orbits of each metal atom are only loosely bound to the nucleus. These electrons are not closely associated with any particular atom and are free to move through the (5)……………. Once an electron has left its (6)…………….round a particular atom, that atom is left with an excess positive charge. The electron-deficient atom is called a (7)……………. The electron that is now free to move is called a free electron. The free electrons in a conductor can be visualized as a cloud of electrons surrounding fixed positive ions.

At normal temperatures, the ions possess energy and vibrate. Collisions between (8)………………and free electrons cause the electrons to move in a random manner. Over a long period of time, the net motion of these free electrons is zero.

If an electric field is applied to the (9)………………, the free electrons will acquire additional energy and will tend to move in the direction dictated by the field. There will be a resulting net motion of free electrons. The (10)..................of charge carriers constitutes an electric current.

5. Read the second part of the text and put the rearranged words in the correct order.

In an insulator, (1) bound very electrons nearly tightly are all to their respective atoms. There are practically no electrons that are able to move under the influence of an applied electric field. Therefore, (2) current conduct an cannot any insulator electric under normal conditions.

As its name implies, a semiconductor (3) current is a that material conducts, but only partly. The conductivity of a semiconductor is somewhere between that of an insulator, which has almost no conductivity, and a conductor, (4) conductivity almost which full has. The conductivity of a semiconductor increases with increasing temperature, (5) of opposite metal that to behaviour a. Semiconductors can display a range of useful properties such as passing current more easily in one direction than the other.

Speaking

6. Imagine that you have an interview with a semiconductor engineer what questions concerning the topic you would ask him. Work in pairs, make dialogues.

TYPES OF SEMICONDUCTORS

Start here

1. Write the abbreviations from the box next to the words and phrases. Which word or phrase is not about semiconductors? Underline it.

Si InP Cu SiGe GaAs Ge GaP InAs SiC GaN InSb C

 

1) indium phosphide

2) silicon carbide

3) copper

4) gallium nitride

5) indium arsenide

6) gallium phosphide

7) silicon

8) indium antimonide

9) silicon germanium

10) carbon

11) gallium arsenide

12) germanium

Listening

2. Listen to this text and name the types of semiconductors.

Semiconductors are mainly classified into two categories: intrinsic and extrinsic. An intrinsic semiconductor material is chemically very pure and possesses poor conductivity. It has equal numbers of negative carriers (electrons) and positive carriers (holes). Holes and electrons are responsible for current in semiconductor materials. A hole is the absence of an electron in a particular place in an atom. These carriers are produced as a result of thermal agitation of the atoms, even at room temperature. Some bound electrons can acquire sufficient energy to escape from their atoms, becoming free electrons and leaving holes behind. This process of producing hole-electron pairs is called thermal generation.

Whereas an extrinsic semiconductor is an improved intrinsic semiconductor with a small amount of impurities added by a process, known as doping which alters the electrical properties of the semiconductor and improves its conductivity. Introducing impurities into the semiconductor materials (doping process) can control their conductivity. Doping process produces two groups of semiconductors: the negative charge conductor (n-type) and the positive charge conductor (p-type).

Semiconductors are available as either elements or compounds. Silicon and Germanium are the most common elemental semiconductors. Si and Ge both have a crystalline structure called the diamond lattice. That is, each atom has its four nearest neighbors at the corners of a regular tetrahedron with the atom itself being at the center.

In addition to the pure element semiconductors, many alloys and compounds are semiconductors. Compound semiconductors include InSb, InAs, GaP, GaSb, GaAs, SiC, GaN. The advantage of compound semiconductor is that they provide the device engineer with a wide range of energy gaps and mobilities, so that materials are available with properties that meet specific requirements. Some of these semiconductors are therefore called wide band gap semiconductors.

3. Listen again and mark the statements below (T) true or (F) false.

1) There are intrinsic and extrinsic semiconductors.

2) Intrinsic semiconductors have good conductivity.

3) A hole is a positive charge.

4) Doped semiconductors are poor conductors.

5) The process of producing hole-electron pairs is called doping.

6) Introducing impurities into the semiconductor materials is called thermal generation.

7) Semiconductors are available as elements and compounds.

Vocabulary

4. Find the antonyms or opposite meanings of such words:

1) elements

2) intrinsic

3) negative carriers

4) n-type semiconductor

5) impure

Grammar

5. “To be, or not to be: that is the question…”

The Greek sea god Proteus was, like the sea, capable of changing form in an instant. Proteus knew all things – past, present, and future – but disliked telling what he knew. In order to get any decent information out of him, you had to grab him and hold on tight while he went through his various forms – lion, wild bear, snake, tree, running stream – it wasn't easy.

The verb “To be” is the most “protean” of the English language, constantly changing form, sometimes without a clear pattern. Considering that we use it so often, it is really too bad that the verb “To be” is the most irregular, slippery verb in the language.

Here are some forms of the verb “To be” in the sentences from the text you have listened to about the types of semiconductors.

An intrinsic semiconductor material is chemically very pure.

Whereas an extrinsic semiconductor is an improved intrinsic semiconductor.

Holes and electrons are responsible for current in semiconductor materials.

These carriers are produced as a result of thermal agitation of the atoms, even at room temperature.

Semiconductors are available as either elements or compounds.

 

The verb “To be” is used in the following patterns:

1) with a noun:

Silicon is a semiconductor.

2) with an adjective:

The electrical conductivity of intrinsic semiconductors is poor.

3) with the - ed form to make the Passive Voice:

This process of producing hole-electron pairs is called thermal generation.

Semiconductors were discovered in 1874.

4) There is / There are:

There is a semiconductor material in each modern electronic device.

There are intrinsic and extrinsic semiconductors.

5) This is / These are and That is / Those are:

This is a p-type semiconductor.

These are doped semiconductors.

That is germanium.

Those are compound semiconductors.

Basic Tense Forms of the verb “To Be”
	Present	Past	Future
Simple	I	Am	was	will be
he/she/it	Is
you/we/they	Are	were
Perfect	I/you/we/they	have been	had been	will have been
he/she/it	has been

 

6. Let’s take a closer look at the Present tense forms of the verb “To be”. The following tables include the affirmative or positive form (+), the negative form (–) and the interrogative or question form (?). Complete the tables.

Affirmative (+)
The Verb “To be”	Contracted	Example
I am	I’m	I am a teacher.
You …	You’re	You are a student.
He …	He’s
She is	She…
It …	It’s	It … a semiconductor.
We …	We’re …
You are	You …	You … students.
They …	They …

 

Negative (–)
The Verb “To be”	Contracted	Example
I am not	I’m not	I am not a student.
You are not	You’re not	You aren’t	You … not a teacher.
He …	He’s not	He isn’t
She is not	She’s not	She …
It is not	It …	It isn’t	It is not a semiconductor.
We …	We’re not	We aren’t
You are not	You…	You aren’t	You are not teachers.
They …	They’re not	They …

 

Interrogative (?)
The Verb “To be”	Short answer (+)	Short answer (–)
Am I a teacher?	Yes, I am.	No, I am not.	No, I’m not.
Are you a student?	Yes, you are.	No, you … not.	No, you’re not.	No, you aren’t.
Is he …?	Yes, he …	No, he …	No, he’s …	No, he isn’t
… she …?
… it …?
Are we...?	Yes, we are.	No, we …	No, we’re not	No, we …
…you…?
…they…?

Speaking

7. Work in pairs. Discuss the difference between the types of semiconductors, agreeing or disagreeing, rather than just making short statements. Use the correct forms of the verb “to be”.