Electricity: 4 - Semiconductors and P-N junctions.

Question 1

What are the three groups that elements can be classified as due to there electrical properties?

Answer 1

Conductors,semiconductors and insulators.

Question 2

What is the description of a conductor material?

Answer 2

Materials with many free electrons. These electrons can flow easily. They have a really low resistance. (Metals, graphite).

Question 3

What is the description of a insulator material?

Answer 3

Materials with very few electrons and have a very high resistance. (Plastic, wood, rubber and glass).

Question 4

What is the description of a semiconductor material?

Answer 4

Materials which lie somewhere between conductors and insulators in terms of their conductivity. They are insulators when in their purest form (behave like insulators when pure), but will conduct when an impurity is added. (Silicon, germanium).

Question 5

Where are electrons in atoms contained?

Answer 5

In energy levels.

Question 6

When atoms come together to form solids what happens to the electrons?

Answer 6

The electrons become contained in energy bands separated by energy gaps.

Question 7

How can electrons reach higher energy levels?

Answer 7

By gaining more energy.

Question 8

In terms of electrical conductivity what two energy bands are of particular importance?

Answer 8

The valence band and the conductive band.

Question 9

What is the valence band?

Answer 9

The valence band is usually the highest band that electrons will normally occupy at room temperature.

Question 10

What is the conduction band?

Answer 10

The conduction band is the highest occupied energy band above the valence band.

Question 11

What can the conduction band do?

Answer 11

The conduction band can accept electrons from the valence band under the right conditions.

Question 12

What are the lector’s in the conduction band allowed to do?

Answer 12

The electrons in the conduction band are free to move.

Question 13

How are metals conductive with reference to conduction bands?

Answer 13

In conductors, the valence band and the conduction band overlap, which allows the valence electrons to move freely through the material.

Question 14

How are some metals highly conductive?

Answer 14

Some metal are highly conductive as they have free electrons and are partially filled, therefore they are highly conductive.

Question 15

How full are the conduction bands in conductors/metals?

Answer 15

One or more of the bands are partially filled.

Question 16

In insulators how is the valence band filled?

Answer 16

In an insulator the highest occupied band/valence band is full.

Question 17

In insulators what is the first infilled band above the valence band?

Answer 17

The conduction band.

Question 18

For an insulator what is the energy gap between the valence band and the conduction band like and how does this contribute to the conduction of insulators?

Answer 18

For an insulator, the energy gap between the valence band and the conduction band is large and at room temperature there is not enough energy available to move electrons from the valence band into the conduction band where they would be able to contribute to conduction. For this reason insulators do not normally conduct.

Question 19

Can insulators be made to conduct?

Answer 19

If the temperature is high enough or the supplied voltage is sufficiently large, some electrons can be lifted to the conduction band to allow current to pass, but this will often damage the material.

Question 20

What is the energy gap between the conduction and valence band like in a semiconductor and what does this mean for the conduction properties of a semiconductor?

Answer 20

In semiconductors the energy gap between the valence band and the conduction band is relatively small (a lot smaller than that of an insulator). At room temperature there is sufficient energy to move electrons from the valence band to the conduction band, allowing some current to pass and some conduction to take place.

Question 21

What does an increase in temperature do to a semiconductor?

Answer 21

An increase in the temperature of a conductor will increase the conductivity of a semiconductor.

Question 22

What can reduce the resistance and increase the conductivity of a semiconductor?

Answer 22

A process called doping.

Question 23

What valency do materials that are used as semiconductors have?

Answer 23

A valency of 4

Question 24

What are some materials that are used as semiconductors?

Answer 24

Silicon or germanium.

Question 25

How many outer electrons do elements that have a valency of 4 have?

Answer 25

4 outer electrons.

Question 26

As semiconductor materials have a valency of 4, what does this mean for the structure and conductivity of compounds formed by just these elements, and how can this resistance be reduced?

Answer 26

They bond to form a crystal lattice structure where each atom is bonded to four other atoms of the same type. This means that they are very few free electrons available to conduct. The resistance of these structures are really high. Increasing the temperature of the semiconductor will result in more free electrons, thus reducing the resistance.

Question 27

During manufacture what may be used to increase their conductivity and what does this result in?

Answer 27

During manufacture semiconductors may be doped with specific impurities to increase their conductivity, resulting in two types of semiconductors: p-type and n-type.

Question 28

What is the process of n-type doping and how does it work?

Answer 28

An impurity with five outer electrons such as arsenic is placed in the semiconductor with 4 outer electrons (silicon, germanium). Four of the electrons of the arsenic atoms will bond covalently with the silicon/germanium, leaving the fifth outer electron of the arsenic free to move and conduct.

Question 29

What happens to the conductivity and resistance of a semiconductor after n-type doping?

Answer 29

The conductivity of the crystal lattice structure has increased with the addition of an impurity, therefore decreasing the resistance of the semiconductor.

Question 30

Why does n-type doping get its name?

Answer 30

As the majority of charge carriers are negative.

Question 31

What is the process of p-type doping and how does it work?

Answer 31

An impurity with 3 outer electrons like boron replaces one of the silicon/germanium atoms which have 4 outer electrons. The 3 outer electrons of the boron will bond covalently to 3 outer electrons of the silicon/germanium, producing a positive ‘hole’ where there is an absence of an electron. An electron will move to fill this hole, and conduction can take place through the movement of positive holes.

Question 32

What happens to the conductivity and resistance of a semiconductor after p-type doping?

Answer 32

The conductivity of the crystal lattice structure has increased with the addition of an impurity, therefore decreasing the resistance of the semiconductor.

Question 33

Why does n-type doping get its name?

Answer 33

Ask the majority of charge carriers are positive.

Question 34

In n-type or p-type doping how many impurities are placed in lattice structure?

Answer 34

One in every million or so silicon/geranium atoms are replaced with an impurity.

Question 35

When a voltage is connected across n-type doping what happens?

Answer 35

The germanium ‘free’ electrons flow towards the positive end of the supply producing an electric current.

Question 36

When an electron moves its position in the crystal lattice what happens and what does this produce?

Answer 36

It leaves a space behind it that is positively charged. This absence of an electron is called a positive hole.

Question 37

What happens when positive holes are created in a semiconductor?

Answer 37

An electron will move to fill any positive holes in the crystal structure and in turn leave a positive hole.

Question 38

What is the overall charge of p-type and n-type semiconductors and why is this?

Answer 38

Neutral as for every electron with a negative charge added in n-type there is a corresponding proton with a positive charge. Similarly in p-type for every electron taken away so too its corresponding proton is also taken away.

Question 39

What is a diode?

Answer 39

A semiconductor which is grown with two half’s where one half is made of p-type and the other n-type, the product is called a p-n junction and is the basis of diodes.

Question 40

What does the the circuit symbol for a diode look like and what is the positive and the negative side.

Answer 40

A horizontal line with a triangle, then a vertical line at the end of the triangle and another horizontal line. The positive end is the vertical line of the triangle and the negative side is the vertical line at the end of the triangle.

Question 41

What is produced in the middle of the p-n junction?

Answer 41

A depletion layer.

Question 42

How is the depletion layer created?

Answer 42

Very close to the junction of the two materials, the excess holes from the p-type material combine with the excess electrons from the n-type material. The region where the charges combine is called the depletion layer.

Question 43

What does the depletion layer create and how does this affect the depletion layer?

Answer 43

This creates a region of positive charge on the side of the n-type material and a region of negative charge on the side of the p-type material, resulting in an area of no charge carriers. Since like charge repel it is now difficult for charges to move across the depletion area.

Question 44

What is said to have happened to the diode if an external voltage is applied to the diode?

Answer 44

We say that the diode is biased.

Question 45

What are the 2 ways that a diode can be biased?

Answer 45

Reverse and forwards biased.

Question 46

What is the process and how does reverse biased work?

Answer 46

To reverse bias the diode, a positive voltage is applied to the n-type side of the material and a negative voltage is applied to the p-type side of the material. This has the effect of increasing the size of the depletion layer as the electrons and holes are repelled from the junction. Very few electrons have enough energy to reach the conduction band so very little current can pass.

Question 47

What is the process and how does forward biased work?

Answer 47

To forward bias the diode, a negative voltage is applied to the n-type side of the material and a positive voltage is applied to the p-type side of the material. This has the effect of narrowing the depletion layer and allowing the p-n junction to conduct. Electrons have enough energy to reach the conduction band, allowing current to be transferred.

Question 48

What is the depletion layer in a p-n junction sometimes referred to as?

Answer 48

An electric field.

Question 49

How doe LEDs work?

Answer 49

When a p-n junction is forward biased, the potential difference across the junction causes electrons to move from the conduction band of the n-type semiconductor towards the conduction band of the p- type semiconductor (the holes combine with the electrons. When electrons gain enough energy to reach the conduction band electrons with less energy in the conduction band fall back down into the valence band and the electrons with greater energy reach the conduction band. When the electrons fall down into rather valence band their energy is released in the form of a photon which produce the light in an LED.

Question 50

What are solar cells and what is this known as?

Answer 50

Solar cells are p-n junctions designed so that a potential difference is produced when photons are absorbed. This is known as the photovoltaic effect.

Question 51

What are photodiodes?

Answer 51

A photodiode is a very thin sandwhich with a layer of p-type material at the top, a junction in the middle and an n-type material at the bottom.

Question 52

What is the basis for solar cells?

Answer 52

When the photodiode is operating in photovoltaic mode.

Question 53

When is the photodiode said to be operating in photovoltaic mode?

Answer 53

When the photodiode uses light to produce energy, it is said to be operating in photovoltaic mode.

Question 54

How do photodiodes work in photovoltaic mode/solar cells?

Answer 54

Light in the form of photons pass through the p-layer and is absorbed at the junction. The absorption of photons provide enough energy to ‘raise’ electrons from the valence band to the conduction band. The p-n junction causes the electrons in the conduction band to travel towards the n-type semiconductor and a potential difference is produced across the solar cell.