In the previous section we completed a discussion on colours of light. In this chapter we will see some basics about electronics.
♦ Conductors readily allow the passage of electric current through them
♦ Insulators do not allow the passage of electric current through them
• There are certain substances which fall in between conductors and insulators
• Silicon is such a substance. It is a common substance found in sand and quartz.
Let us learn about it in a bit more detail. We will write the steps:
1. Silicon is an element in the periodic table
• It is below carbon and above germanium
2. Carbon, silicon and germanium has a common property:
• They have 4 electrons in the outermost shell
• So each of their atoms need four more electrons to attain octet.
3. Consider any one silicon atom. It already has 4 atoms in the outermost shell. It is shown in fig.14.1(a) below:
• The 'extra 4 electrons required' can be obtained by 'sharing' electrons with other silicon atoms
4. When this 'sharing' takes place, strong bonds are formed between individual atoms.
• These bonds are called covalent bonds. We have seen it in our chemistry classes. Details here.
5. So the single silicon atom in fig.14.1(a) will obtain four new silicon neighbours. This is shown in fig.14.1(b) above.
• The structure in fig.b can be shown in a simplified form as in fig.c
6. But each of the four new neighbours are in need of 3 electrons to attain octet.
• So they enter into bonds with other silicon atoms.
• This pattern continues, and we get a large number of silicon atoms bonded together to form a 'crystal lattice'. This is shown in fig.14.2 below:
• A crystal lattice is a three dimensional symmetric arrangement of atoms.
• The same process takes place in carbon also. In chemistry classes, we have seen that the crystalline form of carbon is diamond. Details here.
• Here, we are discussing about silicon. It is a semiconductor. Why do we call it a semi conductor? Let us analyse. We will write the steps:
1. Metals are good conductors of electric current. This is because, metals have free electrons
2. A silicon crystal is shown below:
• It looks metallic. But it is not.
• This is because, all the 'four outer electrons' of the atoms take part in bonding. There are no free electrons
3. We can change this behaviour of silicon by adding some impurities to it. The commonly used impurities are phosphorus, arsenic, boron, gallium etc.,
4. Let us see what happens when phosphorus is added:
(i) The P atoms takes up position among the Si atoms. This is shown in fig.14.3 below:
(ii) But any P atom has 5 outer electrons. We need only 4. So one electron becomes free.
• The free electrons thus produced are indicated by the independent red dots in the fig.14.3 above
(iii) The newly obtained free electrons can move around. So the silicon crystal can now conduct electricity.
(iv) But we have to distinguish between this 'modified silicon' from the 'original silicon'.
• For that, the modified silicon is given a special name: n-type silicon
or, in general: n-type semiconductor
• 'n' stands for 'negative'. The conductivity is achieved by electrons, which are 'negatively charged particles'. Hence the name.
5. Let us connect this semiconductor in a circuit and see what happens. It is shown in fig.14.4 below:
(i) Actually, there is nothing special. It is just like connecting an ordinary metallic conductor. The bulb will glow.
• We can see that free electrons are available just as in the case of a metallic conductor
(ii) But the conductivity will be less than a metallic conductor
• If instead of phosphorus, we use arsenic as the impurity, we will get the same result.
• This is because, like phosphorus, arsenic also has 5 outer electrons
■ So we can write:
N-type semiconductor is made by using phosphorus or arsenic as the impurity
6. Let us see what happens when boron is used as the impurity:
(i) The B atoms takes up position among the Si atoms. This is shown in fig.14.5 below:
(ii) But any B atom has only 3 outer electrons. We need 4. So we are one electron short.
• The 'shortage of electrons' thus produced are indicated by the 'holes' in the fig.14.5 above
7. Let us connect this semiconductor in a circuit and see what happens. It is shown in fig.14.6 below:
(i) When the switch is turned on, the electrons, although bonded between atoms, will experience a pull towards the positive terminal of the cell
(ii) So, in the above fig.14.6, electrons will break bonds and move towards the left. Because they want to reach the positive terminal of the cell
(iii) When they move to the left, they fall into the holes. The holes are filled up.
• But the position previously occupied by the electrons now become holes
(iv) This process continues and we find that:
• Electrons move towards the left
• Holes move towards the right
■ This is shown in the animation below:
In general:
• Electrons move towards the positive terminal
• Holes move towards the negative terminal
■ We can say that, the electrons and holes are moving towards each other. Because:
♦ Electrons on the right side are moving towards the left and
♦ Holes on the left side are moving towards the right
• The crystal will conduct electricity only if the electrons and holes move towards each other in this way
• If they move away from each other, no current will pass
(v) Thus, even though there are no free electrons, we get a flow of electrons. The bulb glows.
(vi) Here also, the conductivity will be less than a metallic conductor
8. So the silicon crystal can now conduct electricity.
• But we have to distinguish between this 'modified silicon' from the 'original silicon' and also from the N-type.
• For that, this modified silicon is given a special name: P-type silicon
or, in general: P-type semiconductor
• 'P' stands for 'positive'. The conductivity is achieved by 'holes'
♦ A 'hole' indicate a deficiency of electron
♦ That is., a deficiency of negative charge
♦ 'Deficiency of negative' is equivalent to 'gain in positive'.
• Hence the name.
• If instead of boron, we use gallium as the impurity, we will get the same result.
• This is because, like boron, gallium also has 3 outer electrons
■ So we can write:
P-type semiconductor is made by using boron or gallium as the impurity
■ The semiconductors are obtained by the addition of impurities to silicon. The process of adding impurities to silicon to alter it's conductivity is called doping.
Semiconductors
• Substances can be classified as conductors and insulators.♦ Conductors readily allow the passage of electric current through them
♦ Insulators do not allow the passage of electric current through them
• There are certain substances which fall in between conductors and insulators
• Silicon is such a substance. It is a common substance found in sand and quartz.
Let us learn about it in a bit more detail. We will write the steps:
1. Silicon is an element in the periodic table
• It is below carbon and above germanium
2. Carbon, silicon and germanium has a common property:
• They have 4 electrons in the outermost shell
• So each of their atoms need four more electrons to attain octet.
3. Consider any one silicon atom. It already has 4 atoms in the outermost shell. It is shown in fig.14.1(a) below:
 |
| Fig.14.1 |
4. When this 'sharing' takes place, strong bonds are formed between individual atoms.
• These bonds are called covalent bonds. We have seen it in our chemistry classes. Details here.
5. So the single silicon atom in fig.14.1(a) will obtain four new silicon neighbours. This is shown in fig.14.1(b) above.
• The structure in fig.b can be shown in a simplified form as in fig.c
6. But each of the four new neighbours are in need of 3 electrons to attain octet.
• So they enter into bonds with other silicon atoms.
• This pattern continues, and we get a large number of silicon atoms bonded together to form a 'crystal lattice'. This is shown in fig.14.2 below:
 |
| Fig.14.2 |
• The same process takes place in carbon also. In chemistry classes, we have seen that the crystalline form of carbon is diamond. Details here.
• Here, we are discussing about silicon. It is a semiconductor. Why do we call it a semi conductor? Let us analyse. We will write the steps:
1. Metals are good conductors of electric current. This is because, metals have free electrons
2. A silicon crystal is shown below:
 |
| Source: Wikimedia commons |
• It looks metallic. But it is not.
• This is because, all the 'four outer electrons' of the atoms take part in bonding. There are no free electrons
3. We can change this behaviour of silicon by adding some impurities to it. The commonly used impurities are phosphorus, arsenic, boron, gallium etc.,
4. Let us see what happens when phosphorus is added:
(i) The P atoms takes up position among the Si atoms. This is shown in fig.14.3 below:
 |
| Fig.14.3 |
• The free electrons thus produced are indicated by the independent red dots in the fig.14.3 above
(iii) The newly obtained free electrons can move around. So the silicon crystal can now conduct electricity.
(iv) But we have to distinguish between this 'modified silicon' from the 'original silicon'.
• For that, the modified silicon is given a special name: n-type silicon
or, in general: n-type semiconductor
• 'n' stands for 'negative'. The conductivity is achieved by electrons, which are 'negatively charged particles'. Hence the name.
5. Let us connect this semiconductor in a circuit and see what happens. It is shown in fig.14.4 below:
 |
| Fig.14.4 |
• We can see that free electrons are available just as in the case of a metallic conductor
(ii) But the conductivity will be less than a metallic conductor
• If instead of phosphorus, we use arsenic as the impurity, we will get the same result.
• This is because, like phosphorus, arsenic also has 5 outer electrons
■ So we can write:
N-type semiconductor is made by using phosphorus or arsenic as the impurity
6. Let us see what happens when boron is used as the impurity:
(i) The B atoms takes up position among the Si atoms. This is shown in fig.14.5 below:
 |
| Fig.14.5 |
• The 'shortage of electrons' thus produced are indicated by the 'holes' in the fig.14.5 above
7. Let us connect this semiconductor in a circuit and see what happens. It is shown in fig.14.6 below:
 |
| Fig.14.6 |
(ii) So, in the above fig.14.6, electrons will break bonds and move towards the left. Because they want to reach the positive terminal of the cell
(iii) When they move to the left, they fall into the holes. The holes are filled up.
• But the position previously occupied by the electrons now become holes
(iv) This process continues and we find that:
• Electrons move towards the left
• Holes move towards the right
■ This is shown in the animation below:
• Electrons move towards the positive terminal
• Holes move towards the negative terminal
■ We can say that, the electrons and holes are moving towards each other. Because:
♦ Electrons on the right side are moving towards the left and
♦ Holes on the left side are moving towards the right
• The crystal will conduct electricity only if the electrons and holes move towards each other in this way
• If they move away from each other, no current will pass
(v) Thus, even though there are no free electrons, we get a flow of electrons. The bulb glows.
(vi) Here also, the conductivity will be less than a metallic conductor
8. So the silicon crystal can now conduct electricity.
• But we have to distinguish between this 'modified silicon' from the 'original silicon' and also from the N-type.
• For that, this modified silicon is given a special name: P-type silicon
or, in general: P-type semiconductor
• 'P' stands for 'positive'. The conductivity is achieved by 'holes'
♦ A 'hole' indicate a deficiency of electron
♦ That is., a deficiency of negative charge
♦ 'Deficiency of negative' is equivalent to 'gain in positive'.
• Hence the name.
• If instead of boron, we use gallium as the impurity, we will get the same result.
• This is because, like boron, gallium also has 3 outer electrons
■ So we can write:
P-type semiconductor is made by using boron or gallium as the impurity
■ The semiconductors are obtained by the addition of impurities to silicon. The process of adding impurities to silicon to alter it's conductivity is called doping.
The invention of semiconductors led to the development of many important components in electronics like diodes, transistors etc., In the next section, we will see some of those components.
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