Sr no: 000 class: CBSE + Engg sub: Electronics chap: Materials lesson: 01
We as engineers are interested in atom and some electrons to the extent that they tell us about the materials, their ELECTRICAL properties. There may be many other aspects but we are not concerned.
And by electrical properties we mean how easy of difficult it is to REMOVE an electron from its valence shell and make it FREE, free for doing work.
What are electrical properties of a material: resistance to flow of current. How can one material offer more resistance while the other less?
Answer lies in the electron arrangements.
We know a material is made up of atoms. Atoms make molecules and molecules make material.
What differentiates one material from another is the number of electrons in an atom of the material, how they are arranged, how many of these are in valence shell. You may already be knowing what is atomic number etc. Most simply put it is the number of electrons in one atoms.
The following table shows how electrons are arranged in an atom. There are several shells containing different number of electrons which are rotating. Nucleus has all the positive charge which attracts the electrons and keeps them from running away. Naturally force of attraction is MAXIMUN on electrons shells near the nucleus.
There are shells which are FULL, full to contain certain maximum number of electrons allowed in each. And then there is last or valence shell which may or may not be FULL. But it follows the rule: IT CANNOT CONTAIN MORE THAN 8 electrons. It can have 1,2,3,4,5,6,7 or 8. Can it have 0 electrons?
It is NOT possible to detach electrons from FULL shells. They are held together strongly.
If valence shell has one electron we say, being alone, it is not stable; it is easiest to detach it by some means. WHAT are they?
If there are two, then each has company, they hold on and resist more. Likewise, 7 or 8 offer most resistance to detaching of electrons.
We know unless there are detached electrons (FREE) to carry charge there cannot be any current flow. When we say more resistive material we only mean LESS or NO free electrons available. This is what describes RESISTANCE ( to flow of charge).
We know how to make an electron FREE.
If we are right then good conductors should have minimum number of valence electrons in valence shell. Look at the electron distribution for Gold, silver, copper. If we are right they must have minimum number of electrons in valence shell.
WE FIND ARE RIGHT! They all have only one.
Now to make a molecule ( atom can not exist independently) and a material large number of atoms are arranged (interconnected) in some special manner that attaches valence electrons of atoms of the same matter.
It may take large number of possible arrangements.
Let us say we are interested only in combination arrangements which gives us 8 electrons in the valence shell in the easiest possible manner. For our end use.
We can form groups like :
Group I: 1,2,3 electrons in valence shell – good conductors
Group II: 4 electrons in valence shell
Group III: 5,6,7 electrons in valence shell – bad conductors
8 worst conductor
** Let us concentrate on Gr II with 4 electron only. This means that valence shell has 4 electrons. These are quite strongly bonded to each other, these are not unstable. At the same time 4 electrons are still not so strongly bonded as are 7 or 8 electrons. Thus it is neither very easy and also not very diifcult to detach an electron from such materials. You can now say that such matrials have CONSIDERABLE resistance but using some technique we can REDUCE their resistance,
Imagine there are friends Ram, Sham,Sita, Geeta, ….. each bringing 4 chapaatis for lunch. They sit on a table with their chapaatis with plates placed around them.
We know each can eat max of 8 chapaatis, but actually have brought only four. So, four plates (spaces) are empty with each of them. Suppose teacher places Shyam’s 4 chapatis on the vacant plates of Ram. Still Shyam has his 4 vacant plates. Here teacher places 4 chapaatis of Sita and so on.
Ram who had brought 4 chapaatis from home now when he looks around at his plates finds them all full. He thinks he has 8 chapaatis. That is all valence shell FULL. Similarly, everyone thinks like this. We know what 8 electrons in valence shell means. A worst conductor!
An atom that had 4 valence electrons required much less energy to dislodge an electron, where as now it is an insulator! We are lucky such materials are there. So, we have
Gr I: conductors
Gr III: Bad conductors
Gr II: SEMI conductors. Midway between both.
This is the group that led us to make mobiles, radios, pocket TVs and everything imaginable.
*** WHY DID WE NEED A SEMI CONDUCTING MATERIAL?
We know electric wires are made of copper or aluminum. Computers use gold plated terminals. You buy what you need from the market and use it. Pure copper or Al has certain conductivity. You see companies claiming 99.9 % pure copper cables. You could even make 100% pure cables. Or 50% pure, and so on. But that is all you could do. Pure copper shall have some resistance based on some formula (FIND IT). Once you have connected such a wire there is nothing more that we can do. Only certain amount of maximum current can flow.
You know Ohm’s law.
If we applied a voltage V on this wire a current =V/R shall flow.
If we wanted to control current using same V, we cannot do.
But suppose even after an item has been connected into a circuit and is enclosed inside a box still, we are able to change its resistance by doing something then we have achieved our goal of creating a VARIABLE resistor. A resistor that can be controlled from outside.
We told you about potentiometer whose value we were able to change manually in our lab experiments. But suppose we wanted to change its value 1,00,000 times in one second then we need something more than a simple potentiometer.
You might say we can change resistance of copper wire by adding something to pure copper. Yes, but you can do it only once and that is it. It is like using poorer quality copper.
Bond that connects the atoms of such materials (with 4 valence electrons) is called CO-VALENT bond.
Other materials have different bonds but we are interested in this bond only. Note a relatively un-stable electron configuration of atom (4 vacancies) has become a very stable configuration after bonding into molecules. It is an insulator.
From our knowledge of atom we can easily imagine that such material will be requiring MORE energy input than good conductors (1,2,3 electrons) but much less energy than poor conductors (5,6,7,8) to release an electron to become free and available for work ( carrying charge from one place to the other).
Remember ALL materials will be of no use at 0 deg Kelvin. WHY? Nothing ever happens.
The importance and usage of such semi conducting materials arises from the fact that we can actually DESIGN a material having newer characteristics from the same pure thing. Increasing CONDUCTIVITY is not possible with other materials like copper, gold except that we can at best make it 100% pure. Adding anything only makes a pure thing IMPURE.
Here we “by making a semiconductor material IMPURE” are able to reduce or control its electrical properties. That is what makes it valuable and that is the reason we are able to carry a TV, mobile, Computer in our pocket!
** now look up your book and see how those 4 + 4 chapaatis are actually arranged so that all children think they each have 8 chapaatis to eat.
** you should be able to choose some diagrams from your book that fit in with this lesson.
*** This and many more lessons are available at website: easyfundamentals.in
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