Sub: Engineering materials Physics/ Electronics class XI/ XII/ Engg lesson 03
Thus far; we found some material whose electrical properties we could control. Main electrical property is : electrical resistance ie resistance to flow of current.
Without such SEMI conducting materials all we could do was to keep on making a piece of copper or aluminum etc purer, nothing else.
Now suddenly we find we can use the new materials like potentiometers, all from outside!
We know what current is; it is number of electrons flowing through the wire. More electrons means larger current, less number of electrons means smaller current. Thus if we could control the number of electrons flowing through the wire ( or an element) we control the current.
We said we measure the current by COUNTING number of electrons through a DOOR in one second. If we open the door fully large number of electrons shall pass. If we could open the door less than less number of electrons. That is all our effort to find a new materials , a semi conductor was to be able to open or close the door or control the opening width of the door. This is what we do with SEMIconductors. This is what makes them useful.
NOW READ OUR LESSON ON AMPLIFIER, how it works. So Easy!!
Read about potentiometer also.
There are TWO materials which have 4 valence electrons: Silicon and Germanium. Check their atomic number position from your book.
We have learnt that we can change electrical properties of some materials like Silicon and Germanium, the two semiconductor materials most useful in Electronics. Since both these materials have 4 valence electrons, but THINK that they have 8, they are basically insulators. WHY?
We learnt that by adding dome impurity material which is having more than 4 valence electrons into the pure semiconductor (SC) we can increase capacity of this material to carry higher current. These impure materials release of FREE electrons (waiters) available for work. Thus now there are many more guests ( and waiters dressed like guests) than the number of chairs. So many more guests are forced to eat standing only.
Now you may ask if we can increase number of free electrons then can we DECREASE number of free electrons? How can we decrease them? Simple.
Put more chairs. Now guests can quickly find vacant chair and sit sown.
How do we do it? Think of adding impurity material having LESS than 4 valence electrons to the pure SC. Say a material having 3 valence electrons. Now the reverse happens.
Tis new atom is like a jhoola in garden where one chair is vacant. Now one child can sit here also.
Free electrons is like a guest who is standing.
This type of space provided by second type of impurity is like a chair. Guest can always sit on such a chair.
All other things remain unchanged.
That at room temperature there will always be some number of electrons that are FREE to be used to carry charge.
NOTE: number of electrons FREE inside a given material remain CONSTANT. You can use one electron for carrying current only if you can supply equal number of electrons from OUTSIDE.
This process of adding materials with 5 valence electrons OR materials with 3 valence electrons to a pure SC is called DOPING.
If we add material with 5 valence then the resulting material having excess of free electrons is called N-TYPE SC.
Similarly, if we add material with 3 valence then the resulting material having less of free electrons is called P-TYPE SC.
**************End of lesson 03***********
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