Introduction to PN junction diode

As we know metals are classified into 3 types they are : conductors,insulators and semiconductors. conductors allow the flow of electrons, where as the insulators doesn't allow the flow.
Reason is that in conductors the distance between conduction band and valence band is small where as in insulators it is high .when we look at semiconductors  which  allow partially the flow of electrons. These semiconductors are necessary in these days as they play an important role in modern day electronic goods.
Examples of semiconductors are silicon and germanium which have four valence electrons in their outermost shell.But we mostly use silicon because it has low leakage current(around nA) due to high forbidden gap between conduction band and valence band,and silicon has high temperature withstand capacity(around 1425c) compared to germanium .
construction:
Take a silicon piece and to the left most dope with p-type impurities and to the right, dope with n-type impurities.
Doping means adding of impurities(other metals) to the present one.
Ex:silicon with boran,aluminium or with phosphorus .By proper doping we can design the diode.
working:

when doping is done, a small junction is formed between the P and N regions before biasing .Reason is P-type has majority holes and minority electrons where as the N-type has majority electrons and minority holes due to force of attraction and repulsion small layer is formed and is called as "depletion layer".

=>when the potential at anode is greater than cathode then we can say diode is in "forward bias" and act like as closed switch with threshold voltage of 0.6V

Reason is that when positive potential(greater) is applied at anode due to force of repulsion between positive charges and holes(positive charges) the depletion layer contracts and allow the electrons to recombine with holes.

=>If we apply lower potential at anode compared to cathode then the diode acts as a "Reverse bias" and is  in open state that is it offers infinite resistance.
  Reason is that lesser voltage applied at anode will attract the holes(positive charges) towards negative charge and in turn increases the depletion layer so conduction stops.
=>Applications of a diode is it can act as a open or closed switch.

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wave propagation

Normally in the conductor free electrons are present due to which we have electric fields and if these are moving we also have the magnetic fields around the conductor. These electric and magnetic fields are responsible for the wave to propagate.normally we observe it in the case of antenna propagation,when antenna is fed by the current source due to charge and as this charge is flowing we have electric and magnetic fields which help in wave propagation.we have many types of antenna's in the past like yagi-uda antenna which has a dipole for which the current is fed,in this antenna another two parasitic elements are present called directors and reflectors. reflector is used to reflect back the back side radiation(opposite to the wave propagation) directors are used to place the radiation in the desired direction.we also have reflector antennas,horn antenna,microstrip antenna(mostly used in mobile phones),helical antenna and many more designing up.But the principle remains same.
The antenna height is determined by the wave frequency.if the wave is of smaller frequency it requires very long antenna(since wavelength is inversly proportional to frequency) and if the wave has higher frequency we can easily transmit,but due to tech development we developed some modulation techniques to transmit the wave having lower frequency.
Suppose assume that wave is propagating along x-axis,then we have the electric and magnetic fields in perpendicular direction to x-axis that is it may be along y or z-axes.In the direction of propagation we dont have any types of fields.further if electric field is along is y-axis,it will be independent of 'y' but is a function of 'x'.similarly if magnetic field is along z-axis,it will be independent of 'z' but will be function of 'x'.
 Based on the maxwell's equations we can decide the nature of wave in the charge ,free and conducting region.The nature of the EM waves vary from medium to medium and also upon the angle of incidence whether it is oblique or normal incidence.

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End of world

Now a days we may hearing the word about how the world is going to be destroyed. There are so many ways but i think these are only few that we should fear.
1.water :our planet earth consists of 75 percent of water and 25 percent of land . So we should think the major one is going to affect the minor in which we are leading the life. Among this most of the impure water is present in oceans and seas and the minimum quantity of pure water is present as ice caps. The land near to these would think that they are in danger because the water is melting due to this the land will get destroy. So that every individual should take part in controlling the global warming.
2.Earthquakes:now a days we are watching in our tv's that so many people are dying due to earthquakes . These days it is common the earthquake is occured at this place and at another place . We have device to estimate the amount of earthquake occured but we dont have the machines to discover it first. Animals are able to discover but man could not. So now there is a need to discover the machine and to minimise the destroy.
3.valcones:Due to this the region around is being damaged. Normally the region is forest . If forest gets destroy the equilibrium will be damaged so that at last man suffers. So we should careful in controlling this.
4.brimstone and fire from heaven:The fire coming from the heaven is the major possibility the our living planet will get destroy. This brimstone normally contains 98 percent of sulphur . The amount of damage we can see in the holy bible which is happened in the city called sodom. So the man should learn from this.
>conclusion:If we want to survive in the we should be in control of this four ways and we should search a way in protecting the beautiful planet earth.

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