When a p-type semiconductor is joined with n-type is called pn junction. The pn junction has great importance because it is in effect the control element for semiconductor devices.
The formation of the pn junction. In practice, it is not that kind of simple but pn junction is fabricated by some special technique. One general method of making pn junction is alloying. In this method, small indium block is placed on the n-type germanium or silicon block as shown in the above figure. After that system is heated at 500°C.
At this temperature germanium and indium melt and form a small puddle as shown in the second figure. Then the temperature is lowered and puddle begins to solidify. After this, under proper conditions, indium impurity will be suitably adjusted in germanium slab and form a single crystal. The addition of the indium overcomes the excess of electrons in the n-type germanium to such an extent that it creates a p-type region.
As the process goes on the remaining material appears as indium button which is frozen on the outer surface of the crystalline structure as shown in the last figure. This button serves as the suitable base for soldering on leads.
Properties Of pn junction:-
After the pn junction formation, the free electrons near the junction in the n region begin to cross the junction and go into the p region which forms an electron-hole pair. The result is that n region loses the free electrons as they diffuse into the junction. which creates layers of positive charges near the junction. As the electrons move across the junction the p region loses holes as electrons and holes are combined. Which results in the formation of negative charges near the p region of the junction. The two layers of positive and negative charges are created called the depletion region.
It may be noted that the thickness of the depletion region is very less. once pn junction is formed.Then depletion layer created. After the formation of the depletion region, free electrons cannot cross the junction. This depletion region generates an electric field and also have a potential difference across the depletion layer which is called barrier potential. The barrier potential of pn junction depends on different factors.
Barrier potential for silicon approximately V0=0.7 V and For germanium V0=0.3 V
Biasing of pn junction:-
When external dc voltage applied to the junction in such a way that current can flow from the pn junction it is called forward basing.
To apply forward bias connect the positive terminal of the battery to the p-type and negative terminal to the n-type as shown in the first figure. The applied forward potential establishes an electric field which acts as the potential barrier. Due to the barrier height is reduce at the junction. Therefore small forward voltage is sufficient to break the potential barrier. Once the potential barrier is break small currents flow In the circuit which also creates low resistance path in entire circuit. Therefore current flows in the circuit called forward current.
Important note for forward bias
1)The potential barrier is reduced at some voltage approximately 0.1 to 0.3 V.
2)The junction offeres low resistance called forward resistance which is responsible for current flow.
3)The magnitude of the current depends upon the applied forward voltage.
When the external dc voltage appied to the junction in such a way that potential barrier is increased it is called reverse biasing.
To apply reverse bias connect negative terminal of the battery to p-type and positive terminal to the n type.After Applying the reverse voltage electric flied is generated in the same direction of the potential barrier.Therefore resultant field is increased which caused the barrier height to increase.These incerese potential barrier prevents the flow of current.
Important point for reverse bias:
1)The potential barrier is increased when forward voltage is applied across the junction.
2)The junction offeres high resistnce which is called the reverse resitance
3) No current is flow in the circuit when reverse voltage is applied.