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DIODES

Diodes are two-terminal semiconductor device that allows current to flow only in one direction.
Because of the depletion layer created in between the PN-junction of the diode a voltage greater than the potential has to be applied between the 2-terminal of the diode to get it to the depletion zone.
This voltage is 0,7v for a Silicon type diode and 0.3v for a germanium type diode. This however means  that a diode will not conduct immediately in a circuit when a voltage is first apply to it.

It will only conduct when the potential voltage of or greater than 0.7v is supply to it.

 Forward Biased
A diode in the forward biased mode allows current to flow freely in a circuit




Reversed  Biased 
A diode in the reversed biased mode does not allow current flow, it blocks the flow of current in a circuit because the voltage applied to the diode is of opposite polarity. However, all diodes have a characteristics known as the breakdown voltage.

Also, when diode is connected in the reverse biased mode, majority carriers migrate towards the opposite side of the power supply (positive and negative side terminal) leaving behind a large depletion layer.
Notice also the migration of the minority charge carrier to the depletion layer.

 The breakdown voltage of the diode is the minimum reverse voltage that makes the diode conduct appreciably in reverse.When this reverse voltage is greater and have exceeded the actual/normal breakdown voltage of the diode as specified on the diode datasheet, the diode will breakdown and current will flow through the diode in the reverse direction.
Breakdown occurs the reverse voltage as increased beyond limit which enables the minority carrier overcome the potential barrier created by the depletion region of the diode.
Reverse voltage of diodes varies depending on the type and size.
At this point the diode act like a wire
NOTE: Breakdown of diode only occurs when applied voltage is greater than the rated reverse voltage of the diode as specified on the diode datasheet. 


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