Working of a DIODE or Crystal Diode.....

In a symbol of the diode, the arrowhead represents the direction of the conventional current Flow.

1. A simple PN junction diode can be created by doping donor impurity to one part of the semiconductor and acceptor impurity to the other part of the semiconductor of silicon or germanium crystal blocks. The middle part of the Sandwich of these two semiconductors makes the PN junction beside which one part is the p-type semiconductor and the other is the n-type semiconductor. 
2. In p-type it is doped with the trivalent or acceptor impurity.
3. In n-type it is doped with the pentavalent or donor impurity.
4. Hence it is a device with two elements Anode formed by p-type and Cathode formed by the n-type.
5. These two terminals are used to make the external connections.

UNBIASED DIODE:

1. In n-side there are a significant number of electrons available and few holes these holes are due to the thermal excitation.
2. In p-side, it will have the huge amount of holes and a few electrons. due to this, a process called diffusion happens in a diode.
3. In the diffusion process, free electrons from n-side will spread into the p-side and recombine with the holes present there.
4. leaving the positive immobile ions in n side and creating the not movable ions on the p side of the crystal diodes 
5. Due to this number of +ve ions and -ve ions will accumulate on the p-side and n-side receptively.
6. The Region formed is known as the depletion region because of the depletion of the free carriers in this region.
7. due to the presence of these +ve and -ve ions, a static electric field is created across the PN junction of the diode and this static electric field is known as barrier potential. 
8. this barrier potential is known as barrier potential because it opposes or blocks the further combination of the holes and electrons across the junction.
   

Forward Biased Diode 

1. In a PN junction diode when the +ve terminal of the battery is connected to the anode terminal of the diode and the negative terminal of the battery is connected the cathode of the diode. the diode is said to be in forward biased state. 
2. We know that in an unbiased state there is barrier potential across the junction. 
3. This barrier potential is directed in the opposite of the forward applied voltage.
4. Due to this diode can only allow current flow in the forward biased state or mode.
5. It only happens when the applied voltage is greater than the barrier voltage.
6. This voltage is known as forward biased voltage.
7. For silicon (Si) diode it is 0.7 and for the Germanium (Ge) diode it is 0.3 volts.
8. when a forward applied voltage is greater than the barrier potential a forward current will flow through the diode and the will behave like a short-circuited.
9. Hence there will be no more voltage drop across the diode after this forward voltage.
10. The forward current will only be limited by the external resistance connected in series with the diode.
11. Thus if the forward applied voltage increases from the 0, the diode will start conducting only when this forward voltage becomes more higher than the barrier potential. 
12. So the time taken by this is input to overcome the forward biased voltage is known as the recovery time of a diode.

Reverse Biased Diode 

1. In Reverse-biased diode the positive terminal of the battery is connected to the cathode terminal of the diode, and -ve terminal of the battery is connected to the Anode terminal of the diode.
2. There will be no flow of electric current through the diode except the reverse saturation current.
3. This is due to in reverse biased state of the diode the depletion layer of the junction becomes wider with the increase in the reverse-biased voltage.
4. There will be a tiny amount of current flowing from n-type to p-type in diode due to minority charge carriers.
5. This small amount of current is known as reverse saturation current. 
6.  Minority charge carriers are mainly thermally generated hole and electrons in p-type and n-type semiconductors.
7. when the reverse applied voltage is continuously increasing with the increase in time after certain limit depletion layer will be destroyed and a huge amount of reverse current will flow through the diode.
8. If the amount of current is not limited externally then it will reach beyond the safe region of the diode and the diode may be destroyed permanently.
9. This will happen because as the magnitude of the reverse voltage increases the kinetic energy of the minority charge carries will also increase.
10. These fast-moving electrons will collide with the other atoms in a diode and knock off more electrons from the device.

due to this the Covalent bonds of an atom are broken and due to this minority charge carriers gets multiplied and this process is known as carrier multiplication and this leads to a considerable increase in the flow of current through the p-n junction and this phenomenon is called Avalanche Breakdown.       

     

V-I Characteristics of PN Junction Diode 

Types of Diodes 

1. P-N Junction Diode
2. Zener Diode 
3. Tunnel Diode 
4.  Schottky Diode 
5. LASER Diode 
6. Varactor Diode 
7. PIN Diode 
8. Photo Diode 
9. Light Emitting Diode (LED)
10. Avalanche Diode