The V-I characteristics or voltage-current characteristics of the p-n junction diode is shown in the below figure. The horizontal line in the below figure represents the amount of voltage applied across the p-n junction diode whereas the vertical line represents the amount of current flows in the p-n junction diode.
IF = Forward Biased Current of diode
VF = Forward Biased Voltage of diode
Io = Reverse biased Saturation Current of Diode
IR  Reverse biased Current of Diode

Forward Biase V-I  characteristic of P-N diode.
When anode is positive with respect to cathode , diode is said to be forward biased. with increase of the source voltage Vs from zero value , initially diode current is zero. from Vs=0 to cut-in voltage , the forward current is very small .cut-in voltage is also known as threshold voltage or turn-on voltage. beyond cut-in voltage ,the diode current rises rapidly and diode said to conduct. for silicon diode, the cut-in voltage is around 0.7. when diode conducts, there is a forward voltage drop of the order of 0.8 to 1V

Reverse Biase V-I  characteristic of P-N diode.
When cathode is positive with respect to anode the , the diode said to be reverse biased. In the reverse biased condition. a small reverse current leakage current , of the order of microampers or milliampers flow . the leakage current is almost independent of the reverse voltage until this voltage reach breakdown voltage at this reverse breakdown, voltage remains almost constant but reverse current becomes quite high limited only by the external circuit resistance . a large reverse break down voltage associated with high reverse current, leads to excessive power loss that may be destroy the diode.

At point a reverse breakdown of the diode occurs and current increase sharply damaging the diode . this point is called knee of  the  reverse   characteristics.             
V-I Characteristics of typical Ge And Si Diode:
  • the cut in voltage voltage for germanium ( Ge ) diode is about  0.3 while for silicon ( Si ) diode is as about 0.7 V.
  • the potential at which current starts increasing exponentially is called offset potential, threshold potential or firing potential of a diode.
  • the reverse saturation current is of order nA for silicon diode while it is of the order of uA for germanium diode.
  • Reverse breakdown voltage for Si diode is higher than that of Ge diode of a comparable rating. 
If the external reverse voltage applied on the p-n junction diode is increased, the free electrons from the n-type semiconductor and the holes from the p-type semiconductor are moved away from the p-n junction. This increases the width of depletion region.

The wide depletion region of reverse biased p-n junction diode completely blocks the majority charge carrier current. However, it allows the minority charge carrier current. The free electrons (minority carriers) in the p-type semiconductor and the holes (minority carriers) in the n-type semiconductor carry the electric current.The electric current, which is carried by the minority charge carriers in the p-n junction diode, is called reverse current. 

In n-type and p-type semiconductors, very small number of minority charge carriers is present. Hence, a small voltage applied on the diode pushes all the minority carriers towards the junction. Thus, further increase in the external voltage does not increase the electric current. This electric current is called reverse saturation current. In other words, the voltage or point at which the electric current reaches its maximum level and further increase in voltage does not increase the electric current is called reverse saturation current. 

The reverse saturation current is depends on the temperature. If temperature increases the generation of minority charge carriers increases. Hence, the reverse current increases with the increase in temperature. However, the reverse saturation current is independent of the external reverse voltage. Hence, the reverse saturation current remains constant with the increase in voltage. However, if the voltage applied on the diode is increased continuously, the p-n junction diode reaches to a state where junction breakdown occurs and reverse current increases rapidly. 

In germanium diodes, a small increase in temperature generates large number of minority charge carriers. The number of minority charge carriers generated in the germanium diodes is greater than the silicon diodes. Hence, the reverse saturation current in the germanium diodes is greater than the silicon diodes.


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