Diode Characteristics

Fig. 1 Diode symbol

Diode is a combination of p-type and n-type semiconductor. This combination act as a rectifier. Fig. 1 shows the symbol of a diode and also called junction diode.

Fig. 2 p-n junction

Fig. 2 shows the schematic diagram of a p-n junction.

Open circuited p-n junction :

The donor ion represented by plus (+) sign because when the impurity atom donates an electron, it becomes in a positive ion and the acceptor ion represented by minus (-) sign because, after this atom accepts an electron, it becomes in a negative ion. Hence only p-type carriers to the left of the junction and only n-type carrier to the right. Fig. 3 shows the schematic detail.

Fig. 3 Schematic diagram of a p-n junction

   

The p-n junction Rectifier 

The p-n junction diode act as a rectifier it allows to flow of charges in one direction and resist to flow of charges in the opposite direction. Here we describe revers bias and forward bias by changing the applied voltage to the p-n junction diode.

Reverse Bias :
Fig. 4 shows the p-n junction diode biased with a battery by connecting negative terminal with p side and n side is connected with positive terminal.

Fig. 4

Hence the negative charge density is spread to the left side of the junction and positive charge density in the right side of the junction.

Fig. 5

But this process is not continue for long and zero current results. But a small current flows due to thermal energy. This small current is the reverse saturation current. This current will increases with increase of temperature. The voltage applied in the direction shown in fig. 4 is called reverse bias or blocking bias. Fig. 5 shows the rectifier symbol.
Forward Bias : 
Fig. 6 shows the p-n junction diode biased with a battery by connecting positive terminal with p side and n side is connected with negative terminal.

Fig. 6

 Here the holes cross the junction from the p-type to n-type and the electrons cross the junction from the n-type to p-type, there for a minority current is injected into the both side. 

Fig. 7

Hence the current flows across the junction is the sum of holes and electrons minority current. The voltage applied in the direction shown in fig. 6 is called reverse bias. Fig. 7 shows the rectifier symbol.