Complete VI Characteristics of a PN Junction Diode

Complete V‒I Characteristics of a Diode

• The complete V‒I characteristics of a diode is the combination of its forward as well as reverse characteristics. This is shown in Fig. 2.7.1.

• In forward characteristics, it is seen that initially forward current is small as long as the bias voltage is less than the barrier potential. At a certain voltage close to barrier potential, current increases rapidly.

• The voltage at which diode current starts increasing rapidly is called as cut‒in voltage. It is denoted by Vγ.

Fig. 2.7.1 Complete V‒I characteristics of a diode

• Below this voltage, current is less than 1% of maximum rated value of diode current. The cut‒in voltage for germanium is about 0.2 V while for silicon it is 0.6 V.

• It is important to note that the breakdown voltage is much higher and practically diodes are not operated in the breakdown condition.

• The voltage at which breakdown occurs is called reverse breakdown voltage denoted as V_BR.

Key Point: Reverse current before the breakdown is very very small and can be practically neglected.

V‒I Characteristics of Typical Ge and Si Diodes

• The potential for Germanium (Ge) diode is about 0.3 V while for Silicon (Si) diode is as about 0.7 V.

• The potential at which current starts increasing exponentially is also called Offset potential, Threshold potential or Firing potential of a diode.

• Fig. 2.7.2 shows the V‒I characteristics of typical Ge and Si diodes.

• The reverse saturation current in a germanium diode is about 1000 times more than the reverse saturation current in a silicon diode of a comparable rating. The reverse saturation current I₀ is of the order of nA for silicon diode while it is of the order of μA for germanium diode.

• Reverse breakdown voltage for Si diode is higher than that of the Ge diode of a comparable rating.

Review Questions

1. Draw and explain the V‒I characteristics of a PN junction diode.

2. Draw and compare V‒I characteristics of typical Ge and Si diodes.