Question

Draw and explain V-I characteristics of a P-N junction diode.

Answer 1

Forward V-I characteristics of p-n junction diode

If the positive terminal of the battery is connected to the p type semiconductor and the negative terminal of the battery is connected to the n-type semiconductor, the diode is said to be in forward bias. In forward biased p-n junction diode, VF represents the forward voltage whereas IF represents the forward current.

Forward V-I characteristics : If the external voltage applied on the silicon diode is less than 0.7 volts, the silicon diode allows only a small electric current. However, this small electric current is considered as negligible. When the external voltage applied on the silicon diode reaches 0.7 volts, the p-n junction diode starts allowing large electric current through it. At this point, a small increase in voltage increases the electric current rapidly. The forward voltage at which the silicon diode starts allowing large electric current is called cut-in voltage. The cut-in voltage for silicon diode is approximately 0.7 volts.

Reverse V-I characteristics : If the negative terminal of the battery is connected to the p-type semiconductor and the positive terminal of the battery is connected to the n-type semiconductor, the diode is said to be in reverse bias. In reverse biased p-n junction diode, VR represents the reverse voltage whereas IR represents the reverse current. The wide depletion region of reverse biased p-n junction diode completely blocks the majority charge carrier current.  

Answer 2

V-I Characteristics of a P-N junction:

V-I characteristics of a Diode:

The V-I characteristics can be divided in two parts.

1. Forward characteristics.

2. Reverse characteristics

Forward characteristics of P-N Junction Diode.

The forward characteristic is the graph of the anode to cathode forward voltage ‘Vf’ verses the forward current through the diode ‘If’.

The forward characteristics are divided into two portions AB and BC as shown in the fig below.

Region A to B:

In this region A to B of the forward characteristics shown in the fig, the forward voltage is small and less than the cut in voltage.

Therefore the forward current flowing through the diode is small.

With further increase in the forward voltage, it reaches the level of the cut in voltage and the width of depletion region grows on decreasing.

Region B to C:

As soon as the forward voltage equals the cut in voltage, current through the diode increases suddenly.

The nature of this current is exponential.

The large forward current in the region B-C of the forward characteristics is limited by connecting a resistor ‘R’ in series with the diode. Forward current is of the order of a few mA.

The forward current is a conventional current that flows from anode to cathode.

Therefore it is considered to be positive current, and the forward characteristics appears in the first quadrant as shown in the fig.

Cut in voltage (Knee Voltage):

The voltage at which the forward diode current starts increasing rapidly is known as the cut-in voltage of a diode. As shown in fig above, the cut in voltage is very close to the barrier potential. Cut-in voltage is denoted by . Cut-in voltage is also called as Knee voltage.

Generally a diode is forward biased above the cut-in voltage. The cut-in voltage for a silicon diode is 0.6V and that for germanium diode is 0.3V.

Reverse Characteristics of Diode:

The minority electrons in the p-region are attracted by the positive end of the dc supply. Hence these electrons will cross the junction and constitutes reverse current ‘Ir’ of the diode as shown in the fig.

Since reverse current is very much less the resistance offered by reverse biased diode is very high of the order of few ‘KΩ’.

Reverse Characteristics:

The circuit arrangement for obtaining the reverse characteristics of the diode is shown in the fig above.

The milli-ammeter is replaced by micro-ammeter.

The applied reverse voltage is gradually increased above zero in suitable steps and the values of diode current are recorded at each step.

Because of the minority carrier there will be small amount of current flowing through the diode is called reverse saturation current.

Now, if we plot a graph with reverse voltage on horizontal axis and current on vertical axis, we obtain reverse characteristics.