What are V-I Characteristics?

Voltage-Ampere or V-I characteristics of a pn junction or semiconductor diode is the curve between voltage across the junction and the current through the circuit. Normally the voltage is taken along the x-axis of the graph in volts and current taken along the y-axis in amperes.

The Characteristics can be explained in three cases

1. Zero bias condition

2. Forward bias condition

3. Reverse bias condition

Case 1: Zero Bias

In zero bias condition, no external voltage is applied to the semiconductor diode, i.e., the circuit is open. Hence, the potential barrier at the junction does not permit current flow. Therefore, the current in the circuit is zero at V=0 V.

Case 2: Forward Bias

In forward bias condition, p-type of the semiconductor diode is connected to the positive terminal and the n-type is connected to the negative terminal of the external voltage. This results in reduced potential barrier. At some forward voltage (0.7 V for Si or Silicon and 0.3 V for Ge or Germanium) the potential barrier is almost eliminated and the current starts flowing in the circuit. From this instant onwards the current increases in forward voltage.

From the forward conditions it can be notes when graphically representing forward bias condition, the current first increases slowly and the curve in non-linear. It is because in this region the external voltage applied to the pn junction is used in overcoming the potential barrier. However, once the external voltage exceeds the potential barrier voltage, the potential barrier is eliminated and the pn junction behaves as an ordinary conductor. Hence, the curve increases very sharply with increase in external voltage and the curve is almost linear.

Case 3: Reverse Bias

In reverse bias condition, the p-type of the pn junction is connected to the negative terminal and the n-type is connected to the positive terminal of the external voltage. This results in increased potential barrier at the junction. Hence, the junction resistance becomes very high and as a result practically no current flows through the circuit. However, a very small current of order microampere, flows through the circuit in practice. This is known as reverse saturation current and it is due to minority charge carriers in the junction. Free electrons in the p-type and holes in the n-type are called minority carriers.

The reverse bias applied to the pn junction acts as forward bias to their minority carriers and hence, small current flows in the reverse direction. If the applied reverse voltage is increased continuously, the kinetic energy of the minority carriers may become high enough to knock out electrons from the semiconductor atom. At this stage breakdown of the junction may occur. This is characterized by a sudden increase of reverse current and a sudden fall of resistance of the barrier region. This may destroy the junction permanently.