Volt-Ampere characteristics of a PN junction diode are the curves between the voltage across the junction and the circuit current. Usually, voltage is taken along the x-axis and current along the y-axis.
Volt-Ampere Characteristic of a PN Junction Diode
The figure below shows the circuit arrangement for determining the V-I characteristics of a PN junction diode. The characteristics can be studied under three heads i.e.
- Zero external voltage
- Forward bias
- Reverse bias
Zero external voltage
When the external voltage is Zero, i.e circuit is open at K, the potential barrier at the junction does not permit current flow. The circuit current in this case is indicated by point O in the figure.
With the forward bias to the PN junction diode, i.e., p-type connected to the positive terminal and n-type connected to the negative terminal, the potential barrier is reduced. When the potential barrier is eliminated at a certain voltage, current starts to flow in the circuit. The current increases with the increase in forward voltage. Thus, the rising curve OB is obtained with a forward bias.
In Figure, OA, the current increases very slowly, and the curve is non-linear because the external voltage is used to overcome the potential barrier. Once the external voltage exceeds the potential barrier voltage, the PN junction diode behaves like an ordinary conductor. Therefore, the current starts to rise very sharply with an increase in external voltage (AB) and is almost linear.
With the reverse bias applied to the PN junction diode, i.e., p-type connected to the negative terminal and n-type connected to the positive terminal, the potential barrier is increased at the diode junction. There is no flow of current as the junction resistance becomes very high. A very small current (A) flows in the circuit with reverse bias. This current is due to the minority charge it carries.
There are a few free electrons in p-type material and a few holes in n-type material. These undesirable free electrons in the p-type and holes in the n-type are called minority carriers. For the minority carriers, the applied reverse bias appears as forward bias. Therefore, a small current flows in the reverse direction.
If reverse voltage is increased continuously, the kinetic energy of electrons (minority carriers) may become high enough to knock out electrons from the semiconductor atoms. At this stage, breakdown of the junction occurs, characterized by a sudden rise in reverse current and a sudden fall in the resistance of the barrier region, which may destroy the junction permanently.
The forward current through a PN junction is due to the majority of carriers produced by the impurity. But the reverse current is due to the minority carriers produced due to the breaking of some covalent bonds at room temperature.