V-I Characteristics of p-n Junction Diode
The Volt-Ampere or V-I characteristics of a p-n junction diode is basically the curve between voltage across the junction and the circuit current.
Usually voltage is taken across x-axis and current along y-axis.
Fig.1 shows the circuit arrangement for determining the V-I characteristics of a p-n junction diode.
The characteristics can be explained under three conditions namely zero external voltage, forward bias and reverse bias.
(i) 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. Therefore, circuit current is zero as indicated by point O in fig.2.
(ii) Forward Bias:
With forward bias to the p-n junction i.e. p-type is connected to positive terminal and n-type is connected to negative terminal, the potential barrier is reduced.
At some forward voltage (0.7 V for Si and 0.3 V for Ge), the potential barrier is altogether eliminated and current starts flowing in the circuit.
From now onwards, the current increases with the increase in forward voltage. Thus a rising curve OB is obtained with forward bias as shown in fig.2.
From the forward characteristics, it is seen that at first (i.e region OA ), the current increase very slowly and curve is non-linear. It is because the external applied voltage is used to overcome the potential barrier.
However, once the external applied voltage exceeds the potential barrier voltage, the p-n junction behaves like an ordinary conductor. Therefore, current rises very sharply with increase in voltage (region AB). The curve is almost linear.
(iii) Reverse Bias:
With reverse bias to the p-n junction i.e. p-type connected to negative terminal and n-type connected to positive terminal, potential barrier at the junction is increased.
Therefore, the junction resistance becomes very high and practically no current flows through the circuit.
However, in practice, a very small current (of the order of μA) flows in the circuit with reverse bias as shown in fig.3.
In n-type and p-type semiconductors, very small number of minority charge carriers is present. Hence, a small voltage applied on the diode pushes all the minority carriers towards the junction.
Thus, further increase in the external voltage does not increase the electric current.
This electric current is called reverse saturation current.
In other words, the voltage or point at which the electric current reaches its maximum level and further increase in voltage does not increase the electric current is called reverse saturation current.
To the free electrons in p-type and holes in n-type, the applied reverse bias appears as forward bias. Therefore, a small current flows in the reverse direction.
The reverse saturation current depends on the temperature. If temperature increases the generation of minority charge carriers increases. Hence, the reverse current increases with the increase in temperature.
However, the reverse saturation current is independent of the external reverse voltage. Hence, the reverse saturation current remains constant with the increase in voltage.
However, if the voltage applied on the diode is increased continuously, the kinetic energy of minority carriers may become high enough to knock out electrons from the semiconductor atom. At this stage breakdown of the junction occurs. This is characterized by a sudden rise of reverse current and a sudden fall of the resistance of barrier region. This may destroy the junction permanently.
In germanium diodes, a small increase in temperature generates large number of minority charge carriers. The number of minority charge carriers generated in the germanium diodes is greater than the silicon diodes. Hence, the reverse saturation current in the germanium diodes is greater than the silicon diodes.