Volt-Ampere or VI characteristic of PN junction diode is the relationship between the voltage across the diode and the current through the diode. The relationship between voltage and current of a diode is mathematically given by
ID= Diode current
VD= Diode voltage
VT= Thermal voltage
IS= Reverse saturation current
n= Function of VD whose value depends on the material 1<=n<=2
K= Boltzmann’s constant=1.38*10^-23 J/K
T= Temperature in kelvin
Q= Electron charge=1.67*10^-19 Coulombs
The curve of VI characteristics of PN junction diode
The above equation shows that with an increase in diode voltage VD, the diode current ID increases. The relationship between ID and VD is exponential. This relationship of the voltage and the current is shown in the characteristics curve below.
With the application of forward biasing voltage on the PN junction diode, the depletion width gets narrower. This will provide a low impedance path through the junction thereby allowing high current to flow. Very small current starts to flow across the diode when the forward-biased voltage is greater than the junction barrier potential. The junction barrier potential is approximately 0.7V for Si (Silicon) and 0.3V for Ge (Germanium). The voltage at which the current increases abruptly is known as knee voltage. Theoretically, the diode can conduct infinite current above this knee point and can effectively become a short circuit, therefore resistors are used in series with the diode to limit its current flow. Beyond the knee point, the V-I characteristic curve is almost linear and the potential barrier is eliminated.
When the PN junction is reverse biased, the size of the depletion width increases along with the barrier potential. Hence a PN junction is the reverse-biased region that offers very high impedance to the flow of current and there is no possibility of flow of majority charge carriers. But the minority charge carriers contribute to the flow of a small amount of current. This current is known as reverse saturation current (IS). –VZ on the reverse biased region is the reverse breakdown voltage above which diode will operate in reverse breakdown region with the flow of a very high amount of current.