A graph showing the variation of current flowing
through a pn junction with the voltage applied across it, both when it is
forward and reverse biased is called V-I
characteristics of Pn Junction diode.
Forward bias characteristics of Pn Junction diode
The figure shows the experimental arrangement for studying the characteristic curve of a pn junction when it is forward biased. Voltage across the diode can be varied using the potentiometer. For different values of voltages the value of current is noted and a graph between V and I is plotted.
The so obtained graph is called forward characteristic.
From the graph
:-
(1) The V-I graph is not a straight line i.e a pn junction
diode does not obey Ohm 's law. Therefore, it is classified as a
non-linear device.
(2) The current through the forward biased diode is always
negligible upto a certain voltage called cut-in voltage. The value of cut-in
voltage is 0.7 V for silicon diode
and 0.3 V for germanium diode.
(3) After the cut-in voltage, the diode current increases
exponentially given by the equation,
I = IS [exp(eV/ɳKBT) - 1]
Here, IS = reverse saturation current, e = charge
of electron, ɳ = emission co-efficient, KB = Boltzmann constant, T =
temperature V=voltage across the diode which has positive value for forward
bias and negative value for reverse bias.
In fact, complete characteristics can be ploted by
this formula by putting appropriate values.
The diode junction offers negligible resistance to
charge carriers, therefore it behaves like a closed switch.
During this time, diode resistance varies with applied
voltage. The resistance is defined as the dynamic resistance Rd of
diode. It is defined as the ratio of small change in applied voltage ΔV to the
corresponding change in current ΔI.
i.e Rd
= ΔV / ΔI
Reverse bias characteristics of Pn Junction diode
The experimental arrangement is similar to the fb case, only the polarity of the supply voltage is reversed. Procedure is same.
From the graph
:-
(1) When diode is reverse biased, for small voltages, the
diode conducts a very small, almost constant, current called reverse saturation current IS .
This is due to drift of minority charge carriers (holes in n region and
electrons in p region) across the junction. Here diode behaves like an open switch.
(2) When the reverse voltage across the pn junction
reaches a sufficiently high value, the reverse current suddenly increases to a
large value. This voltage at which breakdown of the diode occurs is called zener
breakdown voltage or peak-inverse voltage of diode. It
ranges from 1 volt to 100s of volts depending upon dopant density.
It can be seen from the graph that beyond zener
breakdown voltage, the voltage remains almost constant for large changes in
current values. Thus, in this region diode can be used as a votage regulator.
It is clear by now, ignoring zener case, that pn junction conducts only in one direction. So, it is
classified as a unilateral element.
The two characteristics can be joined at the origin to
get the complete I-V Characteristics of Pn Junction diode.
