In chapter 1 – Understanding the PN junction
, we have seen how a PN junction is formed from a p-type and n-type
semiconductor. We have also learned about diffusion current, depletion
region, drift current and barrier potential. If you find these terms
foreign, just read the chapter about “understanding the pn junction” once more. Lets just make some questions. What is the use of a PN junction?
Why have scientists created a pn junction device? What kind of problem
it solves ? Learning anything is really fun when we question it. So
these are our questions. Why there exists a pn junction in this world!
?;)
PN junction diode is symbolically represented as shown in picture. The direction of arrow is the direction of conventional current flow (under forward bias). Now lets try applying an external voltage to the pn junction diode. The process of applying an external voltage is called as “biasing”. There are two ways in which we can bias a pn junction diode.
To get an answer to all these questions, lets first try to understand the characteristics of a PN junction.
We know a pn junction has a “barrier potential”. Only if we overcome
this “barrier potential” by applying an external voltage to the pn
junction, we would be able to make it conducting. This simply means,
current will pass through the pn junction only if we apply an external
voltage higher than the “barrier potential” of pn junction. In chapter
1, we have seen that net current inside a pn junction is
zero. Inorder to understand the behavior of a pn junction we need to
make it conducting by applying an external voltage over a range (say
from 0 volts 5 or 10 volts ), and then we study how the current passed
through the pn junction varies with increasing voltage levels. To apply
an external voltage, we usually connect 2 metallic contacts at the two
ends of the pn junction (known as terminals); one on the p-side and other on the n-side. A PN junction with two metallic contacts is known as a pn junction diode or a semiconductor diode.
Note:- I have written
an interesting article which tells the story behind invention &
discovery of PN Junction diode. If you like to read the story, follow
here:- Story behind Invention & Discovery of PN Junction
PN junction diode is symbolically represented as shown in picture. The direction of arrow is the direction of conventional current flow (under forward bias). Now lets try applying an external voltage to the pn junction diode. The process of applying an external voltage is called as “biasing”. There are two ways in which we can bias a pn junction diode.
1) Forward bias and 2) Reverse bias
The basic difference between a forward
bias and reverse bias is in the direction of applying external voltage.
The direction of external voltage applied in reverse bias is opposite to
that of external voltage applied in forward bias.
Forward biasing a PN Junction diode
Forward biasing a pn junction diode is
very simple. You just need to take a battery whose values can be varied
from (o to V volts), connect its positive terminal to the p-side of pn
junction diode and then connect the negative terminal of battery to the
n-side of the pn junction diode. If you have done upto this, the forward
bias circuit of pn junction diode is complete. Now all we need to do is
understand how the pn junction diode behaves when we increase the
voltage levels from 0 to say 10 volts or 100 volts. We have learned that
if we apply an external voltage higher than the barrier potential of pn
junction diode, it will start conducting, which means it will start
passing current through it. So how we are going to study the behavior of pn junction diode under forward biased condition?
Lets get a voltmeter and ammeter and connect it to the forward biased
circuit of pn junction diode.A simple circuit diagram is shown below,
which has a pn junction diode, a battery (in picture it is not shown as
variable. keep in mind we are talking about a variable power source), an
ammeter (in milli ampere range) and a voltmeter.
Note:- Assume that the
pn junction diode is made from Silicon. The reason is difference in
barrier potential for a diode made from Germanium and Silicon. (For a
silicon diode – barrier potential is 0.7 volts where as for a Germanium
diode barrier potential is low ~ 0.3 volts)
How to plot the characteristics of a pn junction ?
What we are going to do is, vary the
voltage across diode by adjusting the battery. We start from o volts,
then slowly move 0.1 volts, 0.2 volts and so on till 10 volts. Lets just
note the readings of voltmeter and ammeter each time we adjust the
battery (in steps of 0.1 volts). Finally after taking the readings, just
plot a graph with voltmeter readings on X-axis and corresponding
Ammeter readings on Y axis. Join all the dots in graph paper and you
will see a graphical representation as shown below. Now this is what we call “characteristics of a pn junction diode” or the “behavior of diode under forward bias”
How to analyse the characteristics of a pn junction diode ?
Image Source
Its from the “characteristics graph”
we have just drawn, we are going to make conclusions about the behavior
of pn junction diode. The first thing that we shall be interested in is
about “barrier potential”. We talked a lot about
barrier potential but did we ever mention its value ? From the graph, we
observe that the diode does not conduct at all in the initial stages.
From 0 volts to 0.7 volts, we are seeing the ammeter reading as zero!
This means the diode has not started conducting current through it. From
0.7 volts and up, the diode start conducting and the current through
diode increases linearly with increase in voltage of battery. From this
data what you can infer ? The barrier potential of silicon diode is 0.7
volts
What else ? The diode starts conducting at 0.7 volts and current
through the diode increases linearly with increase in voltage. So that’s
the forward bias characteristics of a pn junction diode. It conducts current linearly with increase in voltage applied across the 2 terminals (provided the applied voltage crosses barrier potential).
What happens inside the pn junction diode when we apply forward bias ?
We have seen the characteristics of pn junction diode through its graph. What really happens inside the diode during the forward bias ? We know a diode has a depletion region with a fixed barrier potential. This depletion region has a predefined width, say W.
This width will vary for a Silicon diode and a Germanium diode. The
width highly depends on the type of semiconductor used to make pn
junction, the level of doping etc. When we apply voltage to the
terminals of diode, the width of depletion region slowly starts
decreasing. The reason for this is, in forward bias we apply voltage in a
direction opposite to that of barrier potential. We know the p-side of
diode is connected to positive terminal and n-side of diode is connected
to negative terminal of battery. So the electrons in n-side gets pushed
towards the junction (by force of repulsion) and the holes in p-side
gets pushed towards the junction. As the applied voltage increases from 0
volts to 0.7 volts, the depletion region width reduces from ‘W’
to zero. This means depletion region vanishes at 0.7 volts of applied
voltage. This results in increased diffusion of electrons from n-side
to p-side region and the increased diffusion of holes from p-side to
n-side region. In other words, “minority carrier”
injection happens on both p-side (in a normal diode (without bias)
electrons are a minority on p-side) and n-side (holes are a minority on
n-side) of the diode.
How current flow takes place in a pn junction diode ?
This is another interesting factor, to
explain. As the voltage level increases, the electrons from n-side gets
pushed towards the p-side junction. Similarly holes from p-side gets
pushed towards the n-side junction. Now there arises a concentration
gradient between the number of electrons at the p-side junction region
and the number of electrons at the region towards the p-side terminal. A
similar concentration gradient develops between the number of holes at
the n-side junction region and the number of holes at region near the
n-side terminal. This results in movement of charge carriers (electrons
and holes) from region of higher concentration to region of lower
concentration. This movement of charge carriers inside pn junction gives rise to current through the circuit.
Reverse biasing a PN junction diode
Why should we reverse bias a pn diode ?
The reason is, we want to learn its characteristics under different
circumstances. By reverse biasing, we mean, applying an external voltage
which is opposite in direction to forward bias. So here we connect
positive terminal of battery to n-side of the diode and negative
terminal of the battery to p-side of the diode. This completes the
reverse bias circuit for pn junction diode. Now to study its
characteristics (change in current with applied voltage), we need to
repeat all those steps again. Connect voltmeter, ammeter, vary the
battery voltage, note the readings etc etc. Finally we will get a graph
as shown.
Analysing the revere bias characteristics
Here the interesting thing to note is
that, diode does not conduct with change in applied voltage. The current
remains constant at a negligibly small value (in the range of micro
amps) for a long range of change in applied voltage. When the voltage is
raised above a particular point, say 80 volts, the current suddenly
shoots (increases suddenly). This is called as “reverse current” and this particular value of applied voltage, where reverse current through diode increases suddenly is known as “break down voltage“.
What happens inside the diode ?
We connected p-side of diode to negative terminal of battery and n-side of diode to positive terminal of battery. So one thing is clear, we are applying external voltage in the same direction of barrier potential. If applied external voltage is V and barrier potential is Vx , then total voltage across the pn junction will be V+Vx.
The electrons at n-side will get pulled from junction region to the
terminal region of n-side and similarly the holes at p-side junction
will get pulled towards the terminal region of p-side. This results in
increasing the depletion region width from its initial length, say ‘W’
to some ‘W+x’. As width of depletion region increases, it results in
increasing the electric field strength.
How reverse saturation current occurs and why it exists ?
The reverse saturation current is the
negligibly small current (in the range of micro amperes) shown in graph,
from 0 volts to break down voltage. It remains almost constant
(negligible increase do exist) in the range of 0 volts to reverse
breakdown voltage. How it occurs ? We know, as
electrons and holes are pulled away from junction, they dont get
diffused each other across the junction. So the net “diffusion current”
is zero! What remains is the drift due to electric field. This reverse
saturation current is the result of drifting of charge carriers from the
junction region to terminal region. This drift is caused by the
electric field generated by depletion region.
What happens at reverse breakdown ?
At breakdown voltage, the current
through diode shoots rapidly. Even for a small change in applied
voltage, there is a high increase in net current through the diode. For
each pn junction diode, there will be a maximum net current that it can
withstand. If the reverse current exceeds this maximum rating, the diode
will get damaged.
Conclusion about PN junction characteristics
To conclude about pn junction characteristics, we need to get an answer to the first question we have raised – What is the use of pn junction?
From the analysis of both forward bias and reverse bias, we can arrive
at one fact – a pn junction diode conducts current only in one direction
– i.e during forward bias. During forward bias, the diode conducts
current with increase in voltage. During reverse bias, the diode does
not conduct with increase in voltage (break down usually results in
damage of diode). Where can we put this characteristics of diode into use ?
Hope you got the answer! Its in conversion of alternating current to
direct current (AC to DC). So the practical application of pn junction
diode is rectification!
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