We are going to tell you what diode is. A diode is a device which only allows
unidirectional flow of current if operated within a rated specified voltage level. We said within a rated specified voltage level because diode only blocks
current in the reverse direction. While the reverse voltage is within a limited range. Otherwise reverse barrier
breaks and the voltage at which this breakdown occurs is called reverse
breakdown voltage. The diode acts as a valve in the electrical and electronic circuit
A p-n junction is the simplest form of the diode which behaves as ideally short
circuit when it is in forward biased. And behaves as an ideal open circuit when it is in the reverse biased
The symbol of a diode is shown here. The arrowhead points in the direction of the conventional current flow
A simple p-n junction diode can be created by doping donor impurity in one portion
and acceptor impurity in the other portion of a silicon or germanium
crystal block. These make a p-n junction at the middle portion of the block
besides which one portion is p-type which is doped by trivalent or acceptor
impurity and other portion is n-type which is doped by pentavalent or donor
impurity. Hence it is a device to element. The p-type forms anode and the
n-type forms the cathode. These terminals are brought out to make the external connections
Let us discuss the working principle of diode. You
The n-side will have a large number of electrons and very few holes due to thermal excitation
Whereas the p-side will have a high concentration of holes
and very few electrons. You Due to this a process called diffusion takes place. In this process free
electrons from the n-side will diffuse or spread into the p-side and combine
with the holes present there leaving positive immobile ions in the n-side
You Hence some atoms on the p-side adjacent to junction are converted into negative ions
Similarly due to diffusion of electrons from n-side to p-side there will be lack
of electrons in some atoms on the n-side adjacent to the junction and as a result
these atoms on the n-side adjacent to junction will get converted to positive
ions. Due to this a large number of positive ions and negative ions will
accumulate on the n-side and p-side respectively. This region so formed is
called a depletion region. You Due to the presence of these positive and negative ions a static electric field
called barrier potential is created across a p-n junction of the diode. It is
called a barrier potential because it acts as a barrier and opposes the
further migration of holes and electrons across the junction. In a p-n junction when the forward voltage is applied that is positive
terminal of a source is connected to the p-type side and the negative terminal of
the source is connected to the n-type side the diode is said to be in forward
biased condition. You We know that there is a barrier potential across the junction. This
barrier potential is directed in the opposite of the forward applied voltage
So a diode can only allow current to flow in the forward direction when
forward applied voltage is more than barrier potential of the junction. Voltage at which the forward current just starts to flow is called forward
biased voltage and it is equal to the barrier potential. For silicon diode it
is 0.7 volts. For germanium diode it is 0.3 volts. When forward applied voltage
is more than this forward biased voltage there will be forward current in
the diode and the diode will become short-circuited. Hence there will be no
more voltage drop across the diode beyond this forward biased voltage and
forward current is only limited by the external resistance connected in series
with the diode. Thus if forward applied voltage increases from zero the diode
will start conducting only after this voltage reaches just above the barrier
potential or forward biased voltage of the junction. Now if the diode is reverse biased
that is positive terminal of the source is connected to the n-type end
and the negative terminal of the source is connected to the p-type end of the
diode there will be no current through the diode except reverse saturation
current. This is because at the reverse biased condition the depilation layer of
the junction becomes wider with increasing reverse biased voltage. Although there is tiny current flowing from n-type end to p-type end in the
diode due to minority carriers. This tiny current is called reverse saturation current. Minority carriers are
mainly thermally generated electrons and holes in p-type and n-type semiconductor respectively
Now reverse applied voltage across a diode and continually increase it then
after certain applied voltage the depletion layer will be destroyed which will cause a huge reverse current to flow through the diode. If this
current is not externally limited and it reaches beyond the safe value the diode
may be permanently destroyed. This is because as a magnitude of the reverse voltage increases
the kinetic energy of the minority charge carriers also increases. you
These fast-moving electrons collide with the other atoms in the device to knock
off some more electrons from them. The electrons so released further release
much more electrons from the atoms by breaking the covalent bonds. This process
is termed as carrier multiplication and leads to a considerable increase in the
flow of current through the p-n junction. The associated phenomenon is called
avalanche breakdown. you That is why we already said the current can be blocked by a diode when the
applied reverse voltage is less than a rated or specified voltage. This specified voltage is less than the voltage at which avalanche breakdown
occurs. This is the basic theory of diode. Thank you