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1.1. PN JUNCTION DIODE

A diode is a device which only allows unidirectional flow of current if operated
within a rated specified voltage level.
A 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
electronic and electrical 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 ideally open
circuit when it is in the reverse biased.
SYMBOL OF DIODE
The name diode is derived from "di-ode" which means a device having two
electrodes.

Fig:1.1.1 Symbol of PN junction diode

A simple PN junction diode by doping donor impurity in one portion and acceptor
impurity in other portion of silicon or germanium crystal block. These make a p n junction
at the middle part of the block beside which one portion is p-type (doped with trivalent
or acceptor impurity), and another portion is n-type (doped with pentavalent or donor
impurity). It can also be formed by joining a p-type (semiconductor doped with a trivalent
impurity) and n-type semiconductor (intrinsic semiconductor doped with a pentavalent
impurity) together with a special fabrication technique such that a p-n junction is formed.
It can also be formed by joining a p-type (semiconductor doped with a trivalent impurity)
and n-type semiconductor (intrinsic semiconductor doped with a pentavalent impurity)
together with a special fabrication technique such that a p-n junction is formed.

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CONSTRUCTION
The p-type forms anode and the n-type forms the cathode. These terminals are
brought out to make the external connections.
N-side will have a significant 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. Due to this, a process called diffusion takes place. In this process free
electrons from n side will diffuse (spread) into the p side and recombine with holes
present there, leaving positive immobile (not moveable) ions in n side and creating
negative immobile ions in p side of the diode. Hence, there will be uncovered positive
donor ions in n-type side near the junction edge.

Fig:1.1.2 Construction of PN Diode

Similarly, there will be uncovered negative acceptor ions in p-type side near the
junction edge. Due to this, numbers of positive ions and negative ions will accumulate
on n-side and p-side respectively. This region so formed is called as depletion region due
to the “depletion” of free carriers in the region. Due to the presence of these positive and
negative ions a static electric field called as barrier potential is created across the pn
junction of the diode. It is called as "barrier potential" because it acts as a barrier and
opposes the further migration of holes and electrons across the junction.
FORWARD BIAS
In a PN junction diode when the forward voltage is applied i.e. 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. 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

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direction when forward applied voltage is more than barrier potential of the junction. This
voltage is called forward biased voltage. For silicon diode, it is 0.7 volts. For germanium
diode, it is 0.3 volts.

Fig:1.1.3 PN Diode Forward Bias

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. The time, taken by this input voltage to reach that value
or in other words, the time, taken by this input voltage to overcome the forward biased
voltage is called recovery time.
REVERSE BIAS
The diode is reverse biased i.e. 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 a tiny current flowing

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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 semiconductor and n-type semiconductor respectively.

Fig:1.1.4 PN Diode Reverse Bias

Now if reverse applied voltage across the diode is continually increased, then after
certain applied voltage the depletion layer will destroy 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 the
magnitude of the reverse voltage increases, the kinetic energy of the minority charge
carriers also increases. 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.

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V- I Characteristics
Forward Bias
When, P terminal is more positive as compared to N-terminal i.e. P-terminal
connected to positive terminal of battery and N-terminal connected to negative terminal
of battery, it is said to be forward biased. Positive terminal of the battery repels majority
carriers, holes, in P-region and negative terminal repels electrons in the N-region and
push them towards the junction. This result in increase in concentration of carriers near
junction, recombination takes place and width of depletion region decreases. As forward
bias voltage is raised depletion region continues to reduce in width, and more and more
carriers recombine. This results in exponential rise of current.

Fig:1.1.5 V-I Characteristics of PN Diode

Reverse Bias
In reverse biasing P- terminal is connected to negative terminal of the battery and
N- terminal to positive terminal of battery. Thus applied voltage makes N-side more
positive than P-side. Negative terminal of the battery attracts majority carriers, holes, in
P-region and positive terminal attracts electrons in the N-region and pull them away from
the junction. This result in decrease in concentration of charge carriers near junction and
width of depletion region increases. A small amount of current flow due to minority
carriers, called as reverse bias current or leakage current. As reverse bias voltage is raised

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depletion region continues to increase in width and no current flows. It can be concluded
that diode acts only when forward biased. Operation of diode can be summarized in form
of I-V diode characteristics graph.
For reverse bias diode,

Where, V = supply voltage

ID = diode current
IS = reverse saturation current
For forward bias,

Where, VT = volt’s equivalent of temperature = KT/Q = T/11600

Q = electronic charge
K = Boltzmann’s constant
N = 1, for Ge = 2, for Si

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