S4.
1 Theory of PN Junction
Ajunction is formed between a sample of 'P type semiconductor
and a sample of 'N' type semiconductor, both joined together. This
device is called the PN junction.
The formation of PN junction is also called as Diode, because it
has two electrodes one for P region named as, Anode and the
other for 'N region named as, Cathode.
concentration of free electrons
The N type semiconductor has high
concentration of holes as shown
while 'P type semiconductor has high
figure 3.21 (a). At the junction, there is a tendency for the free
in and holes to the ' N side
electrons to mnove towards the P' side Difusion. The diffusion
known as
4nd vice versa. This process is carrier move from high concentration
charge
Is the process by which
area to low concentration area.
diffusing from 'N' side into "P side recombine
Wnen free electrons charged immobile ions near
with the holes and leaves negatively
side. Similarly, holes diffusing from 'P side into
ejunction of 'P electrons and leaves positively charged
;Side recombine withjunction of 'N' side.
immobile ions near theimmobile positive ions deposited across the 'N'
After certain extent, the carrier diffusion from 'P region into
region prevents further charge
N regions.
ELECTRICAL AND ELECTRONICS
3.30 BASIC
N- Type
ENGIHE P.
Positively
P-Type
4
}oo00 |
Goo00 Neelecharcrons
gathgveedyn
charged
holes as
majority carriers
majorty cai
(hole)
Minority charge carrier
Minority charge carrier (electron) (b)
(a)
Migrated holes from P -type
Migrated electrons from N-type
ooooo
P N
Ooo0 ;
-Space charge region or depletion region
, Potential Barrier
(c)
Figure 3.21 Operation of PN junction
Similarly, the immobile negative ions deposited across then 'N regions
into 'P' region is restricted. These immobile ions forms a region.
known as depletion region. i.e., the region over which all the mobile
or free charge carriers are depleted. The region is also known as
Space charge region or Charge free region because, there is no
free charge carriers are available for conduction.
The existence of these immnobile ions develops the potential differenct
across the junction, this potential acts as barrier for further conduction
between the junction. Thus, this potential is named as barrier potential
or cut in voltage of semiconductor diode.
The value of barrier potential is 0.3 Vfor germanium iodes anu
0.7 Vfor sillcon diodes.
Junction Voltage or Barrier Voltage
When the depletion layer is formed there are negative immobile ions
in P-type semiconductor and positive immobile ions in N-tvne
as shown in figure 3.22. Due to this charge separation, asemiconduct
voltage V; is
developed across the junction under equilibrium condition. This voltage
is known as "junction potential or
barrier potential
It is clear from the figure 3.22, that the potential barrier VB set upin
this manner gives rise to an electric field. This electric field preventsthe
Renective majority carriers from crossing the barrier region.
ANALOG ELECTRONICS
3.31
The potential barrier
is in the order of 0.1 Vto N
0.3 V for Ge and 0.7 V to |o000
1.1 Vfor St. The barrler
potential of a PN junction Holes
Electron
depends upon three factors
namely, density, charge and Immobile Immobile
temperature. For a given PN negative ions positive ions
junction, the first two factors - -
are constant. Thus, making L Barrier
Va Potential
the value of VB dependent
only on temperature. It has
been observed that both Barrier width
germanum and stlicon iodes
Depletion i
decrease their barrier region
potential by 2 mVPC. Figure 3.22 Formation of barrier voltage
3.4.2 Operation of PN Junction Diode
In order to understand the working of the PN junction diode, we shall
consider the effect of forward bias and reverse bias across the P-N junction.
Migrated electrons Migrated holes
from N-type from P-type
oo
Under forward bias
No biasing
Va
Figure 3.23 Forward biased PN junction
3.4.2.1 Forward Bias
In an unbiased PN junction, there is no flow of current. A PN junction
connected to an external voltage Source is called as biased PN
Junction". By this biasing, the width of depletion region is
which results in control of its resistance and current flow iscontrolled
possible.
When an external voltage is applied to the P-N junction, in
way that it cancels the potential barrier and such a
tlow, it is called as biasing. permits the current
BASIC ELECTRICAL AND ELECTRONICS
3.32
terminal of a battery connected with
P-type ENGINE RING
The positive
semicomducto,
and the negative terminal connected with N-type semiconductor a
shown in the figure 3.23, provides the forward
bias to PN
The applied forward potential establishes an
electric ju nction
field opposite
barrier is
to the potential barrier. Therefore, the potential
As the potential barrier is very small (0.3 Vfor Ge and 0.7 Vfor Si, reduced.
a small forward voltage is sufficient to completely eliminate
barrier potential, thus the junction resistance becomes zero.
In other words, the applied positive potential repels the holes in the D
region so that the holes move towards the junction and applied negative
potential repels the electrons in the 'N region towards the junction
results in reduction of depletion region. When the applied potential
is more than the internal barrier potential, then the depletion region
completely disappear, thus the junction resistance becomes zero.
Once the potential barrier is eliminated by a forward voltage, junction
establishes the low resistance path for the entire circuit, thus a
current flows in the circuit, it is called as forward current.
3.4.2.2 Reverse Bias
When an external voltage is applied to P-N junction in such a way that
it increases the potential barrier then it is called as reverse bias". Fr
reverse bias, the negative terminal is connected to P type semiconductor
and positive terminal is connected to N type semiconductor as shown
in figure 3.24.
Migrated electrons Migrated holes
from N-type from P-type
oo oo o09
HUnder reverse bias
1 No biasing
Figure 3.24 Reverse biased P-Njunction
ANALOG ELECTRONICS
3.33
When reverse bias voltage is applied to the
carriers of P region are attached towardsjunction, all the majority
of the battery and the majority carriers of the the negative terminal
N region are attached
towards the positive terminal of the battery, hence the
region increases. depletion
The applied reverse voltage establishes an electric field
acts in the same direction of the potential which
barrier. Therefore,
the resultant field at the junction is strengthened and the
width is increased. This increased potential barrier barrier
flow of charges carriers across the junction, results prevents the
in a high
resistance path is established.
From the above discussion, we conclude that when a P-N
is forward biased, it has a low resistance path and
junction
hence current
flows in the circuit due to the majority carriers. On the other hand,
when it is reverse biased, it has high resistance path and no current
flows in the circuit. This process cannot be continued indefinitely
because after certain extent, the junction break down occurs. As
a result, a small amount of current flows through it due tominority
carriers. This current is known as reverse saturation current"
Thus, P-N junction diode is a unilateral device which offers a low
resistance when forward biased and behaves like an insulator when
reverse biased.
The holes traveling from 'P' region to 'N' region and electrons travelling
tom 'N' region to 'P region constitute the conventional currents in the
Same
5
direction namely from 'P' region to 'N region. So, the resultant current
the summation of the two currents.
D4.3 V4 Characteristics of P-N Junction Diode
Figure 3.25 shows the V-I characteristics of a P-N junction diode.
) Porward Bias
For the forward bias of a P-N junction, P-type is connected to the
Positive terminal while N-type is connected to the negative terminal
Of the battery. On varying this voltage slowly,at some forward voltage
the potential barrier is eliminated and current starts flowing, This
voltage is known as "thershold voltage (VTh) Or cut in voltage or
knee voltage". It is practically same as barrier voltage VR. For V< Vrh
the current flow is negligible.
the
the
forward applied voltage increases beyond threshold voltage,
It forward current rises exponentially as shown in the figure 3.25.
should be remembered that, if the forward voltage is increased
beyond a certain safe value, it produces an extremely large Current
which may destroy the junction due to overheating.
3.34 BASIC ELECTRICAL AND ELECTRONICS
Reverse bias
IAmA) Forward bias
ENGINE RING
Ge Si
Breakdown
voltage A'
V,+ AV,
Si Ge
Figure 3.25 V-I characteristics of PN junction diode
In portion OA or OA' (non linear operating region), even if there is
large variation in applied voltage there will be small variation in
the current flowing through the diode, because of the depletion region.
At point 'A', V =VTh V hence, the depletion region disappears
result in linear increase of current in portion AB or AB. This portion
is known as linear operating region of diode. The forward resistance
of the diode is obtained from the slope of the curve
AV,
R=
AI,
ii) Reverse Bias
For the reverse bias of P-N junction, P-type semiconductor is connected
to the negative terminal and N-type semiconductor is
connected to
the positive terminal of the battery.
Under this condition, a strong depletion region is formed across tne
iunction. It offers very high resistance, thus verv small current flow
OC and OC' shown in figure 3.25.
In this case the junction resistance becomes very high and practically
no current flOws through the circuit. If the reverse voltage is further
increased, the kinetic energy of the electrons beconme so high that
they knock out electrons from semiconductor atons. At this stage
breakdown of junction occurs which results in sudden rise of reverse
current. This current is known as reverse saturation current. The
reverse resistance of the diode is obtained from the slope of the curve
R, =
