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DEPARTMENT OF ECE
ELECTRONIC DEVICES AND CIRCUITS
UNIT-I
P-N JUNCTION DIODE
Classification of materials (Exp Q. What is an insulator, a
semiconductor and a
metal? Explain with the help of energy band diagrams)
Materials are broadly classified into
1. Conductors
2. Insulators
3. Semiconductors
CONDUCTOR:
Conductor is one in which Conduction band and valence band are
overlapped with
each other (i.e.) no energy gap between Conduction band and
valence band. Ex: Copper,
Aluminium
INSULATOR:
An insulator is one in which large energy gap between conduction
band and valance
band. In this material, Forbidden energy gap is large (EG 6e.V).
Practically it is not possible to
jump electrons from valance band to conduction band. Ex: Diamond
is a perfect insulator.
Conductor
Semi-
Conductor
Insulator
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SEMI CONDUCTOR:
A semiconductor material is one in which the forbidden energy
gap is greater than
conductors but less than insulators. Ex: Germanium, Silicon
(Energy gap of Germanium is
about 0.785 eV and for silicon it is 1.21ev).
TYPES OF SEMICONDUCTORS (Exp Q. What is meant by doping? Explain
about
intrinsic & extrinsic semiconductors, N-type material &
P-type material)
Doping
Adding impurities to a semiconductor is called Doping. Doping is
mainly used to increase the
conductivity. Pure semiconductor is called intrinsic
semiconductor. Semiconductor with
impurities added is called extrinsic semiconductor.
EXTRINSIC SEMICONDUCTOR MATERIALS:
The characteristics of semiconductor materials can be altered
significantly by the
addition of certain impurity atoms into the relatively pure
semiconductor material. These
impurities, although only added to perhaps 1 part in 108, can
alter the band structure
sufficiently to totally change the electrical properties of the
material. A semiconductor
material that has been subjected to the doping process is called
an extrinsic material. There
are two extrinsic materials of immeasurable importance to
semiconductor device fabrication:
n-type and p-type.
N-Type Material: both the n- and p-type materials are formed by
adding a predetermined
number of impurity atoms into a germanium or silicon base. A
small amount of pentavalent
impurity such as antimony, arsenic or phosphorus is added to the
pure semiconductor
(Germanium or Silicon) to get N type Semiconductor. Ge atom has
four valence electrons
and antimony has five valence electrons. The effect of such
impurity element is indicated in
the following figure. Each antimony atom forms a covalent bond
with surrounding four Ge
atoms. Thus four valence electrons of antimony atom form
covalent bond with four valance
electrons of individual Ge atoms and fifth valance electron is
left free which is loosely bound
to the antimony atom. This loosely bound electron can be easily
excited from the valence
band to the conduction band by the application of external
energy. Thus every antimony atom
donates one electron without creating a hole. Such pentavalent
impurities are called donor
impurities because it donates one electron to the conduction
band. The donor atom becomes
positively charged ion after giving an electron to the
conduction band.
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DEPARTMENT OF ECE
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Thus the addition of pentavalent impurity increases the number
of electrons in the conduction
band thereby increasing the conductivity of N type
semiconductor. So free electrons are
majority charge carries and holes are minority charge
carriers.
P-Type Material:
A small amount of trivalent impurity such as boron is added to
the pure semiconductor
(Germanium or Silicon) to get P type Semiconductor. Ge atom has
four valence electrons and
boron has three valence electrons. The effect of such impurity
element is indicated in the
above figure. Each boron atom forms a covalent bond with
surrounding four Ge atoms
leaving one bond incomplete which gives rise to a hole. Thus
trivalent impurity when added
to the intrinsic semiconductor (Ge) introduces large number of
holes in the valence band.
Such trivalent impurities are called acceptor impurities because
it accepts free electron in the
place of hole. The acceptor atom becomes negatively charged ion
after accepting electron to
the hole.
Thus the addition of trivalent impurity increases the number of
holes in the valence
band thereby increasing the conductivity of P type
semiconductor. So holes are majority
charge carriers and free electrons are minority charge
carries.
Concept of Majority and Minority Carriers (Exp Q. Explain the
concept of majority
and minority carriers)
Majority and Minority Carriers
In the intrinsic state, the number of free electrons in Ge or Si
is due only to those few
electrons in the valence band that has acquired sufficient
energy from thermal or light sources
to break the covalent bond or to the few impurities that could
not be removed. The vacancies
left behind in the covalent bonding structure represent our very
limited supply of holes. In an
n-type material, the number of holes has not changed
significantly from this intrinsic level.
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The net result, therefore, is that the number of electrons far
outweighs the number of holes.
For this reason:
In an n-type material the electron is called the majority
carrier and the hole the minority
carrier.
For the p-type material the number of holes far outweighs the
number of electrons, as shown
in Therefore: In a p-type material the hole is the majority
carrier and the electron is the
minority carrier. When the fifth electron of a donor atom leaves
the parent atom, the atom
remaining acquires a net positive charge: hence the positive
sign in the donor-ion
representation. For similar reasons, the negative sign appears
in the acceptor ion. In N type
material Fermi level is just below the conduction band. In P
type material Fermi level is just
above the valence band
Open Circuited P-N Junction:
If donor impurities are introduced into one side and acceptor
impurities into other side of a
single crystal of a semiconductor, a P-N junction is formed.
Such a system is shown in the
following figure. The donor ion is indicated by a plus sign
because after this impurity atom
donates an electron, it becomes a positive ion. The acceptor ion
is indicated by a negative
sign, because after this atom accepts an electron, it becomes a
negative ion. Every acceptor
atom accept one electron from valence band, resulting one
immobile negatively charged
acceptor ion and one mobile hole in the valence band occurred.
Similarly every donor atom
donates one free electron to the conduction band, resulting one
immobile positively charged
donor ion and one mobile electron in the conduction band
occurred.
Due to concentration gradient, electrons diffuse across the
junction from N region to the P
region while holes diffuse across the junction from p region to
N region. The holes on
diffusing from P region to the N region combine with electrons
in the vicinity of the junction.
Similarly the electrons on diffusing from N side the P side
combines with holes in the vicinity
of the junction. As a result of displacement of these charges, a
potential will appear across the
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junction. This is called barrier potential. This value is equal
to 0.2 Volts for Ge and 0.6 Volts
for Si. The region of the junction is depleted of mobile
charges, it is called depletion region.
It is also called as space charge region or transition region.
The thickness of the region is of
the order of one micro meter.
P-N Junction Diode (Exp Q.Explain PN diode characteristics in
forward bias and
reverse bias regions)
The P-N junction diode permits the easy flow of current in one
direction but restricts
the flow current in the opposite direction. In order to
understand the working of P-N junction
diode, we shall consider the effect of forward bias and Reverse
bias across the junction.
REVERSE BIAS: If an external voltage is applied in such a way
that positive terminal to n
side and negative terminal to P side of a P-N junction as shown
below. The junction is said to
be reverse biased. In this arrangement electrons from the N side
are attracted towards the
positive terminal and holes from the P side are attracted
towards the negative terminal. Thus,
both the holes in the p type and electrons in the N type to move
away from the junction.
Resultant, the region of negative charge density on the P side
and region of positive charge
density on the N side become wider (i.e.) the width of depletion
layer increases. As depletion
region widens, barrier potential across the junction also
increases. This increased barrier
potential, reduce the flow of majority charge carriers to the
other side(i.e) holes from the P
side to the N side and electrons from N side to the P side.
However the flow of minority
charge carriers remains uninfluenced by the increased barrier
potential. But there are very
few minority charge carriers (i.e.) hole in the N region and an
electron in P region crosses the
junction. Thus a small amount of current flows through the diode
which is called Reverse
Saturation Current (I0) order of micro amperes. This reverse
saturation current will increase
with increasing temperature.
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DEPARTMENT OF ECE
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FORWARD BIAS:
If an external voltage is applied in such a way that positive
terminal to P side and negative
terminal to N side of a P-N junction as shown above. The
junction is said to be forward
biased. In this arrangement electrons on the N side are repelled
from the negative terminal
and driven towards the junction. Similarly holes on the P side
are repelled from the positive
terminal and driven towards the junction. Resultant, the width
of depletion layer decreases
and the barrier potential also decreased. Thus majority charge
carriers crossing the junction
(i.e.) electrons flow from n side to p side and holes flow from
P side to N side. Since the
barrier potential is very small, a small forward voltage is
sufficient to eliminate the barrier
completely. Once the barrier is eliminated by the application of
forward voltage, junction
resistance becomes almost zero. Resultant large current flows
through the diode which is
called forward current.
VOLT AMPERE CHARACTERISTICS OF P-N JUNCTION DIODE:
The current passing through a P-N junction diode is given
by:
The diode is forward biased if V is positive (i.e.) P side of
the junction is positive and N side
of the junction is negative. The symbol is unity for Ge and is 2
for Si. The symbol stands
for volt equivalent of temperature and is given by:
At room temperature, T=27+273= ,
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The volt ampere characteristics are shown in the following
figure. In order to display forward
and reverse volt ampere characteristics of a P-N junction on a
single graph, it is necessary to
two different current scales. Forward current scale is in milli
amperes and reverse current
scale is in micro amperes.
In forward characteristics, up to some applied voltage, the
current passing through the P-N
junction diode is almost zero. The voltage up to which no
current passing through the P-N
junction diode is called cut in voltage. It is denoted by . It
is equal to 0.2 V for Ge and 0.6
V for Si.
In reverse characteristics, up to some applied voltage, current
passing through P-N junction
diode is almost constant. The voltage up to which constant
current passing through P-N
junction diode is called Peak Inverse Voltage (PIV) of a P-N
junction. Beyond PIV, the
junction breaks and enormous current passing through P-N
junction diode.
CURRENT COMPONENTS IN A P-N JUNCTION DIODE:
When a forward bias is applied to a P-N junction diode, holes
are injected in to the N side and
electrons in to the P side. After injecting, these are minority
charge carries move away from
the junction exponentially decreases with the distance as shown
below. There are two
minority currents as indicated in the following figure.
represents the hole
current in N side and represents the electron current in the P
side as a function of x.
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Electrons crossing the junction at x=0 from right to left
constitute a current in the same
direction as holes crossing the junction from left to right.
Hence the total current I at x =0 is
given by:
Since current is same throughout series circuit, I independent
of x and is shown as a
horizontal line in the following figure. Consequently in the P
side, there must be a second
component of current which combining with gives the total
current I. hence the hole
current in the P side is given by:
Similarly the electron current in the N side is given by:
For away from the junction in P side, the current is a drift
current ( ) of holes. As the holes
approaching the junction, some of them recombine with electrons
which are injected in to the
P side from N side. Thus the current decreases towards the
junction. The rate of decrease
is such that the total current remains constant independent of
distance. The decreased hole
current at the junction now enters the N side and becomes the
hole diffusion current ( ).
Similar remarks can be made with respect to current ( ). Hence
in a forward biased P-N
junction diode, the current enters in to the P side as a hole
current and leaves from the N side
as an electron current of the same magnitude.
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DEPARTMENT OF ECE
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DIODE RESISTANCE:
Static resistance and dynamic resistance (Exp Q. Explain static
resistance and dynamic resistance)
DC or Static Resistance:
The static resistance of a diode is defined as the ratio of
voltage and current. Any
point on the V-I characteristics of the diode, the static
resistance is equal to reciprocal of the
slope of a line joining the operating point to the origin. The
static resistance varies with V and
I and is not a useful parameter.
Dynamic resistance:
It is defined as the reciprocal of slope of the V-I
characteristics of a diode.
But we know that
But
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Law of Junction:
hole concentration in P side
hole concentration in N side
thermal equilibrium hole concentration in P side
thermal equilibrium hole concentration in N side
Total hole concentration in N side (i.e.) thermal
equilibrium
hole concentration + Injected hole concentration in N side
Barrier Potential
Volt temperature equivalent =
At room temperature, T =27+273 = ,
According to Boltzmann relationship of kinetic gas theory,
In the case of open circuited P-N junction,
In the case of forward biased P_N junction diode by an applied
voltage V,
But equations (1) and (2) are equal.
This boundary condition is the law of junction.
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DEPARTMENT OF ECE
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This law states that for a forward biased P-N junction diode,
the injected hole concentration
at the junction increases over thermal equilibrium value . It is
given by:
Diode current equation:
Let us derive the expression for the total current as a function
of applied voltage assuming
that the width of the depletion region is zero. When a forward
bias is applied to a P-N
junction diode, holes are injected from the P side to N side.
Due to this, the concentration of
holes in the N side ( ) is increased to thermal equilibrium hole
concentration in N side
( ) plus injected hole concentration in N side [ ].
(1)
Where is the diffusion length for holes in the N side.
At the junction (i.e.) x=0
(2)
Where is the injected hole concentration at the junction.
But according to law of junction
Substitute this value in equation (2)
(3)
The diffusion hole current in the N side is given by:
From equation (1),
Where diffusion coefficient for holes in
A Cross sectional area of a semiconductor
D Charge of an electron
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DEPARTMENT OF ECE
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The diffusion hole current at x=0 is given by:
But from(3)
(4)
Similarly, the diffusion electron current crossing the junction
in to P side with x=0 is given
by:
(5)
The total diode current is sum of equations (4) and (5)
[
]
But [
] = reverse saturation current
The general equation of the P-N junction diode current equation
is given by:
(6)
But correct equation for the P-N junction diode current equation
is given by:
(7)
Where V external applied voltage
volt temperature equivalent =
is a constant
= 1 for Ge and 2 for Si.
Temperature dependence of V-I characteristics:
Electron hole pairs are generated in semiconductors whenever
temperature increases as a
result conductivity increases. Thus, current passing through the
diode increases with
temperature as given by the diode current equation
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DEPARTMENT OF ECE
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The reverse saturation current becomes doubles for every rise in
temperature. Hence,
the temperature increased at constant voltage, current I
increases. The reverse saturation
current increases with temperature according to the following
equation.
Where Reverse saturation current of the diode at temperature
Reverse saturation current of the diode at temperature
Volt-Ampere Characteristics of P-N junction diode:
The total current as a function of applied voltage for a P-N
junction diode is given by:
The diode is forward biased if V is positive indicating that the
P side of the junction is
positive with respect to N side. The symbol is unity for Ge and
2 for Si. The symbol
stands for volt equivalent of temperature and is given by:
At room temperature, T= 27+273= ,
The V-I characteristics of P-N junction diode is shown below.
When diode is reverse biased,
V is negative.
In order to display forward and reverse V=I characteristics of a
P-N junction diode on a
single graph, it is necessary to use two different current
scales. Forward current scale is in
milli amperes and reverse current scale is in micro amperes.
In forward biased characteristics, up to some applied voltage,
the current passing through P-N
junction diode is almost zero. The voltage up to which no
current passing through the diode is
called Cut-in voltage and is denoted by . is approximately equal
to 0.2 V for Ge and
0.6 V for Si. In reverse biased characteristics, up to some
applied voltage, the current passing
through p-N junction diode is almost constant. The voltage up to
which constant current
passing through P-N junction diode, is called Peak Inverse
Voltage (PIV). Beyond PIV, the
junction breaks and enormous current passing through the
diode.
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TRANSITION (CT) AND DIFFUSION CAPACITANCE (CD) (Exp Q.
Explain
Transition capacitance and Diffusion capacitances)
In the p-n semiconductor diode, there are two capacitive effects
to be considered. In
the reverse bias region we have the transition- or
depletion-region capacitance (CT), while in
the forward bias region we have the diffusion (CD) or storage
capacitance. Recall that the
basic equation for the capacitance of a parallel-plate capacitor
is defined by C=A/d, where
is the permittivity of the dielectric (insulator) between the
plates of area A separated by a
distance d. In the reverse bias region there is a depletion
region (free of carriers) that behaves
essentially like an insulator between the layers of opposite
charge. Since the depletion width
(d) will increase with increased reverse-bias potential, the
resulting transition capacitance will
decrease. The fact that the capacitance is dependent on the
applied reverse-bias potential has
application in a number of electronic systems. The capacitive
effects described above are
represented by a capacitor in parallel with the ideal diode, as
shown below.
Temperature effects on p-n diode (Exp Q. Explain temperature
effects on p-n diode characteristics)
Temperature can have a marked effect on the characteristics of a
silicon
semiconductor diode. It has been found experimentally that the
reverse saturation current Io
will just about double in magnitude for every 10C increase in
temperature. Typical values of
Io for silicon are much lower than that of germanium for similar
power and current levels. The
result is that even at high temperatures the levels of Io for
silicon diodes do not reach the same
high levels obtained for germanium. As the temperature increases
the forward characteristics
are actually becoming more ideal.
Zener Diode (Exp Q.Explain break down mechanisms in
semiconductor diodes)
Zener diode is a reverse biased heavily doped whose doping
concentration is one part of
impurity is added to every 105 parts of pure semiconductor
material P-N junction diode.
Zener Diode operates in the breakdown region. When a Zener Diode
is operated in forword
biased, its characteristics are same as that of ordinary P-N
junction diode.
Following are two mechanisms of Zener breakdown.
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1) Zener Breakdown: Zener breakdown generally occur in very thin
junctions
(i.e.) when both sides of the junction are heavily doped and
resultant the depletion layer is
narrow. When a small reverse bias is applied, a very strong
electric field is set up across the
junction. This field is enough to break the covalent bonds. Now
large number of free
electrons and holes are produced which constitute the reverse
saturation current.
2) Avalanche breakdown: This type of breakdown occurs when both
sides of
junction are lightly doped and resultant the depletion layer is
large. In this case, the electric
field across the junction is not so strong to produce Zener
breakdown. In this case, free
electrons acquire sufficient energy from applied potential and
collide with the semiconductor
atoms in the depletion region. Due to the collision with valance
electrons, covalent bonds are
broken and electron hole pairs are generated. These new charge
carriers again acquire
sufficient energy from the applied potential and collide with
semiconductor atoms. In turn
produce additional charge carriers. This forms cumulative
process called an avalanche
multiplication. The breakdown is called avalanche breakdown.
In general, at reverse voltages less than 6 volts, Zener
breakdown occurs while greater
6 volts, Avalanche breakdown occurs. When the breakdown voltage
is reached in Zener
diode, the current increases rapidly with small change in
voltage. The V-I characteristics of
Zener Diode is shown below. The complete equivalent circuit of
the Zener diode in the Zener
region includes a small dynamic resistance and dc battery equal
to the Zener potential
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Tunnel Diode:
A Tunnel diode also called Esaki diode is a heavily heavily
doped P-N junction diode whose
doping concentration is one part in 103.
In the case of lightly doped P-N junction diode, the
Fermi level lies inside the forbidden energy gap. But in the
case of heavily heavily doped P-N
junction diode, the Fermi level lies outside forbidden energy
gap. In heavily heavily doped n
type material, the Fermi level lies in the conduction band. In
heavily heavily doped P type
semiconductor, the Fermi level lies in the valence band.
Under open circuit conditions, the energy band diagram of a
heavily heavily doped P-N
junction diode is shown in the following figure (a). The Fermi
level in the P side is at same
energy as the Fermi level in the N side. Above the Fermi level
in the valence band in P side
indicates completely filled with holes. Below the Fermi level in
the conduction band in N
side indicates completely filled with free electrons.
Reverse Biased Condition: Let us consider that the P type
semiconductor is grounded and
that a voltage applied across a diode shift the N side with
respect to the P side. If a reverse
bias voltage is applied across the tunnel diode, the height of
barrier is increased above its
open circuit value . Hence N side levels must shift downward
with respect to the P side
levels as shown in the following figure (b). Hence, electrons
will tunnel from the P side to the
N side, giving rise to a reverse diode current. As the magnitude
of the reverse bias increases,
the reverse current also increases as shown in the following
figure (c).
Forward Bias Condition: If a forward bias voltage is applied
across a tunnel diode, the
height of barrier is decreased below its open circuit value .
Hence N side levels must shift
upward with respect to the P side levels as shown in the
following figure (D). Resultant,
electrons will tunnel from the N side to the P side, giving rise
to a forward diode current as
shown below figure (C).
As the forward bias is increased further, the condition is shown
in the following figure (E).
Now maximum number of electrons will tunnel from the N side to
the P side, giving rise to
the peak current as shown in the following figure (C). If still
more forward bias voltage is
applied, the condition is shown in the following figure (F). Now
number of electrons will
tunnel from the N side to the P side decreases. Resultant, the
tunnelling current decreases as
shown in the following figure (C). On further increasing forward
bias voltage, the condition
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is shown in the following figure (G). Now electrons will not
tunnel from the N side to the P
side, giving rise to zero current as shown in the following
figure (C).
The solid curve gives the tunnelling current which is shown in
the following figure (C). But
in addition to this, the P-N junction diode current also flows
and is shown by dotted lines in
the following figure (c0. The sum these two currents is the
tunnel diode current which is
shown in the following figure (H). The symbol for tunnel diode
is shown in the following
figure (I).