Transcript Reversed Biased PN junction

Semiconductor Devices
Lecture 05
Prepared by: Eng.Maira Naz
Date:06-06-11
Lecture Contents:
PN junction(Diode)
Forward biased PN junction.
Hole current in forward biased PN junction.
Electron current in forward biased PN junction.
Reverse biased PN junction.
Peak Inverse voltage.
Reverse saturation current.
Hole current in reverse bias PN junction.
PN Junction (Diode)
 We begin our study of semiconductor devices with
pn junction for three reasons.
The device finds application in many electronic
devices, e.g. in adaptors that charge the batteries of
cell phones
The pn junction is among the simplest semiconductor
devices, thus provide a good entry point into the
study of the operation of such complex structures as
transistors.
The pn junction also serves as part of transistors.
We also use the term “diode” to refer to pn junction.
PN Junction (Diode)
A pn junction is formed when the acceptor impurities
are introduced into one side and donor impurities are
introduced into the other side of a single crystal
structure of a semiconductor.
Single crystal means that the continuity of structure is
maintained.
When a single crystal of semiconductor contains both
the impurities, the new material we get is called as
PN-junction.
PN Junction (Diode)
The dotted cross sectional area in the middle of
junction is called a metallurgical junction where the
effective doping densities changes fro acceptor to
donor.
PN-junction is not created by joining together P-type
and N-type semiconductors because such a creation
cannot result into a single crystal structure.
PN-junctions are formed by carefully controlled
manufacturing process.
PN Junction (Diode)
PN Junction (Diode)
 Only one dimensional behavior will be considered,
meaning the charge carrier density is a function of x only.
 The doping levels of P and N-regions are uniform, i.e.
NA=ND.
 The transition from NA to ND is abrupt near the junction
meaning semiconductor changes its characteristics from
P-type to N-type at the boundary where the concentration
of either P-type or N-type is zero.
 This is a called as step-graded junction.
PN Junction (Diode)
Fig shows the virgin PN-junction (no external voltage
is applied) just after PN-junction is manufactured.
Holes are the majority charge carriers in P-region
while the electrons are minority charge carriers in Pregion.
Electrons are the majority charge carriers in N-region
while the holes are minority charge carriers in Nregion.
PN Junction (Diode)
PN Junction (Diode)
So holes (or electrons) having a tendency to move
from the region of higher concentration towards the
region of lower concentration (due to diffusion).
So, the diffusion current will occur immediately just
after the formation of PN-junction.
The total diffusion current will bee the sum of two.
PN Junction (Diode)
Once, the holes diffuse across the junction from P to
N-region they are surrounded by a lot of electrons
and are quickly neutralized and will disappear from
the scene.
In identical manner electrons diffuse into P-region
from N-region.
They immediately find a block of holes in P-region,
they fall into them and vanish from arena.
PN Junction (Diode)
 This means N-region will be deprived of electrons
near the junction.
 Similarly P-side will loose an equal number of holes
near the junction.
 This will eventually produces a thin layer of equal but
opposite charges near the junction.
 The thin charge region across the junction is termed
as depletion charge layer.
 It is also called as space charge layer, dipole layer or
transition zone.
PN Junction (Diode)
PN Junction (Diode)
 Consequently, a potential barrier, Vo, is created across
the junction.
 Because of this Vo, electric field will develop ant its
direction will be from N to P-region.
 The development of this electric field will give a birth
to another phenomena i.e. drift current.
 The direction of electric field support the flow of
minority carriers.
 So, the movement of minority current is due to drift
current.
PN Junction (Diode)
PN Junction (Diode)
PN Junction (Diode)
 Since, no external dc battery is connected to PNjunction, So, the net current is zero.
 This means the diffusion and drift current in an
unbiased PN-junction are equal and opposite in
direction

I diffusion = -I drift
 This means both the currents are in opposite
direction.
PN Junction (Diode)
PN Junction (Diode)
Equilibrium condition:
If diffusion current is going to increase, its means
holes cross junction from P to N-region.
This will cause an expansion in space charge layer.
As a result, Vo and associated electric field also
increase.
PN Junction (Diode)
So, a sort of equilibrium condition will always prevail
and there will be no net current flow in an un biased
PN-junction.
Potential barrier across PN-junction is typically 0.6 to
0.7V for silicon and 0.2 to 0.3V for germanium.
Forward Biased PN junction
When the positive terminal of battery is connected to
P-side and negative terminal to the N-side of diode, it
is said to be forward biased.
In this condition, diode is capable of carrying a
substantial amount of current and is said to be in on
state.
Forward Biased PN junction
Forward Biased PN junction
If the magnitude of battery is more than the internal
built in potential barrier, Vo, then the negative side of
battery will repel the free electrons of N-region
towards he junction.
Similarly, positive side of battery will repel the holes
of P-region towards the junction.
This correspond to the majority carrier current.
Forward Biased PN junction
 So, holes from P-region are compelled to move into
depletion layer, they will neutralize some electrons.
 Similarly, the electrons from N-region are compelled
to move into depletion layer, they will neutralize
some holes.
 Consequently depletion layer will reduce.
Forward Biased PN junction
Forward Biased PN junction
Hole current:
Forward Biased PN junction
Electron current:
Reversed Biased PN junction
When positive terminal of battery “VBB” is connected
to N-region and the negative terminal of battery is
connected to the P-region, then the diode is said to be
reverse biased.
In, an reverse biased diode majority current does not
flow.
Instead of that minority current can flow.
Reversed Biased PN junction
The free electrons in N-region are attracted by the
positive terminal of battery and they move away from
the junction.
In a similar manner, free hoes in P-region are
attracted by negative terminal of battery and move
away from the junction.
So, they leave behind immobile ions.
They are negative ions in P-region and positive ions
in N-region.
Reversed Biased PN junction
This corresponds to the widening of space charge
layer across the junction.
So, the potential barrier becomes large compared to
unbiased PN-junction.
Reversed Biased PN junction
Reversed Biased PN junction
Punch through:
 Whatever the magnitude of battery, it will simply
appear across the space charge layer as reverse bias.
 At a sufficiently large reverse bias the space charge
layer can expand so much that it touches the
boundaries of PN-junction.
 This phenomena is called as punch through and
should be avoided.
Reversed Biased PN junction
Reversed Biased PN junction
Peak reverse voltage:
Various diodes having the capability of withstanding
various amount of reverse bias.
They are rated by device manufacturer.
Typical values of reverse bias rating of diodes may
range from a tens of volts to thousands of volt.
Diodes are selected for use in practical circuits
keeping in view their reverse bias rating.
Reversed Biased PN junction
Peak reverse voltage (Cont..):
Low voltage circuits such as used in radio receivers
and tape recorders will require diodes with low
reverse bias rating.
High voltage circuits used in television receivers and
oscilloscope will deploy diodes with high reverse bias
voltage.
Reverse bias rating of diode is termed as Peak reverse
voltage(PRV) Or Peak inverse voltage(PIR).
Reversed Biased PN junction
Reverse saturation current:
 The increased space charge layer will also increase the
intensity of associated electric field.
 The direction of electric field aids the flow of minority
charge carriers across the junction.
 Minority charge carriers i.e. holes in N-region and
electrons in P-region cross the space charge layer and go
to the other side.
 At the same time, the increased space charge layer and its
increase electric field will completely stop the majority
carriers to migrate across the junction.
Reversed Biased PN junction
Reverse saturation current (Cont..):
Therefore, the majority electrons in N-region will
face an enhanced opposition from the electric field
intensity of reverse bias diode.
These electrons will not be able to overcome this
opposition.
Hence, they will never cross the junction and migrate
to P-region.
Same situation is valid for holes in P-region.
Reversed Biased PN junction
Reverse saturation current (Cont..):
Reverse bias diode does not support the majority
carrier current.
It will allow the minority carrier current to flow
across the junction.
This current is called as reverse current or reverse
saturation current or reverse leakage current.
Its magnitude is smaller than the forward majority
current of forward biased diode.
Reversed Biased PN junction
Why the silicon semiconductors are used frequently in the
manufacturing of diodes and transistors?
The generation of minority charge carriers in
semiconductors is highly temperature dependent and
increased at elevated temperature.
Therefore, the minority current of reverse bias diode
is also temperature dependent.
The reverse current in small purpose silicon diodes is
typically in the range of nano-amperes and can be
ignored.
Reversed Biased PN junction
It may be of the order of tens of micro-amperes for
medium sized silicon diodes.
It is not as frivolous even in small purpose
germanium diode.
It may be in the range of tens of micro-amperes to
hundred of micro-amperes.
Because of this low reverse current, silicon
semiconductors are replacing germanium diode in the
manufacture of diodes and transistors.
Reversed Biased PN junction
Hole current: