Transcript KIRCHHOF’S LAW KIRCHOOF’S CURRENT LAW

BSIC SEMICOCONDUCTOR
CONCEPTS
INTRINSIC SILICON:
 A crystal of pure or intrinsic silicon
has a regular lattice structure. Where
the atoms are held at their fixed
positions by bonds, called Covalent
bonds, formed by four valence
electrons associated with four each
silicon atom.
BSIC SEMICOCONDUCTOR CONCEPTS
INTRINSIC SILICON:

At sufficiently low temperature are
covalent bond are intact and not( very
few) free electrons are available to
conduct electric current.
INTRINSIC SILICON:

Thermal ionization results in free
electrons and holes in equal numbers
and hence equal concentrations.
These free electrons and holes move
randomly through silicon crystal
structure, and in this process some
electrons may fill some of the holes.
INTRINSIC SILICON:
This process, called RECOMBINATION.
 It results in disappearance of free
electrons and holes. The recombination
rate is proportional to the number of free
electrons and holes, which in turn is
determined by ionization rate.

Drift
The process whereby charged particles
move under the influence of electric field.
Diffusion
The process of flow of particles from
a region of high concentration to a
region of low concentration.
Diffusion Current
The current that results from the
diffusion of charged particles.
Drift Current
The current that results from the drift of
charged particles.
Drift Velocity
The average velocity of charged particles
in the presence of an electric field.
INTRINSIC SILICON
The ionization is a strong function of
temperature.
 In thermal equilibrium the
recombination rate is equal to the
ionization or thermal-generation rate.

DOPED SEMICONDUCTORS.
Doped semiconductors are the
materials in which carrier of one kind
(electrons or holes) predominate.
 Doped silicon in which the majority of
charge carriers are negatively
charged electrons is called n type.

DOPED SEMICONDUCTORS.
While silicon doped so that majority of
charge carriers are positively charged
holes is called p type.
 Doping of a silicon to turn it into p
type or n type is achieved by a small
number of impurity atoms.

DOPED SEMICONDUCTORS.

For instance, introducing impurity
atoms of a penta-valent element such
as phosphorus results in n type
silicon.
NO BIAS
THE DIFFUSION CURRENT ID

Because the concentration of holes is
high in the p region and low in the n
region, holes diffuse across the
junction from the p region to the n
region.
THE DIFFUSION CURRENT ID
Similarly, electrons diffuse the
junction from n side to p side.
 These two current components add
together to form the diffusion current
ID .Whose direction is from p side to n
side, as indicated in the figure.

THE DEPLETION REGION

The electrons that diffuse across the
region quickly recombine with
recombine with some of the majority
holes present in the p region and thus
disappear from the scene.
THE DEPLETION REGION

This results also in disappearance of
some majority holes, causing some of
the bound negative charge to be
uncovered (i-e no longer neutralized
by holes). Thus in the p material close
to the junction that is depleted of free
electrons.
THE DEPLETION REGION
This p region will contain uncovered
bound negative charge, as indicated
in the figure.
 From the above it follows that a
carrier depletion region will exist on
both sides of the junction.

THE DEPLETION REGION

With n side of this region positively
charged and p side negatively
charged. This carrier depletion region
or simply, depletion region is also
called the SPACE CHARGE
REGION.
THE pn JUNCTION UNDER
REVERSE BIAS CONDITION.

The pn junction is
excited by a
constant current
source I in the
reverse direction.
To avoid
breakdown, I kept
smaller than IS.
THE pn JUNCTION UNDER
REVERSE BIAS CONDITION.
Note that the
depletion layer
widens and the
barrier voltage
increases by VR
volts, which
appears between
the terminals as
reverse voltage.

THE pn JUNCTION UNDER
REVERSE BIAS CONDITION.

The current I will
be carried by
electrons flowing in
the external circuit
from the n
material to p
material (that is, in
direction opposite
to that of I).
THE pn JUNCTION UNDER
REVERSE BIAS CONDITION

This will cause electrons to leave the
n material and holes to leave p
material . Thus reverse current I will
result in an increase in the width of,
and the charge stored in the
depletion layer. Which will increase
the voltage across depletion region.
THE pn JUNCTION UNDER
FORWARD BIAS CONDITION.

The pn junction is
excited by a
constant current
source supplying a
current I in forward
direction. The
depletion layer
narrows and
barrier voltage
THE pn JUNCTION UNDER
FORWARD BIAS CONDITION.
decreases by V
volts, which
appears as an
external voltage in
the forward
direction.
THE pn JUNCTION UNDER
FORWARD BIAS CONDITION.

This current causes
majority carriers to be
supplied to both sides
of the junction by the
external circuit. Holes
to the p material and
electrons to the n
material.
THE pn JUNCTION UNDER
FORWARD BIAS CONDITION.

These majority carriers will
neutralized some of the uncovered,
causing less charge to be stored in
the depletion region. Thus the
depletion layer narrows and the
depletion barrier voltage reduces.
THE pn JUNCTION UNDER
FORWARD BIAS CONDITION.

This reduction in voltage cause more
electrons to move from n side to p
side and more holes to move from p
side to n side. So that diffusion
currents increases until equilibrium is
achieved.