2. p-n Junction
Physical Structure
◼ pn junction is formed
within a single silicon
crystal by creating
regions of different
dopings (p and n
regions).
2
5. ◼ These two current components
add together to form the
diffusion current ID, whose
direction is from the p side to
the n side.
p-n Junction
The Diffusion Current ID
• Concentration of holes is
high in the p region and low
in the n region, holes diffuse
across the junction from the
p side to the n side;
• Similarly, electrons diffuse
across the junction from the
n side to the p side.
5
6. ◼ In the p material close to
the junction, there will be
a region depleted of
holes and containing
uncovered bound
negative charge (-ve
ions)
6
p-n Junction
The Depletion Region
7. ◼ In the n material close
to the junction, there
will be a region
depleted of electrons
and containing
uncovered bound
positive charge (+ve
ion) 7
p-n Junction
The Depletion Region
9. ◼ carrier-depletion region
◼ depletion region (DR)
◼ space-charge Region
9
p-n Junction
The Depletion Region
❑ The charges on both sides of the
DR cause an electric field E to be
established
❑ Potential difference results across
the DR
❑ The resulting electric field
opposes the diffusion of holes
into the n region and electrons
into the p region
11. ◼ Two current components:
electrons moved by drift from
p to n and holes moved by
drift from n to p
◼ Add together to form the drift
current IS, whose direction is
from the n side to the p side
of the junction,
11
p-n Junction
The Drift Current Is
12. Diffusion Current IDiff
◼ Diffusion current happen
due to majority charge
carriers
◼ ID: depends strongly on
the voltage drop V0 across
the depletion region.
Why??
12
◼ Under a forward bias, the
electric field strength is
reduced. Therefore ,each
type of charge carriers can
diffuse more easily, which
means that the diffusion
currents for both types of
carriers increase under a
forward bias.
13. Drift current Is
◼ Drift current is happen due
to thermally generated
minority carriers, its value
is strongly dependent on
temperature; however, it is
independent of the value
of the depletion-layer
voltage V0.
13
➢ Drift current happen due
to minority charge carriers
➢ The drift current is
independent of the
biasing,
➢ Drift current is
temperature dependent.
➢ Why??
15. ◼ Equilibrium state :
Drift component
= Diffusion component
Net current=0
15
p-n Junction
(1) Equilibrium state (No biasing)
16. 1) The +ve terminal will repel the
holes, while –ve terminal will
repel the electrons.
2) Recombination between
immobile ions and charging
carriers
3) Width of depletion region and
barrier voltage will be affected
16
p-n Junction
(2) Forward Biasing
17. 17
p-n Junction
(2) Forward Biasing
◼ Depletion region width
decreases
◼ Net current I=ID-Is
◼ IDiff increased
◼ Is remain the same
◼ Current flow across the junction
through external circuit
Electrons and holes can
cross the junction and ID
increased
exponentially..
18. 1) +ve terminal of battery
will attract electrons
while –ve terminal will
attract holes
2) Uncovered immobile ions
will start to appear in
both regions
18
p-n Junction
(3) Reverse Biasing
19. p-n Junction
(3) Reverse Biasing
▪ Increased the barrier voltage
▪ Increase DR width
▪ IDiffu reduced to almost zero
▪ net current across the junction
becomes the very small reverse
current IS .
If the reverse voltage is increased in magnitude to a value specific to
the particular junction, the junction breaks down, and a large reverse
current flows. The value of the reverse current must be limited by the
external circuit(external resistance ) 19
