My First Ppt presentation of ies college bhopal

1. ECE 663
PPT Presention
Of PN Juction

2. ECE 663
P-N Junctions

3. ECE 663
• So far we learned the basics of semiconductor physics, culminating
in the Minority Carrier Diffusion Equation
• We now encounter our simplest electronic device, a diode
• Understanding the principle requires the ability to draw band-
diagrams
• Making this quantitative requires ability to solve MCDE
(only exponentials!)
• Here we only do the equilibrium analysis

4. ECE 663
P-N junction diode
V
I

5. ECE 663
P-N junction diode
V
I
I = I0(e
qV/kT
-1)
pn v
I
p
0 = q(ni
2/ND) (Lp/p)

6. ECE 663
P-N Junctions - Equilibrium
<= P-type, low EF
- = fixed ionized acceptors
+ = mobile holes, p
What happens when these bandstructures collide?
• Fermi energy must be constant at equilibrium, so bands
must bend near interface
• Far from the interface, bandstructures must revert

7. ECE 663
Time < 0
P-type piece N-type piece
Time < 0: Pieces separated

8. ECE 663
At time = 0, slam the two pieces together

9. ECE 663
Hole gradient
Jp, diffusion = -qDp dp/dx = current right, holes right
Electron gradient
Jn,diffusion = -qDn dn/dx = current right, electrons right
Gradients drive diffusion
left

10. ECE 663
Gradients drive diffusion

11. ECE 663
But charges can’t venture too
far from the interface because
their Coulomb forces pull them back!
++
- -
+
+
+
+
- -
- -
- -
- -
-
+
+
+
++

12. ECE 663
Separation of a sea of charge, leaving
behind a charge depleted region

13. ECE 663
V (stopping e- flow)
V (stopping e+ flow)
Resulting in a barrier across a depletion region

14. ECE 663
E
E
Depletion
Region

15. ECE 663
How much is the Built-in Voltage?
Right
i
F
Left
F
i
bi E
E
E
E
qV )
(
)
( −
+
−
=
P side N side






=
−
=

−
i
a
Left
F
i
kT
E
E
i
a
a
n
N
kT
E
E
e
n
N
N
p
F
i
ln
)
(
)
(






=
−
=

−
i
d
Right
i
F
kT
E
E
i
d
d
n
N
kT
E
E
e
n
N
N
n
i
F
ln
)
(
)
(

16. ECE 663








=







+






=
2
ln
ln
ln
i
d
a
bi
i
d
i
a
bi
n
N
N
q
kT
V
n
N
q
kT
n
N
q
kT
V
Na acceptor level on the p side
Nd donor level on the n side
How much is the Built-in Voltage?

17. ECE 663
Special case: One-sided Junctions
• One side very heavily doped so that Fermi level is at band edge.
• e.g. p+-n junction (heavy B implant into lightly doped substrate)






+
=







=
−
=
−

−
i
d
G
bi
i
D
Right
i
F
G
V
i
Left
F
i
n
N
q
kT
q
E
V
n
N
kT
E
E
E
E
E
E
E
ln
2
ln
)
(
2
/
)
(

18. ECE 663
How wide is the depletion region?
kT
E
E
i
F
i
e
n
p
)
( −
= kT
E
E
i
i
F
e
n
n
)
( −
=

19. ECE 663
Depletion Approximation-step junction
 x

20. ECE 663
Depletion approximation-step junction
Exponentials replaced with step-functions

21. ECE 663
Doping
Charge
Density
Electric
Field
Electrostatic
Potential
NAxp = NDxn
= WD/(NA
-1 + ND
-1)
Ks0Em = -qNAxp = -qNDxn
= -qWD/(NA
-1 + ND
-1)
Vbi = ½|Em|WD

22. ECE 663
Maximum Field
Em = 2qVbi/ks0(NA
-1+ND
-1)

23. ECE 663
How far does Wd extend into each junction?
Depletion width on the n-side depends on the doping on the p-side
Depletion width on the p-side depends on the doping on the n-side
e.g. if NA>>ND then xn>>xp One-sided junction






+
=





 +
=
D
A
A
n
A
D
A
n
N
N
N
W
x
or
N
N
N
x
W






+
=





 +






=





 +
=
D
A
D
p
A
D
A
D
A
p
A
D
A
n
N
N
N
W
x
or
N
N
N
N
N
x
N
N
N
x
W

24. ECE 663

25. ECE 663
"P+ - N" => N a >> Nd
"P - N+" => N a << Nd

26. ECE 663
P-N Junction with applied voltage

27. ECE 663
Reverse Bias
• +Voltage to the n side and –Voltage to the p side:
holes (+) out of P-side
Vapplied is sucking more:
electrons (-) out of N-side
Depletion region will be larger

28. ECE 663
Reverse Bias Band Diagram

29. ECE 663
Reverse Bias depletion
( )( )
2
1
0
2






+
+

= rev
bi
A
D
D
A
S
V
V
N
N
N
N
q
K
W
( ) 





+
=
D
A
A
rev
n
N
N
N
V
W
x ( ) 





+
=
D
A
D
rev
p
N
N
N
V
W
x
Applied voltage disturbs equilibrium EF no longer constant
Reverse bias adds to the effect of built-in voltage

30. ECE 663
Forward Bias
holes (+) out of P-side
Vapplied is sucking more:
electrons (-) out of N-side
+ -
Negative voltage to n side positive to p side
More electrons supplied to n, more holes to p
Depletion region gets smaller

31. ECE 663
Forward Bias Depletion
( )( )
2
1
0
2






−
+

= fwd
bi
A
D
D
A
S
V
V
N
N
N
N
q
K
W
( ) 





+
=
D
A
A
fwd
n
N
N
N
V
W
x ( ) 





+
=
D
A
D
fwd
p
N
N
N
V
W
x

32. ECE 663
General Expression
• Convention = Vappl= + for forward bias
Vappl= - for reverse bias
( )( )
2
1
0
2






−
+

= appl
bi
A
D
D
A
S
V
V
N
N
N
N
q
K
W
( ) 





+
=
D
A
A
appl
n
N
N
N
V
W
x ( ) 





+
=
D
A
D
appl
p
N
N
N
V
W
x

33. ECE 663
Bands = plots of electron energy
Voltage = potential energy per (+) charge
Positive voltage pulls bands down- bands are plots of electron energy
Fermi level is not constant  Current Flow
Fn
Fp
n = nie(Fn-Ei)/kT
p = nie(Ei-Fp)/kT

34. In summary
A p-n junction at equilibrium sees a depletion width
and a built-in potential barrier. Their values depend on
the individual doping concentrations
Forward biasing the junction shrinks the depletion width
and the barrier, allowing thermionic emission and higher
current. The current is driven by the splitting of the
quasi-Fermi levels
Reverse biasing the junction extends the depletion width
and the barrier, cutting off current and creating a strong
I-V asymmetry
In the next lecture, we’ll make this analysis quantitative
by solving the MCDE with suitable boundary conditions

35. ECE 663
Thank you
Presented by –Harishankar
Chaurasia