principles of semiconductor devices-l21
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8/8/2019 Principles of Semiconductor Devices-L21
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www.nanohub.org
NCN
Lecture 21: p‐n Diode I‐V CharacteristicsMuhammad Ashraful Alam
.
Alam ECE‐606 S09 1
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Outline
1) Derivation of
the
forward
bias
formula
2) Solution in the nonlinear regime
3) I‐V in the ambipolar regime
4 Conclusion
.
,
Alam ECE‐606 S09 2
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Topic Map
E uilibrium DC Small Lar e Circuits
signal SignalDiode
Schottky
BJT/HBT
MOSFET
Alam ECE‐606 S09 3
8/8/2019 Principles of Semiconductor Devices-L21
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Continuity Equations for p‐n junction Diode
+ −• = − − D A
1n∂ N N N t q
= −∂
J μ = + ∇qn E qD n
1P P P
pr g
∂ = ∇ • − +J
Will focus on this part today …
t q
P P Pqp E qD pμ = − ∇J
Alam ECE‐606 S09 4
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Applying a Bias: Poisson Equation
qV bi
E F -E V
E C -E F
q(V bi - V )
C - n
F p -E V
-q V
Alam ECE‐606 S09 5
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Depletion Widths
GND-V DN AN
xn xp
02 ( )s Abi n k N V V x ε = −
22 AD n pqNqN x x − =
D A pn D A D
0 02 2s sk k ε ε ( )0 ( )s D bi
A A D p V q N N N V = −+
Alam ECE‐606 S09 6
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Flat Quasi ‐Fermi Level up to Junction
E C
E V
E C
Jn
E V
Alam ECE‐606 S09 7
p
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Various Regions of I‐V Characteristics
ln I
n ~ A Bk T
. us on m e
2. Ambipolar transport 2
3. High injection
- 6,7
5. Breakdown A
1
6. Trap-assisted R-G
7. Esaki Tunneling
4
5
8Alam ECE‐606 S09
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Recall: One Sided Minority DiffusionSteady
stateAcceptor doped
Can calculate current q V bi
‐V anywhere, let us solvethe problem where it is the easiest …
1∂ d n J
F p‐E V
‐
= − +∂ n nr g
t q dx
μ = +n nn
dnqn qD Jdx
E 2
20 =n
d nDdx
Alam ECE‐606 S09 9
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Boundary Conditions0 0 0
+ + +
Δ = −n n n
( )( 0 ) β −+= = n i F E i n x n e ( )
2
1 β
=
= −
G G
AqV i ne
N( )
( 0 )β − −+
= =p i F E
i p x n e
( )2 2 β β −= = pn AqV i i
F F n e n en p q(V bi -V A )
F p F n -V A(0 )+ =
A p N
2
(0 ) β + = Aqi
A
V n eN
n AN
Alam ECE‐606 S09 10
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Right Boundary Condition
2
( ) inn x W = ≈
( ) 0 A
pn x W Δ = =
C
V
E V
Alam ECE‐606 S09 11
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Example: One Sided Minority Diffusion
2
20
N D
dx=
( , )n x t C DxΔ = +
2
, ( ) 0 p p p x W n x W C DW = Δ = = ⇒ = −
0 ', ( 0) 1 AqV i
A
x n x e C N
β = Δ = = − =2 ⎛ ⎞
12
( , ) 1 1 Aqi
A p
n x t e N W
Δ = − −⎜ ⎟
⎝ ⎠
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Electron & Hole Fluxes
2 AqV in x⎛ ⎞
− − β n D n= + ∇E
A p N W
⎝ ⎠
( )
2
0 1 AqV n i
n n x A
qD ndn
J qD edx W N β
=
= = − −
Δn
F p
n
Δp
2
Alam ECE‐606 S09 13
( )0'
1 A p qV i p p
x n D
n p J qD e
dx W N
β
=
= − = − −
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Total Current
( )22
1 An i
p
qV T
p i
D A n
D nW N
D nW N
J q e β ⎡ ⎤
= − + −⎢ ⎥⎢ ⎥⎣ ⎦ln ln . J V k T const ≈ +
Forward Bias
Reverse BiasΔn .T J const ≈
ln(I)(2)
(3)F p
n
VA(4)Δp
Alam ECE‐606 S09 14
(5)
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Outline
1 Derivation of the forward bias formula
2) Solution in the nonlinear regime
‐
4) Conclusion
Alam ECE‐606 S09 15
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Nonlinear Regime (3) …
( )2 2
n p AqV pn i i F F D D n n β −Δ Δ−⎡ ⎤= − − ( ) A pn J q V a b J β − −= −
p A n DW N W N ⎢ ⎥
⎣ ⎦
0
2
1
6,7
16
A
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Flat Quasi ‐Fermi Level up to Junction ?
N N N
dnqn qDμ = +J E n n n
n n n
dF J W J F nμ = ⇒ Δ =
dF dn− −
n D
n n iii N i N n n e q q
dx dxn e= = −
⎢ ⎥⎣ ⎦⎣ ⎦
F F n
n N N
dF dnqD qD
dx dxn β ⎡ ⎤= −⎢ ⎥⎣ ⎦
E
n N B N
n
dF D T q ndx q
μ μ
= − =⎢ ⎥⎣ ⎦
∵E
Alam ECE‐606 S09 17
Wn
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Forward Bias: Nonlinear Regime …
2( )(0 ) n pi
junctio
F F
n
nn e β −+ = ( ) ( )( )
2 2
(0 ) 1 p A An pnqV qV i F F F i F n ne n e
N N β β Δ Δ− − − −Δ + Δ= ⇒ Δ = −
( )2 2
n p AqV pn i i F F D D n n β −Δ Δ−⎡ ⎤− −
T p A n DW N W N ⎢ ⎥⎣ ⎦
Δn
n n
n Dn
J W N
F μ
Δ =ΔF
ΔFnV
p p n
n D
F N
J W μ
Δ =
Alam ECE‐606 S09 18
pprox: t us on om nate transport
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Outline
1) Derivation of the forward bias formula
2) Solution in the nonlinear regime
3) I‐V in the ambipolar regime
4) Tunneling and I‐V characteristics
5 Conclusion
Alam ECE‐606 S09 19
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Region (2): Ambipolar Transport
D − −⎡ ⎤
ln(2
) A n pnT i T
p n B
J q n e J W W k T
≈ − + ≈⎢ ⎥⎣ ⎦
2
1
6,7
20
A
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Nonlinear Regime: Ambipolar Transport
( )2
2
n pF F i
q V F F
np n e
n
β
β
−
−Δ −Δ
=
A i
A
n
N
pn e+ =+ −
( ) / 2
1 A n p
A n p
i
q V F F
n e pn
β
− −
−Δ −Δ≈
Δ = −Δ≈
( ) / 2 A n pqV F F n in n
qD nn J qD e
W W β −Δ −ΔΔ= − =
( ) / 2 A n pqV F F p i p p
n n
qD nn J qD e
W W β −Δ −ΔΔ= − =
Alam ECE‐606 S09 21
Note: junction never disappears!
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Outline
1) Derivation of the forward bias formula
2) Solution in the nonlinear regime
3) I‐V in the ambipolar regime4) Tunneling and I‐V characteristics
5) Conclusion
Alam ECE‐606 S09 22
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Forward Bias Nonlinearity (7): Esaki Diode
3ln(I)
F n F p
1
F n
VA
6,7
emptyp
Heavy doping
F n F
X1No states!
Alam ECE‐606 S09
VA
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Reverse Bias (5): Zener Tunneling
+VA
F n
p
4empty
5
4
= υ I qpT empty
2 2
4cosh sinhα⎛ ⎞α + − α⎜ ⎟α⎝ ⎠
k d d k
Alam ECE‐606 S09 24
.
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Conclusion
1) I‐V characteristics of a p‐n junction is defined by many
interesting phenomena including diffusion, ambipolar
2) The separate regions are identified by specific features.
w y , w
other mechanism is dominated for a given technology.
3) In the next class, we will discuss a few more non ‐ideal effects.
Alam ECE‐606 S09 25
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