ece 340 lecture 15 and 16: diffusion of...
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ECE 340Lecture 15 and 16: Diffusion
of Carriers
Class Outline:
•Diffusion Processes•Diffusion and Drift of Carriers
• Why do carriers diffuse?• What happens when we add an electric
field to our carrier gradient?• How can I visualize this from a band
diagram?• What is the general effect of including
recombination in our considerations?• What is the relationship between
diffusion and mobility?M. J. Gilbert ECE 440 – Lecture 1 5 and 1 6 9/26/1 1
Things you should know when you leave…
Key Questions
M. J. Gilbert ECE 440 – Lecture 1 5 and 1 6 9/26/1 1
Diffusion ProcessesWhat happens when we have a concentration discontinuity??
Consider a situation where we spray perfume in the corner of a room…
•If there is no convection or motion of air, then the scent spreads by diffusion.
•This is due to the random motion of particles.•Particles move randomly until they collide with an air molecule which changes it’s direction.•If the motion is truly random, then a particle sitting in some volume has equal probabilities of moving into or out of the volume at some time interval.
T = 0
T1 = 0
T2 = 0T3 = 0
Shouldn’t the same thing happen in a semiconductor if we have spatial gradients of carriers?
M. J. Gilbert ECE 440 – Lecture 1 5 and 1 6 9/26/1 1
Diffusion ProcessesLet’s shine light on a localized part of a semiconductor…
Now let’s monitor the system…
•Assume thermal motion .•Carriers move by interacting with the lattice or impurities.•Thermal motion causes particles to jump to an adjacent compartment.•After the mean-free time (τc), half of particles will leave and half will remain a certain volume.
1024
512384
1024
512 512 256 256384
128 128
320 256256
192
0=t ct τ= ct τ2= ct τ3=
ct τ6=
•Process continues until uniform concentration.•We must have a concentration gradient for diffusion to start.
M. J. Gilbert ECE 440 – Lecture 1 5 and 1 6 9/26/1 1
Diffusion ProcessesHow do we describe this physical process??
We want to calculate the rate at which electrons diffuse in a simple one-dimensional example. Consider an arbitrary electron distribution…
λ λ λ
•Divide the distribution into incremental distances of the mean-free path (λ).•Evaluate n(x) in the center of the segments.•Electrons on the left of x0have a 50% chance of moving left or right in a time, τc.•Same is true for electrons to the right of x0.
( ) ( )AnAn λλ 21 21
21
−Net # of electrons moving from left to right in one τc.
M. J. Gilbert ECE 440 – Lecture 1 5 and 1 6 9/26/1 1
Diffusion ProcessesSo we have a flux of particles…
λ λ
The rate of electron flow in the +x direction (per unit area):
( )212nn
cn −=
τλφ
Since the mean-free path is a small differential length, we can write the electron difference as:
( ) ( )λx
xxnxnnn∆
∆+−=− 21
In the limit of small Δx, or small mean-free path between collisions…
=nφ
( ) ( ) ( )
( )dx
xdnx
xxnxnx
c
xc
n
τλ
τλφ
2
0
2
lim
−=
∆∆+−
=→∆
Diffusion coefficient(cm2/sec)
M. J. Gilbert ECE 440 – Lecture 1 5 and 1 6 9/26/1 1
Diffusion ProcessesBut we already expected this…
Define the carrier flux for electrons and holes:
( ) ( )
( ) ( )dx
xdpDx
dxxdnDx
pp
nn
−=
−=
φ
φ
And the corresponding current densities associated with diffusion…
( )dx
xdnqDJ nndiff =
( )dx
xdpqDJ np
diff −=
Carriers move together, currents opposite directions.
M. J. Gilbert ECE 440 – Lecture 1 5 and 1 6 9/26/1 1
Diffusion and Drift of CarriersHow do we handle a concentration gradient and an electric field?
n(x)
p(x)
x
E( ) pn JJxJ +=
The total current must be the sum of the electron and hole currents resulting from the drift and diffusion processes
( ) ( ) ( ) ( )
( ) ( ) ( ) ( )dx
xdpqDxExpqxJ
dxxdnqDxExnqxJ
ppn
nnn
−=
+=
µ
µ
Drift DiffusionWhere are the particles and currents flowing?
Electrons
Holes
e-
h+
Dashed Arrows = Particle Flow Solid Arrows = Resulting Currents
φp (diff and drift)
Jp (diff and drift)
φn (diff)
Jp (diff)
φn (drift)
Jn (drift)
M. J. Gilbert ECE 440 – Lecture 1 5 and 1 6 9/26/1 1
Diffusion and Drift of CarriersA few extra observations…
Dashed Arrows = Particle Flow Solid Arrows = Resulting Currents
φp (diff and drift)
Jp (diff and drift)
φn (diff)
Jp (diff)
φn (drift)
Jn (drift)
•Diffusion currents are in opposite directions.•Drift currents are in the same direction.•Currents depend on:
•Relative electron and hole concentrations.•Magnitude and directions of electric field.•Carrier gradients.
( ) ( ) ( ) ( )
( ) ( ) ( ) ( )dx
xdpqDxExpqxJ
dxxdnqDxExnqxJ
ppn
nnn
−=
+=
µ
µ •Diffusion currents can be large even if the carriers are in the minority by several orders of magnitude.•Not true for drift currents.
M. J. Gilbert ECE 440 – Lecture 1 5 and 1 6 9/26/1 1
Diffusion and Drift of CarriersCan we relate the diffusion coefficient to the mobility?
We can by using what we know about drift, diffusion, and band bending…
•In equilibrium, no current flows.•Any fluctuation that would begin a diffusion current also sets up an electric field which redistributes the carriers by drift.
( ) ( ) ( ) ( ) 0=+=dx
xdnqDxExnqxJ nnn µ
Solve for the electric field E(x): ( ) ( )( )
dxxdn
xnDxE
n
n 1µ
=
It’s equilibrium, so we know n(x): ( )( )
TkEE
ib
iF
enxn−
=
( ) ( )
nETk
qdxdEe
Tkn
dxxdn
b
iTkEE
b
i b
iF
−=−=− ( ) ( ) 0=− N
bn D
TkqqnEqnE µ
Assuming E is non-zero qTkDqTkD
b
P
P
b
N
N
=
=
µ
µ
M. J. Gilbert ECE 440 – Lecture 1 5 and 1 6 9/26/1 1
Diffusion and Drift of CarriersThese relations are called the Einstein relations…
qTkDqTkD
b
P
P
b
N
N
=
=
µ
µ
dxdE
qdxdE
qdxdE
q
VE
ivc 111===
−∇=
•The balance of drift and diffusion currents creates a built-in electric field to accompany any gradient in the bands.
•Gradients in the bands can occur at equilibrium when:
• the band gap varies.• alloy concentration varies. •dopant concentrations vary.
M. J. Gilbert ECE 440 – Lecture 1 5 and 1 6 9/26/1 1
Diffusion and Drift of CarriersRecall the previous example… Ec
Ei
Ef
Ev
Assume that:
•It is silicon maintained at 300 K.
•Ef – Ei = Eg/4 at ± L and Ef – Ei = Eg/4 at x = 0.
•Choose the Fermi level as the reference energy.
x
-L L0
( )refc EEq
V −−=1
x
-L L0
V
dxdE
qdxdE
qdxdE
q
VE
ivc 111===
−∇=
x-L L0
E
M. J. Gilbert ECE 440 – Lecture 1 5 and 1 6 9/26/1 1
Diffusion and Drift of CarriersQuestion: Is it in equilibrium? Ec
Ei
Ef
Ev
x
-L L0
Energy
Ef
Material 1DOS – N1(E)FD – f1(E)
Material 2DOS – N2(E)FD – f2(E)
•Assume two materials in intimate contact.•In thermal equilibrium.
•No current.•No net energy transfer.
•Carriers moving from 2 to 1 must be balanced by carriers moving from 1 to 2.
( ) ( ) ( ) ( )[ ]EfENEfEN 2211 1−•Rate1-2
( ) ( ) ( ) ( )[ ]EfENEfEN 1122 1−•Rate2-1
Rate1-2 = Rate2-1
Therefore… f1(E) = f2(E) Ef1 = Ef20=
dxdEF YES
M. J. Gilbert ECE 440 – Lecture 1 5 and 1 6 9/26/1 1
Diffusion and Drift of CarriersWhat are the electron and hole current densities at ± L/2:
Ec
Ei
Ef
Ev
x
-L L0
It is in equilibrium, so JP and JN = 0.
Roughly sketch n and p inside the sample:
x
-L L0
ni
p
n
M. J. Gilbert ECE 440 – Lecture 1 5 and 1 6 9/26/1 1
Diffusion and Drift of CarriersWhat are the electron diffusion current at ± L/2? If so, in what direction?
Ec
Ei
Ef
Ev
x
-L L0
0>dxdn
0<dxdn
There is a diffusion current at both L/2 and –L/2.
At –L/2: ndiffJ
At L/2: ndiffJ
What are the electron drift current at ± L/2? If so, in what direction?
EqnvqnJ ndndriftn µ=−=
ndriftJAt –L/2:
At L/2: ndriftJ
What is the diffusion coefficient?
Use Einstein relation q
TkD b
P
P =µ 11.9 cm2/sec
M. J. Gilbert ECE 440 – Lecture 1 5 and 1 6 9/26/1 1
Diffusion and RecombinationSo what does this mean?
Consider this semiconductor:•The hole current density leaving the differential area may be larger or smaller than the current density that enters the area.•This is a result of recombination and generation.•Net increase in hole concentration per unit time, dp/dt, is difference between hole flux per unit volume entering and leaving, minus the recombination rate.
M. J. Gilbert ECE 440 – Lecture 1 5 and 1 6 9/26/1 1
Diffusion and RecombinationHow can we explain this?
The net increase in hole concentration per unit time is the difference between the hole flux entering and leaving minus the recombination rate…
( )p
PP
xxx
px
xxJxJqt
pτ∆
−∆
∆+−=
∂∂
∆+→
)(1
Rate of hole buildup.
Increase in hole concentration in ΔxA per unit time.
Recombination rate
As Δx goes to zero, we can write the change in hole concentration as a derivative, just like in diffusion…
( )
( )N
N
P
P
nx
Jqt
nt
txn
px
Jqt
pt
txp
τ
τ∆
−∂∂
=∂∂
=∂
∂
∆−
∂∂
−=∂∂
=∂
∂
1,
1,Holes
ElectronsThese relations form the continuity equations.
M. J. Gilbert ECE 440 – Lecture 1 5 and 1 6 9/26/1 1
Diffusion and RecombinationAre there any simplifications?
If the current is carried mainly by diffusion (small drift) we can replace the currents in the continuity equation…
xpqDJ
xnqDJ
Pp
diff
Nndiff
∂∂
−=
∂∂
=
We put this back into the continuity equations…
PP
NN
pxpD
tp
nxnD
tn
τ
τ
∆−
∂∂
=∂∂
∆−
∂∂
=∂∂
2
2
2
2Diffusion equation for electrons
( ) ( ) ( ) ( )
( ) ( ) ( ) ( )dx
xdpqDxExpqxJ
dxxdnqDxExnqxJ
ppn
nnn
−=
+=
µ
µ
( )
( )N
N
P
P
nx
Jqt
nt
txn
px
Jqt
pt
txp
τ
τ∆
−∂∂
=∂∂
=∂
∂
∆−
∂∂
−=∂∂
=∂
∂
1,
1,
Diffusion equation for holes
Useful mathematical equation for many different physical situations…
M. J. Gilbert ECE 440 – Lecture 1 5 and 1 6 9/26/1 1
Steady State Carrier InjectionTo this point, we been assuming that the perturbation was removed…
What happens if we keep the perturbation?•The time derivatives disappear
PP
NN
pxpD
tp
nxnD
tn
τ
τ
∂−
∂∂
=∂∂
∂−
∂∂
=∂∂
2
2
2
2
22
2
22
2
PPP
NNN
Lp
Dp
dxpd
Ln
Dn
dxnd
∆≡
∆=
∆≡
∆=
τ
τ Electrons
HolesWhere
PPP
NNN
DL
DL
τ
τ
=
=
Diffusion Length