integrated circuit devices professor ali javey summer 2009 pn junctions reading: chapter 5
TRANSCRIPT
PN Junctions
A PN junction is present in every semiconductor device.
N-type
P-type
Donors
V
I
Reverse bias Forward bias
N P
V
I
diodesymbol
– +
N-region P-region(a)
(b)
(c)
(d)
Depletionlayer
Neutral P-region
NeutralN-region
Ef
Energy Band Diagram and Depletion Layer of a PN Junction
A depletion layer exists at the PN junction. n 0 and p 0 in the depletion layer.
Ec
Ef
Ev
Ec
Ev
Ef
Ec
Ev
Ef
Ev
Ec
When the junction is formed, electrons from the n-side and holes from the p-side will diffuse leaving behind charged dopant atoms. Remember that the dopant atoms cannot move! Electrons will leave behind positively charged donor atoms and holes will leave behind negatively charged acceptor atoms.
The net result is the build up of an electric field from the positively charged atoms to the negatively charged atoms, i.e., from the n-side to p-side. When steady state condition is reached after the formation of junction (how long this takes?) the net electric field (or the built in potential) will prevent further diffusion of electrons and holes. In other words, there will be drift and diffusion currents such that net electron and hole currents will be zero.
Formation of pn junctions
Equilibrium ConditionsUnder equilibrium conditions, the net electron current and hole current will be zero.
NA = 1017 cm3 ND = 1016 cm3
hole diffusion current
hole drift current
net current = 0
E-fieldP-type N-type
2lni
adbi
n
NN
q
kTV
Built-in Potential
d
ckTAqcd N
N
q
kTAeNNn ln
2
2
lni
ackTBqc
a
i
n
NN
q
kTBeN
N
nn
d
c
i
acbi N
N
n
NN
q
kTABV lnln 2
N-region
P-region
(b)
E f
E c
E v
qVbi qB
qA
The Depletion Approximation
We assume that the free carrier concentration inside the depletion region is zero.
We assume that the charge density outside the depletion region is zero and q(Nd-Na) inside the depletion.
On the P-side of the depletion layer, = –qNa
On the N-side, = qNd
(a)
N P
Nd
Na
D eple tion La yer N e utral R egi on
–xn
0 xp
(b)
x x
p
–xn
(c)
qNd
–qNa
x
E
–xn xp
(d)
(f)
Ec
Ef
Ev
bi , built-in potential
P N
0
–xn
xp
x
bi
(e)
N eut ra l Re gion
V
s
aqNdxd
E
E
)()( xxqN
x p
s
a E
)()( n
s
d xxqN
x E
Field in the Depletion Layer
Field in the Depletion Layer
The electric field is continuous at x = 0.
Naxp = Ndxn
A one-sided junction is called a N+P junction or P+N junction
(a)
N P
Nd
Na
D eple tion La yer N e utral R egi on
–xn
0 xp
(b)
x x
p
–xn
(c)
qNd
–qNa
x
E
–xn xp
(d)
(f)
Ec
Ef
Ev
bi , built-in potential
P N
0
–xn
xp
x
bi
(e)
N eut ra l Re gion
V
EXAMPLE: A P+N junction has Na=1020 cm-3 and Nd
=1017cm-3. What is a) its built in potential, b)Wdep , c)xn ?
Solution:a)
b)
c)
V 1cm10
cm1010lnV026.0ln
620
61720
2
i
adbi
n
NN
q
kT
μm 12.010106.1
11085.812222/1
1719
14
d
bisdep qN
W
μm 12.0 depn Wx
Junction Breakdown
A Zener diode is designed to operate in the breakdown mode.
V
I
VB, breakdown
Forward Current
Small leakageCurrent
voltage
Tunneling Breakdown
Dominant breakdown cause when both sides of a junction are very heavily doped.
(a)
(b)
Empty StatesFilled States
V
I
(c)
Breakdown
-
Ev
Ec
Ev
Ec
Ef
Avalanche Breakdown
impact ionization
avalanche breakdown
EcE
fn
Ec
Ev
Efp
originalelectron
electron-holepair generation
The PN Junction as a Temperature Sensor
What causes the IV curves to shift to lower V at higher T ?
)1(0 kTVqeII
an
n
dp
pi NL
D
NL
DAqnI 2
0
Other PN Junction Devices–From Solar Cells to Laser Diodes
Solar Cells
Also known as photovoltaic cells, solar cells can convert sunlight to electricity with 15-30% energy efficiency
Solar Cells
N P
-
+
short circuit
lightIsc
(a)
V0.7 V
–Isc
Maximumpower-output
Solar CellIV
I
Dark IV
0
(b)
Ec
Ev
Eq.(4.9.4)
Eq.(4.12.1)
p-i-n Photodiodes•Only electron-hole pairs generated in depletion region (or near depletion region) contribute to current
•Only light absorbed in depletion region contributes to generation
–Stretch depletion region
–Can also operate near avalanche to amplify signal