Integrated Circuit Devices

1
Integrated Circuit Devices

• Professor Ali Javey
• Summer 2009

PN Junctions
Reading Chapter 5
2
PN Junctions

Donors
N-type
P-type

I
V
I
N
P
V
Reverse bias
Forward bias
diode
symbol
A PN junction is present in every semiconductor
device.
3
Energy Band Diagram and Depletion Layer of a PN
Junction
N-region
P-region
Ef
(a)
Ec
Ec
Ef
(b)


Ev
Ev
Ec
Ef
A depletion layer exists at the PN junction. n ?
0 and p ? 0 in the depletion layer.
(c)
Ev
Neutral

Neutral
Depletion
layer
P-region
N-region
Ec
Ef

(d)
Ev

4
Doping Profile of Idealized Junctions
p
p
n
n
5
Qualitative Electrostatics
Band diagram
Built in-potential
From e-dV/dx
6
Formation of pn junctions
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.
7
Equilibrium Conditions
Under equilibrium conditions, the net electron
current and hole current will be zero.
E-field
N-type
P-type
NA 1017 cm?3
ND 1016 cm?3
hole diffusion current
hole drift current
8
Built-in Potential
N-region
P-region

E
c

q
V

qB
bi

E

(b)
f

E
qA
v

9
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.
10
Field in the Depletion Layer

• On the P-side of the depletion layer, ? qNa

qN
d
E
-

a
e
dx
s
qN
-

a
)
(
)
(
x
x
x
E
p
e
s

• On the N-side, ? qNd

E
qN


)
(
)
(
d
x
x
x
E
n
e
s
11
Field in the Depletion Layer
The electric field is continuous at x 0. Naxp
Ndxn
A one-sided junction is called a NP junction or
PN junction
12
Depletion Width
13
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14

• EXAMPLE A PN junction has Na1020 cm-3 and Nd
  1017cm-3. What is a) its built in potential,
  b)Wdep , c)xn ?
• Solution
• a)
• b)
• c)

15
Reverse-Biased PN Junction
16
Forward Biased PN Junction

17
Junction Breakdown
I
Forward Current
V
, breakdown
B
voltage
V
Small leakage
Current

A Zener diode is designed to operate in the
breakdown mode.
18
Quantum Mechanical Tunneling
Potential energy barrier
E
x
d
19
Tunneling Breakdown

(a)
Ec

Dominant breakdown cause when both sides of a
junction are very heavily doped.
Ef
Ev

(b)

-
Empty States
Filled States
Ec
Ev

I

(c)
V
Breakdown
20
Avalanche Breakdown
impact ionization
avalanche breakdown
21
The PN Junction as a Temperature Sensor
What causes the IV curves to shift to lower V at
higher T ?
22
Other PN Junction DevicesFrom Solar Cells to
Laser Diodes
Solar Cells Also known as photovoltaic cells,
solar cells can convert sunlight to electricity
with 15-30 energy efficiency
23
Solar Cells
short circuit
I
Dark IV
light
N
Eq.(4.9.4)
P
I

sc
0.7 V
-
0
V
E
Solar Cell
c
IV
Eq.(4.12.1)
Maximum


I
E
power-output
sc
v

(a)
(b)
24
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

25
Light Emitting Diodes (LEDs)

• LEDs are typically made of compound
  semiconductors
• Why not Si