This document summarizes the key principles of how a p-n junction diode works. It explains that a p-n junction diode is formed by placing a p-type semiconductor material directly next to an n-type material. When the two materials meet, electrons from the n-type diffuse to the p-type, and holes from the p-type diffuse to the n-type, leaving an area depleted of charge carriers called the depletion region or space charge region. This creates an electric field across the junction and a built-in potential barrier that prevents current flow when no external voltage is applied.

1.  Review of P-N Junction Diodes 
How do they work?
Dr Vikas Mahor

2. p-n Junction Diode
p-n Junction
p-Type Material n-Type Material

3. p-n Junction Diode
 p-n Junction
p-Type Material
p-Type Material n-Type Material
n-Type Material
 A p-n junction diode is made by forming a p-type region of
material directly next to a n-type region.
p-n Junction

4. p-Type Material n-Type Material
x
ND - NA
ND
NA
p-n Junction
 p-n Junction
p-n Junction Diode

5.  In regions far away from the “junction” the band diagram
p-n Junction Diode
p-Type Material n-Type Material
EC
EV
EF
Ei
EC
EV
EF
Ei

6.  But when the device has no external applied forces, no current
can flow. Thus, the Fermi-level must be flat!
 We can then fill in the junction region of the band diagram as:
p-n Junction Diode
p-Type Material n-Type Material
EC
EV
EF
Ei
EC
EV
EF
Ei

7.  But when the device has no external applied forces, no current
can flow. Thus, the Fermi-level must be flat!
 We can then fill in the junction region of the band diagram as:
p-Type Material
n-Type Material
p-n Junction Diode
EC
EV
EF
Ei
EC
EV
EF
Ei

8.  Built-in-potential
p-n Junction Diode
EC
EV
EF
Ei
EC
EV
EF
Ei
- qVBI
p-Type Material n-Type Material
x
Electrostatic Potential
)
(
1
ref
C E
E
q
V 

Built-in-potential
BI
V

9.  Built-in-potential
p-n Junction Diode
x
Electrostatic Potential
)
(
1
ref
C E
E
q
V 

Built-in-potential
BI
V
x
Electric Field
dx
dV
E 

 Electric Field

10. Charge Density
(NOT Resistivity)
 Poisson’s Equation
p-n Junction Diode
0







S
K
E
0





S
K
dx
dE
in 1-dimension
Electric Field
Permittivity of free space
Relative Permittivity
of Semiconductor
(r)
)
( A
D N
N
n
p
q 





11. x
Electric Field
dx
dV
E 

 Built-in-potential
p-n Junction Diode
 Charge Density
x
dx
dE
KS 0

 

qND
qNA
Charge Density
- -
-
+ +
+

12. p-n Junction Diode
Energy
Potential
Electrical Field
Charge Density
dx
dE
KS 0


dx
dV

q
1


13. nno
x
x = 0
pno
p po
npo
lo g (n ), lo g (p )
-e N a
e N d
M
x
E (x)
B -
h +
p n
M
As +
e –
Wp Wn
V o
V (x)
x
P E (x)
x
– W
p
W
n
0
e V o
x
– e V o
Hole Potential Energy PE (x)
o
– E o
E0
 net
M
ni
p-n Junction Principles
Electron Potential Energy PE (x)
Space charge region
M
-Wp -Wn
Neutral n-region
Neutral p-region
Metallurgical Junction