P-N Junctions

1
P-N Junctions
Physical aspects of pn junctions
Circuit Symbol

• Basis for other devices

V -
Mathematical models

• Depletion capacitance
• Breakdown characteristics

2
PHYSICAL ASPECTS OF THE PN JUNCTION
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P-N Junctions
N-type ND
P-type NA

4
P-N Junctions
N-type ND
P-type NA

5
P-N Junctions
N-type ND
P-type NA
Depletion Layer or Region

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P-N Junctions
N-type ND
P-type NA
Depletion Layer or Region
qND
Charge Density
-qNA

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P-N Junctions
N-type ND
P-type NA
Depletion Layer or Region
qND
Charge Density
-qNA
Band Diagram

8
Electrostatics in PN Junction
DQ r x
qND
Charge Density
x
-qNA
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Electrostatics in PN Junction
DQ r x
qND
Charge Density
x
-qNA
Gauss' Law in 1 Dimension Electric Field
DQ / e
Electric Field
x
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Electrostatics in PN Junction
DQ r x
qND
Charge Density
x
-qNA
Gauss' Law in 1 Dimension Electric Field
DQ / e
Electric Field
x
Vbi Built in Voltage
Electron Potential
x
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Why is this the Equilibrium Condition?
Vbi built-in voltage
kT ln ( )
NAND
ni2
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Why is this the Equilibrium Condition?
Vbi built-in voltage
kT ln ( )
NAND
ni2
Too much Vbi
Too little Vbi
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P-N Junctions --- Diodes
N-type ND
P-type NA
First-Principles Model
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P-N Junctions --- Diodes
N-type ND
P-type NA
First-Principles Model
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P-N Junctions --- Diodes
N-type ND
P-type NA
First-Principles Model
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P-N Junctions --- Diodes
N-type ND
P-type NA
First-Principles Model
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Applying Voltage to a Diode
Forward Bias
FN
- V
FP
Forward Bias
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Applying Voltage to a Diode
Forward Bias
FP
FN
- V
V -
FN
FP
Reverse Bias
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Capacitance in pn Junctions
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Capacitance in pn Junctions
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Capacitance in pn Junctions
4.5
4
3.5
Cj0
3
2.5
Capacitance (fF)
2
1.5
1
0.5
0
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
Diode voltage (V)
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Breakdown Voltage
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Breakdown Voltage