Short Notes: P-N Junction & Diodes | Electronic Devices - Electronics and Communication Engineering (ECE) PDF Download

 Page 1


P-N Junction and Dio des
The P-N junction is a fundamen tal building blo c k of semiconductor devices, forming the basis for
dio d es and other electronic comp onen ts. Dio des, whic h utilize P-N junctions, are essen tial for con trolling
curren t flo w in circuits, enabling applications in rectification, signal mo dulation, and protection.
1. In t ro d uction to P-N Junction
A P-N junction is formed b y joining a p-t yp e semiconductor (ric h in holes) with an n-t yp e semiconductor
(ric h in electrons). The in terface b et w een these regions creates unique electrical prop erties due to the
in t eraction of c harge carriers, making the P-N junction critical for semiconductor devices.
2. F ormation of P-N Junction
When p-t yp e and n-t yp e semiconductors are brough t together:
• Diffusion : Electrons from the n-region diffuse in to the p-region, and holes from the p-region diffuse
in to the n-region due to concen tration gradien ts.
• Depletion Region : A t the junction, electrons and holes recom bine, creating a region depleted of
free c harge carriers. This region con tains fixed ionized dopan ts, forming an e lectric field.
• Built-in P oten tial : The electric field opp oses further diffusion, establishing a p oten tial barrier
V
0
giv en b y:
V
0
=
kT
q
ln
(
N
a
N
d
n
2
i
)
where k is Boltzmann’s constan t, T is the temp erature in Kelvin, q is the electron c harge, N
a
and
N
d
are the acceptor and donor concen trations, and n
i
is the in trinsic carrier concen tration.
3. P-N J unction Under Bias
The b eha vior of a P-N junction dep ends on the applied v oltage:
• F orw ard Bias : A p ositiv e v oltage is applied to the p-side relativ e to the n-side, reducing the
p oten tial barrier. This allo ws curren t to flo w, primarily due to ma jorit y carriers. The curren t is:
I = I
s
(
e
qV
kT
-1
)
where I
s
is the rev erse saturation curren t and V is the applied v oltage.
• Rev erse Bias : A negativ e v oltage is applied to the p-side, increasing the p oten tial barrier. Only
a small leakage curren t (I
s
) flo ws due to minorit y carriers.
• Breakdo wn : A t high rev erse v oltages, breakdo wn o ccurs (e.g., Zener or a v alanc he breakdo wn),
allo wing significan t curren t flo w.
4. Di o de Characteristics
A dio de is a t w o-terminal device based on a P-N junction, allo wing curren t to flo w primarily in one
direction. Key c haracteristics include:
• I-V Curv e : The curren t-v oltage relationship is exp onen tial in forw ard bias and nearly constan t
(small I
s
) in rev erse bias.
1
Page 2


P-N Junction and Dio des
The P-N junction is a fundamen tal building blo c k of semiconductor devices, forming the basis for
dio d es and other electronic comp onen ts. Dio des, whic h utilize P-N junctions, are essen tial for con trolling
curren t flo w in circuits, enabling applications in rectification, signal mo dulation, and protection.
1. In t ro d uction to P-N Junction
A P-N junction is formed b y joining a p-t yp e semiconductor (ric h in holes) with an n-t yp e semiconductor
(ric h in electrons). The in terface b et w een these regions creates unique electrical prop erties due to the
in t eraction of c harge carriers, making the P-N junction critical for semiconductor devices.
2. F ormation of P-N Junction
When p-t yp e and n-t yp e semiconductors are brough t together:
• Diffusion : Electrons from the n-region diffuse in to the p-region, and holes from the p-region diffuse
in to the n-region due to concen tration gradien ts.
• Depletion Region : A t the junction, electrons and holes recom bine, creating a region depleted of
free c harge carriers. This region con tains fixed ionized dopan ts, forming an e lectric field.
• Built-in P oten tial : The electric field opp oses further diffusion, establishing a p oten tial barrier
V
0
giv en b y:
V
0
=
kT
q
ln
(
N
a
N
d
n
2
i
)
where k is Boltzmann’s constan t, T is the temp erature in Kelvin, q is the electron c harge, N
a
and
N
d
are the acceptor and donor concen trations, and n
i
is the in trinsic carrier concen tration.
3. P-N J unction Under Bias
The b eha vior of a P-N junction dep ends on the applied v oltage:
• F orw ard Bias : A p ositiv e v oltage is applied to the p-side relativ e to the n-side, reducing the
p oten tial barrier. This allo ws curren t to flo w, primarily due to ma jorit y carriers. The curren t is:
I = I
s
(
e
qV
kT
-1
)
where I
s
is the rev erse saturation curren t and V is the applied v oltage.
• Rev erse Bias : A negativ e v oltage is applied to the p-side, increasing the p oten tial barrier. Only
a small leakage curren t (I
s
) flo ws due to minorit y carriers.
• Breakdo wn : A t high rev erse v oltages, breakdo wn o ccurs (e.g., Zener or a v alanc he breakdo wn),
allo wing significan t curren t flo w.
4. Di o de Characteristics
A dio de is a t w o-terminal device based on a P-N junction, allo wing curren t to flo w primarily in one
direction. Key c haracteristics include:
• I-V Curv e : The curren t-v oltage relationship is exp onen tial in forw ard bias and nearly constan t
(small I
s
) in rev erse bias.
1
• Threshold V oltage : The v oltage at whic h significan t curren t b egins to flo w (e.g., ˜ 0.7 V for
silicon dio des).
• Rev erse Breakdo wn V oltage : The v oltage at whic h the dio de conducts in rev erse bias, designed
for sp ecific applications lik e Zener dio des.
5. T yp es of Dio des
Differen t dio des exploit P-N junction prop erties for sp ecific functions:
• Rectifier Dio des : Used in p o w er supplies to con v ert A C to DC.
• Zener Dio des : Op erate in rev erse breakdo wn for v oltage regulation.
• Sc hottky Dio des : Use a metal-semiconductor junction for lo w forw ard v oltage drop and fast
switc hing.
• LEDs (Ligh t-Emitting Dio des) : Emit ligh t when forw ard-biased, used in displa ys and ligh ting.
• Photo dio des : Generate curren t when exp osed to ligh t, used in sensors.
6. Applications of P-N Junction Dio des
P-N junction dio des are used in:
• Rectification : Con v erting A C to DC in p o w er supplies.
• V oltage Regulation : Zener dio des main tain constan t output v oltage.
• Signal Pro cessing : Clipping, clamping, and mo dulation circuits.
• Protection Circuits : Prev en ting v oltage spik es in sensitiv e comp onen ts.
• Opto electronics : LEDs and photo dio des in comm unication and sensing.
7. Practical Considerations
• T emp erature Effects : The saturation curren t I
s
increases with temp erature, affecting dio de
p erformance. The forw ard v oltage drop decreases b y appro ximately 2 m V/
?
C.
• Rev erse Reco v ery Time : The time tak en for a dio de to stop conducting in rev erse bias, critical
for high-frequency applications.
• Capacitance : The depletion region acts as a capacitor, impac ting high-frequency p erformance.
• P o w er Dissipation : Dio des m ust op erate within p o w er limits to a v oid thermal damage.
8. F abrication of P-N Junction Dio des
Dio des are fabricated using:
• Doping : In tro ducing p-t yp e and n-t yp e impurities via diffusion or ion implan tation.
• Metal Con tacts : A dding ohmic con tacts for electric al connections.
• P ac kaging : Encapsulating the dio de to protect it from en vironmen tal factors.
2
Page 3


P-N Junction and Dio des
The P-N junction is a fundamen tal building blo c k of semiconductor devices, forming the basis for
dio d es and other electronic comp onen ts. Dio des, whic h utilize P-N junctions, are essen tial for con trolling
curren t flo w in circuits, enabling applications in rectification, signal mo dulation, and protection.
1. In t ro d uction to P-N Junction
A P-N junction is formed b y joining a p-t yp e semiconductor (ric h in holes) with an n-t yp e semiconductor
(ric h in electrons). The in terface b et w een these regions creates unique electrical prop erties due to the
in t eraction of c harge carriers, making the P-N junction critical for semiconductor devices.
2. F ormation of P-N Junction
When p-t yp e and n-t yp e semiconductors are brough t together:
• Diffusion : Electrons from the n-region diffuse in to the p-region, and holes from the p-region diffuse
in to the n-region due to concen tration gradien ts.
• Depletion Region : A t the junction, electrons and holes recom bine, creating a region depleted of
free c harge carriers. This region con tains fixed ionized dopan ts, forming an e lectric field.
• Built-in P oten tial : The electric field opp oses further diffusion, establishing a p oten tial barrier
V
0
giv en b y:
V
0
=
kT
q
ln
(
N
a
N
d
n
2
i
)
where k is Boltzmann’s constan t, T is the temp erature in Kelvin, q is the electron c harge, N
a
and
N
d
are the acceptor and donor concen trations, and n
i
is the in trinsic carrier concen tration.
3. P-N J unction Under Bias
The b eha vior of a P-N junction dep ends on the applied v oltage:
• F orw ard Bias : A p ositiv e v oltage is applied to the p-side relativ e to the n-side, reducing the
p oten tial barrier. This allo ws curren t to flo w, primarily due to ma jorit y carriers. The curren t is:
I = I
s
(
e
qV
kT
-1
)
where I
s
is the rev erse saturation curren t and V is the applied v oltage.
• Rev erse Bias : A negativ e v oltage is applied to the p-side, increasing the p oten tial barrier. Only
a small leakage curren t (I
s
) flo ws due to minorit y carriers.
• Breakdo wn : A t high rev erse v oltages, breakdo wn o ccurs (e.g., Zener or a v alanc he breakdo wn),
allo wing significan t curren t flo w.
4. Di o de Characteristics
A dio de is a t w o-terminal device based on a P-N junction, allo wing curren t to flo w primarily in one
direction. Key c haracteristics include:
• I-V Curv e : The curren t-v oltage relationship is exp onen tial in forw ard bias and nearly constan t
(small I
s
) in rev erse bias.
1
• Threshold V oltage : The v oltage at whic h significan t curren t b egins to flo w (e.g., ˜ 0.7 V for
silicon dio des).
• Rev erse Breakdo wn V oltage : The v oltage at whic h the dio de conducts in rev erse bias, designed
for sp ecific applications lik e Zener dio des.
5. T yp es of Dio des
Differen t dio des exploit P-N junction prop erties for sp ecific functions:
• Rectifier Dio des : Used in p o w er supplies to con v ert A C to DC.
• Zener Dio des : Op erate in rev erse breakdo wn for v oltage regulation.
• Sc hottky Dio des : Use a metal-semiconductor junction for lo w forw ard v oltage drop and fast
switc hing.
• LEDs (Ligh t-Emitting Dio des) : Emit ligh t when forw ard-biased, used in displa ys and ligh ting.
• Photo dio des : Generate curren t when exp osed to ligh t, used in sensors.
6. Applications of P-N Junction Dio des
P-N junction dio des are used in:
• Rectification : Con v erting A C to DC in p o w er supplies.
• V oltage Regulation : Zener dio des main tain constan t output v oltage.
• Signal Pro cessing : Clipping, clamping, and mo dulation circuits.
• Protection Circuits : Prev en ting v oltage spik es in sensitiv e comp onen ts.
• Opto electronics : LEDs and photo dio des in comm unication and sensing.
7. Practical Considerations
• T emp erature Effects : The saturation curren t I
s
increases with temp erature, affecting dio de
p erformance. The forw ard v oltage drop decreases b y appro ximately 2 m V/
?
C.
• Rev erse Reco v ery Time : The time tak en for a dio de to stop conducting in rev erse bias, critical
for high-frequency applications.
• Capacitance : The depletion region acts as a capacitor, impac ting high-frequency p erformance.
• P o w er Dissipation : Dio des m ust op erate within p o w er limits to a v oid thermal damage.
8. F abrication of P-N Junction Dio des
Dio des are fabricated using:
• Doping : In tro ducing p-t yp e and n-t yp e impurities via diffusion or ion implan tation.
• Metal Con tacts : A dding ohmic con tacts for electric al connections.
• P ac kaging : Encapsulating the dio de to protect it from en vironmen tal factors.
2
9. Conclusion
The P-N junction is the cornerstone of dio de tec hnology , enabling precise con trol of curren t flo w in
electronic circuits. Dio des, lev eraging the prop erties of P-N junctions, are v ersatile comp onen ts used
in rectification, regulation, and signal pro cessing. Understanding their b eha vior under differen t bias
conditions and their practical limitations is essen tial for designing e?icien t and reliable electronic devices.
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