Transition capacitance and diffusion capacitance of pn junction diode is discussed. Breakdown mechanisms such as zener and avalanche is explained

1. ANIL NEERUKONDA INSTITUTE OF TECHNOLOGY &
SCIENCES(A)
Department of Electronics and Communication Engineering
ECE 125 Basic Electronics Engineering
 Academic year : 2022-23
 Class & Section : 1/4 ECE-A
 Name of the Faculty : Mr.D.Anil Prasad
ANIL PRASAD DADI/ECE/ANITS

2. UNIT-I(Semiconductor Diodes)
• Fermi level in Intrinsic & Extrinsic semiconductors. Mass-Action law.
Mobility and conductivity, Hall effect, Generation and recombination
of charges, Drift and diffusion current, Band structure of open-circuit
p-n junction, V-I characteristics, transition and diffusion capacitance,
reverse recovery time, Avalanche and zener breakdown, zener
diodes, Light Emitting Diodes.
ANIL PRASAD DADI/ECE/ANITS

3. Contents
• Junction capacitance
– Transition capacitance
– Diffusion capacitance
• Breakdown mechanisms
– Zener breakdown
– Avalanche breakdown
ANIL PRASAD DADI/ECE/ANITS

4. Transition capacitance
ANIL PRASAD DADI/ECE/ANITS
• Transition capacitance exists when the
pn junction is reverse biased.
• Change in VR (either increase or
decrease of VR) changes width of
depletion region
• Increase in VR increases depletion regi
• on on either side
• Decrease in VR decreases depletion
region on either side

5. Transition capacitance
ANIL PRASAD DADI/ECE/ANITS
• Change in VR changes charge on
either side of the depletion region
• When RB across pn junction is
changed then depletion width and
depletion charge also get changed.
This change in depletion charge w.r.t
change in applied voltage is known as
a capacitance called junction
capacitance or transition capacitance,
CT

6. Transition capacitance
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7. Diffusion capacitance
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• This capacitance exists when pn junction is forward biased.
• There is net storage of electrons in p-side and net storage of holes
in n-side minority charge carrier charge is stored (electrons in p-side
and holes in n-side)
• In a FB PN junction holes are injected from p to n-side and electrons
are injected from n-side to p-side continuously. Therefore there is
net storage of minority charge carrier across the junction. This
storage of charge is again a capacitance called diffusion
capacitance

8. Diffusion capacitance
ANIL PRASAD DADI/ECE/ANITS

9. Breakdown mechanisms
• Zener breakdown: It occurs in relatively heavily doped abrupt pn junction
at breakdown voltages VZ<6V
ANIL PRASAD DADI/ECE/ANITS
• Increase in VR results in
Breakdown of diodes
• VZ is called breakdown
voltage
• Internal electric field is
very high initially because
doping is high in a heavily
doped PN junction

10. Breakdown mechanisms
• If VR is increased beyond VZ then the net electric field inside depletion
region is very large.
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• This high value of electric field is
sufficient enough to break the
covalent bonds inside the
depletion region. Hence carrier
concentration increases rapidly
inside depletion region. Hence
current increases rapidly. This
mechanism of breakdown is
called Zener breakdown
mechanism.

11. Breakdown mechanisms
• Zener breakdown voltage is negative temperature coefficient
ANIL PRASAD DADI/ECE/ANITS
• If temperature is increased then
breakdown occurs at a voltage
less than VZ. Since Zener
breakdown is due to sudden
breaking of covalent bonds
which becomes easy with
increase in temperature.

12. Breakdown mechanisms
• Avalanche breakdown: It occurs in relatively lightly doped abrupt pn
junction at breakdown voltages VZ>6V
ANIL PRASAD DADI/ECE/ANITS
• Increase in VR results in
Breakdown of diodes
• VZ is called breakdown
voltage
• Internal electric field is
very small because doping
is less in a lightly doped
PN junction

13. Breakdown mechanisms
• If VR is increased beyond VZ then high energy electrons collides with atom and able
to knock out outer orbit electrons of silicon atoms. Hence secondary electrons are
generated
ANIL PRASAD DADI/ECE/ANITS
• The generated electrons in turn
generates further electrons when
colliding with another atoms. This
process continues, huge no of
charge carriers are generated.
Therefore huge current flows and it is
called the avalanche
breakdown(Avalanche means
multiplication).

14. Breakdown mechanisms
• Since energetic electrons are colliding with atoms and generating secondary
electrons. Hence this process is called impact ionization process where as Zener
breakdown is field ionization process.
ANIL PRASAD DADI/ECE/ANITS
• Avalanche breakdown is positive temperature
coefficient.
• Since avalanche breakdown is due to impact
ionization where an energetic electron collides
with atom and knock out outer orbit electrons.
When temperature increases oscillation of atoms
at lattice point increases. Hence collision between
electrons and atom is not so impactful that it can
generate secondary electrons. Hence to get
breakdown we need to apply a higher voltage.

15. Distinguish between Zener and Avalanche
Breakdown mechanisms
• Zener breakdown
1. Exists in heavily doped pn junction
2. VZ<6V
3. Eint is very large
4. Breakdown is due to breaking of
covalent bonds
5. Negative temperature coefficient
6. Field ionization process
• Avalanche breakdown
1. Exists in lightly doped pn junction
2. VZ>6V
3. Eint is very small
4. Breakdown is due to collision of
electrons with atoms
5. Positive temperature coefficient
6. Impact ionization process
ANIL PRASAD DADI/ECE/ANITS

16. References
ANIL PRASAD DADI/ECE/ANITS
• Robert L Boylestad, Electronic Devices And Circuit Theory, Prentice
Hall, seventh edition,2021
• Jacob Millman and Christos Halkias, Electronics Devices and
Circuits, Black edition, October,2017

17. Thank you
for listening
ANIL PRASAD DADI/ECE/ANITS