explains the

2. Silicon has 4 electrons in outer layer
It wants 8
Si

3. • Silicon atoms can share electrons with
other atoms to get 8 electrons in outer
layer.
• Silicon atoms combining together forms a
crystal.
• Pure silicon crystals are not good
electrical conductors, they are insulators.

4. all silicon atoms in a
crystal

5. Doping
• Doping means adding other kinds of
atoms to the silicon crystal.
• Phosphorus and Arsenic atoms have 5
outer electrons.
• Aluminum and Gallium have 3 outer
electrons.
• Doping makes the crystal into a
semiconductor.

6. As As As
As As
As
As
As As
As

7. extra electrons
in Arsenic atoms

8. Al Al
Al
holes (missing electrons)
in Aluminum atoms Al
Al
Al Al Al

9. • N type material = semiconductor with
extra negative charges (extra electrons)
• P type material = semiconductor with extra
positive charged holes (fewer electrons or
missing negative charges)

10. PN Junction
• What happens when P type material and
N type material are placed together?

13. • Electrons from Arsenic atoms in N type
material are attracted to holes (missing
electron positions) in Aluminum atoms in
P type material.
• Electrons from N type material move and
fill in holes in P type.

14. + -
+ -
+ -

15. • Atoms have the same number of electrons
as protons.
• When electrons leave Arsenic atoms, they
now have more protons than electrons.
• Arsenic atoms become positive ions.
• When aluminum atoms pick up extra
electrons, they now have more electrons
than protons.
• Aluminum atoms become negative ions.

16. N Type P Type
extra electrons holes (missing electrons)

17. N Type P Type
extra electrons holes (missing electrons)

18. N Type P Type
extra electrons holes (missing electrons)
depletion zone

19. • Harder and harder to get the depletion
zone to be wider because
– electrons are repelled by negative aluminum
ions
– holes are repelled by positive arsenic ions

20. N Type P Type
extra electrons holes (missing electrons)
depletion zone

21. N Type P Type
extra electrons holes (missing electrons)
depletion zone
stabilized

22. What happens when a voltage source
is applied to the 2 ends of the
diode?

23. • Electrons are pushed from the negative
supply into the N side of the diode.
• Holes are pushed (electrons are drained)
from the P side of the diode into the
positive side of the supply.

24. What happens when a voltage source
is applied to the 2 ends of the
diode?

25. As more and more charges build up,
easier for charges to move across the
depletion zone
Current flows as electrons flow from N type
through holes of P type out to positive source

26. Forward Biased
• To cross depletion zone, must apply
enough voltage (pressure or push) to get
electrons past negatively charged region
of depletion zone.
• Forward bias voltage about 0.7 V for
silicon diodes and 0.3 V for germanium
diodes.

27. What happens if the voltage
polarity is reversed?

28. What happens if the voltage
polarity is reversed?
depletion zone
widens

29. Reversed Bias
• When diodes are reverse biased, depletion
zone widens
• Harder for charges to cross depletion zone
• High enough voltage will force charges
across depletion zone
– Heat from large reverse current destroys diode
– Breakdown voltage = voltage level that causes
diodes to conduct when reverse biased