Semiconductors are materials with conductivity between conductors and insulators. They are important because their conductivity can be controlled and they can produce unidirectional current. Before semiconductors, vacuum tubes were used but were bulky, required high voltages, and had low reliability. Semiconductors like transistors were smaller, required less power, and were more reliable. Semiconductors can be intrinsic, with conductivity changing based on temperature, or extrinsic through doping to increase conductivity. Doping with elements like phosphorus creates n-type semiconductors with extra electrons as charge carriers, while doping with elements like boron creates p-type semiconductors with holes as charge carriers. A p

1. Semiconductors
-renuha

2. -In today’s world SCs are commonly found in all the circuits inside electronics
-they are materials which have conductivity between a conductor and an
insulator.
Why are these important?
-two important properties:
→can control the number/flow of the current carriers/conductivity can be
controlled.
→unidirectional current can be produced.
Textbook

3. Before semiconductors
● before semiconductors, vacuum tubes did the work.
● Vacuum tubes
○ could be diodes: anode and cathode
○ Or triodes which has three electrodes – cathode, plate and grid.
○ Or can be tetrodes and pentodes.
■ In a vacuum tube, electrons are supplied by→heated cathode . the controlled flow of these
electrons in vacuum is obtained by varying the voltage between its different electrodes
■ Vacuum makes sure the electrons don't lose their energy as they collide with the air molecules
DISADVANTAGES:
1.bulky
2.vaccum creation
3.high voltage and power
4.low life, less reliability.
And then later came the transistors
and diodes which were smaller in
size with low power needed and high
reliability.

4. Examples of SC
1.Elementary SC: made up of one element.
Eg: silicon and germanium
2.Compound SC:compounding metals
→organic :
→inorganic
→organic polymers

5. CLASSIFICATION BASED ON RESISTIVITY AND
CONDUCTIVITY:
1.conductors [most conductive as resistance is very low]
2.semiconductors
3.insulators [not conducting at all as resistance is very high]

6. Energy Band
● Inside the crystal each electron has a unique position and no two electrons see exactly the same
pattern of surrounding charges. Because of this, each electron will have a different energy level. These
different energy levels with continuous energy variation form what are called energy bands.
● The energy band which includes the energy levels of the valence electrons is called the valence band.
● The energy band above the valence band is called the conduction band.
energy band theory
● The overlap or the interaction will be felt more by the electrons in the outermost orbit while the inner
electron energies will remain unaffected
One energy band is
made up of 10^23
energy lines.
Energy level refers to a fixed distance from an
atom's nucleus where electrons may exist

7. -The gap between the top of the valence band and bottom of the conduction band is called the energy band
gap.
-It may be large, small, or zero, depending upon the material
There is an energy gap of about 1 eV between the valence and the conduction band
energy bands of different types of materials.

8. TYPES OF SC

9. Intrinsic sc
Pure sc , without impurities
Eg: pure silicon and pure germanium [have 4 valence electrons each and are involved in formation of
covalent bonds,hence have no free electrons.]
-at 0 K this
Semiconductor will not Conduct any electricity
As there are no free electrons And hence act as insulators.
-when heated to high temp, the valence Electrons break
away from The covalent bond and jump from valence
Band to conductivity band gives rise to free electrons to
Conduct electricity and forms holes at its original place.

10. Holes and electrons
What are holes?
-when electrons jump while being heated from the valence band to the conduction band, at the position of the electron in the
valence band, a vacancy/hole is formed.
-The electrons in the conduction band
Are free electrons nd hence conduct
Electricity well.
-holes are positively charged.
-when the electrons leave the valence band The crystal
becomes negatively charged. Since there is separation
of charge, electricity Is conducted.
-holes move in the direction of applied electric
fied.
Current generated
is in the opposite
direction of the
movement of
electrons.

11. ● just like how the electrons leave the valence band and create holes, they can also come back and
recombine.
○ This is called as recombination of thermally generated free electrons and holes.
○ At equilibrium, generation and recombination is equal.The recombination occurs due to an electron colliding with
a hole
○ Recombination takes place at every temperature and recombination and generation are simultaneous.
→number of free electrons=number of holes. [N e=N h] for an intrinsic sc
N i: intrinsic concentration
from 40 to end.

12. Disadvantages of intrinsic sc
-only with increase in temp, the conductivity increases.
in room temp, its conductivity is very poor.
-

13. Extrinsic sc
-Doping an intrinsic semic with some impurity to increase conductivity.
Very small amount of impurity is enough to increase the conductivity
Process of adding impurity: doping ;; impurity:dopant;;being doped : doped sc
-type of impurity decides if its n-type or p-type.

14. How to dope
-heating the sc in the presence of the impurity,which will in return diffuse into the sc.
-bombarding the sc with ions of impurity.

15. What kind of impurity is added?
1.pentavalent atom:
-has 5 valence electrons.
-Eg:phosphorus,arsenic,antimony.
-N-type sc is formed.
2.trivalent impurity atom:
-three valence electrons
-boron,aluminium,indium
-p-type sc is formed.

16. n-type
The impurity atoms replace the pure sc’s atoms.
The impurity atom’s size should be close to the pure sc’s size
Now this free electron
helps with conduction.
But to free the electron
completely, a bit of energy is
required so that it can jump to
the conduction band.
Since it is already free, the
electron is already closer to the
conduction band, a bit of
energy will take it to CB
These free
electrons are called
donor impuriites as
each atom donates
one e

17. -Sometimes due to heat being supplied, a few covalent bonds break and free electrons are formed and in
their place holes are created.
Hence, the number of total electrons are more than that of holes [as there are already free electrons from
phosphorus and now even bond breaking gives rise to electrns]
-Hence, more doping→more donor impurities→more conductivity
-Hence, conductivity of n type doesnt depend on temperature but the
number of donor impurities.

18. The ntype sc are electrically neutral as When a pentavalent impurity atom is added to
a pure semiconductor to make it n-type semiconductor, it donates one free electron to the
crystal, leaving a positive charge on the donor atom, the donated electron remains in the crystal
in free state. As the charge on the donor ion is equal and opposite to the charge of the electron,
the crystal itself remains electrically neutral. With the same reasoning, p-type semiconductor is
also electrically neutral.

19. p-type
-replaces the pure crystals atom.
3 electrons of let's say trivalent aluminum forms 3 bonds with the
neighbouring silicon there will be one hole as it trivalent.
When a bit of heat is given, any electron can jump into the hole
and the bond changes,it doesn't break but just moves, and the
place of the electron becomes a hole now.
This keeps happening and holes keep getting formed and filled and
so on.
These holes are in the valence band.
Trivalent impurity accepts electron from neighbouring atom and
makes bond therefore called ACCEPTOR IMPURTIY.
Since the number of holes are in a higher number, holes act as the
charge carriers.
More impurity→more acceptors→more holes→more
conductivity.
Independent of temperature.
Again here due to heat being supplied, bonds break and free
electrons are formed which may replace holes and hence create
new holes.

22. P-n junction
-when p and n type sc are joined, p-n junction diode is formed.

23. How is it formed
When p and n are brought closer, the electrons diffuse in to the holes due to difference in concentration.
**mobility of electrons is very high.
It is called depletion region as the no of mobile electrons in that part is being depleted.
Width of depletion region: 10^-6m
The potential is not same
everywhere, it is higher in
the n type and lower in
the p type.
There is an electric field
from n to p

24. -potential difference created across the diode is called potential barrier → Vb
Avg potential barrier is 0.5V
e=v/d
Where e=electric field
d=depletion layer
v=vb

25. Diffusion current
This diffusion results in a current
from p to n opposite to the direction
of flow of electrons.

26. Drift currrent
A drift current is given rise to from n to p .

27. biasing
The way the p-n junction diode is connected to the emf.
Allows current to pass through only one direction
1.forward biasing:
-positive terminal connected to p and negative terminal connected to n
-current flows from p to n which means electrons flow from n to p
Therefore less resistance.
reverse and forward biasing
https://www.youtube.com/watch?v=DDalmLJr2_c
rf bias
2.reverse biasing:
-negative terminal to p and positive terminal to n
-Resistance is high,current doesnt flow unless the field is high enough.

28. Doesnt follow ohms law