Download 1) Energy bands in solids:

10TH CLASSES
PHYSICS
DAILY PLAN
SUBJECT: SEMICONDUCTORS
GOALS:
DURATION:
IN PRACTICE:
As temperature of a semiconductor
increases, more electrons jump into the
conduction band. This is why resistivity
of a semiconductor decreases with
temperature.
PRESENTATION:
In a solid we have about 1023 atoms in 1 cm3. Therefore
energy levels are so close that we can take them as a
continious band of energy levels.
Energy
Energy

T
3) n-type and p-type semiconductors:
Conductivity of semiconductors can be increased by adding
“impurities” (= atoms of another element) into it. This is
called “doping”.
a) n-type semiconductors:
Silicon has 4 valance electrons:
forbidden
band
allowed
band
atom in solid
structure
single atom
[ The “width” of a band depends on the interaction among the
neighboring atoms only.
But the number of levels in one band depends on the total
number of atoms in the crystal.
So, within an allowed band there are billions of very closely
spaced energy levels.]
Memorize:
An atom in a solid structure has “energy band”s.
An electron can be in “allowed band”s.
The region between two allowed bands is called a “forbidden
band”.
The uppermost energy band occupied by electrons is called
“valance band”.
The next higher energy band is called “conduction band”.
The forbidden band between valance and conduction bands is
called “energy gap” or “Fermi gap”.
2) Electrical properties:
Electrical properties of a material is determined by features of
energy bands.
a) Insulator:
~ 5eV
Arsenic has 5 valance electrons:
b) p-type semiconductors:
Silicon has 4 valance electrons. Indium has 3 valance
electrons. So when silicon is doped with indium:
The vacancy (empty place) in the crystal structure is called a
“hole”.
The holes act as if they are positive charge carriers. So the
semiconductor is called p-type.
Si
Si
Si
The impurity atom (e.g. indium) is
called “acceptor”.
Si
As
Si
free
electron
Si
Si
empty conduction
band
Si
Si
Si
wide energy gap
Si
In
Si
hole
filled valance
band
Si
b) Metal:
filled valance
band
Ar
Such a solid is called
“n-type”, because the current is carried by negative electrons.
Since the arsenic gives one free electron, it is called “donor”
atom.
Si
partially filled
conduction
band
Si
narrow energy
gap
Si
Si
HOMEWORK:
MULTIMEDIA:
DEMONSTRATION:
EXPERIMENT:
c) Semiconductor:
~ 1 eV
empty conduction
band
TEACHER:
energy gap
DIRECTOR:
filled valance
band
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