Semiconductor Basic

1. Semiconductor Basics
Lecture 1
By : ENGR. SALMAN HAMEED

2. Lecture Overview
⚫ Atomic Structure
⚫ Classification of matter on the basics of
semiconductor theory
⚫ Covalent Bond
⚫ Conduction in semiconductors
⚫ N-type and P-type Semiconductors

3. Atomic Structure
⚫ Elements are made of atoms
– 110 Elements; each has an atomic
structure
⚫ Bohr Model
– Atoms have planetary structure
– Atoms are made of nucleus (Protons
(+) & Neutrons) and electrons (-)
110 th element is called Darmstadtium (Ds)

4. Atomic Structure (Contd..)
⚫ Atoms go around the nucleus in their orbits –
discrete distances
⚫ Each orbit has some energy level
⚫ The closer the orbit to the nucleus the less
energy it has
⚫ Group of orbits called shell
⚫ Electrons on the same shell have similar energy
level
⚫ Valence shell is the outmost shell
⚫ Valence shell has valence electrons ready to
be freed
⚫ Number of electrons (Ne) on each shell (n)
– First shell has 2 electrons
– Second shell has 8 electrons (not shown here)
Ne = 2n2

5. Valence Shell
⚫ Atoms are made of valence
shell and core
⚫ Core includes nucleus and
other inner shells
– For a Carbon atom the atomic
number is 6
– Core charge = 6 P + 2 e =
(+6)+(-2)=(+4)
– Remember the first shell has 2
electrons

6. Elements
⚫ Basic categories
– Conductors
⚫ Examples: Copper, silver
⚫ One valence electron , the e
can easily be freed
– Insulators
⚫ Valence electrons are tightly
bounded to the atom
– Semiconductors
⚫ Silicon, germanium (single
element)
⚫ Gallium arsenide, indium
phosphide (compounds)
⚫ They can act as conductors or
insulators
Conduction band is
where the electron
leaves the valence shell
and becomes free
Valence band is where
the outmost shell is
Always free
electrons
Free
electrons

7. Semiconductors
⚫ Remember the further away from the
nucleus the less energy is required
to free the electrons
⚫ Germanium is less stable
– Less energy is required to make the
electron to jump to the conduction
band
⚫ When atoms combine to form a solid,
they arrange themselves in a
symmetrical patterns
⚫ Semiconductor atoms (silicon) form
crystals
⚫ Intrinsic crystals have no impurities

8. Conduction Electrons and Holes
⚫ Electrons exist only within
prescribed energy bands
⚫ These bands are separated by
energy gaps
⚫ When an electron jumps to the
conduction band it causes a hole
⚫ When electron falls back to its initial
valence recombination occurs
⚫ Consequently there are two
different types of currents
– Hole current (electrons are the
minority carriers)
– Electron current (holes are the
minority carriers)
Remember: We are interested in electrical current!

9. Doping
⚫ By adding impurities to the intrinsic
semiconductor we can change the
conductivity of the material – this is called
doping
– N-type doping
– P-type doping
⚫ N-type: pentavalent (atom with 5 valence
electrons) impurity atoms are added
– [Sb(Antimony) + Si]
– Negative charges (electrons) are generated
– → N-type has lots of free electrons
⚫ P-type: trivalent (atom with 3 valence
electrons) impurity atoms are added
– [B(Boron) + Si]
– Positive charges (holes) are generated
– P-type has lots of holes

10. Reference
- Chapter 01 (Section 1.1 to 1.5)
“Electronic Devices” by Floyd, Seventh Edition