Introduction to Semiconductors

1. Introduction to Semiconductors
Chapter 4

2. Learning Outcome
2
• At the end of this topic, the student should be able to:
1. Understand material classification, atomic structures, energy band, covalent
bonds and conduction in semiconductors.
2. Understand the concept of free electrons and holes as carriers.
3. Explain types of semiconductor : Intrinsic and Extrinsic
4. Explain doping process for the semiconductor material.
5. Explain the formation of P-type, N-type material.
6. Describe the process of depletion region in semiconductor and P-N junction
formation

3. Atomic structures
• Construction of every discrete solid-state electronic device or IC begin with highest
quality semiconductor material
• Semiconductors (S/C) are a special class of elements having a conductivity between
good conductor and insulator
• Two classes of S/C material :
• Single crystal – repetitive crystal structure i.e Ge, Si
• Compound – constructed of 2 or more S/C materials of different atomic structure
i.e GaAs, CdS, GaN, GaAsP
• Most frequently used in electronic devices - Germaniun (Ge), Silicon (Si) and Galium
Arsenic (GaAs)

4. Crystal structure of silicon Compound : Crystal structure of GaN (Gallium Nitride)

5. • Atoms are bound together to form a crystalline structure
• Fundamental components of atoms are electron, proton and neutron
• In the lattice structure, neutron and protons form the nucleus
• Electrons appear in fixed orbits around the nucleus
• Bohr model of atomic structure are shown in Fig.1
• Si has 14 orbiting electrons, Ge has 32, Gallium has 31 and arsenic has 33 orbiting
electron respectively
• Si and Ge has four valence electrons, Ga has three and As has five valence electrons
• Atom with four valence electrons called tetravalent, those with three are called trivalent
and those with five are called pentavalent.

6. Gallium
Arsenic
Silicon Germanium
Fig.
1:
Atomic
structure
Atomic Number of,
Silicon – 14
Germanium – 32
Gallium – 31
Arsenic - 33

7. Covalent Bonding
• In a pure Si or Ge crystal, the four valence electrons of one atom form a
bonding arrangement with four adjoining atoms.
• This bonding of atoms, strengthened by the sharing of electrons is called
covalent bonding.
• Since GaAs is a compound semiconductor, there is sharing between two
different atoms.
• The sharing between GaAs is five electron provided by the As atom and three
by the Ga atom.
• Covalent bonding of the Silicon atom and GaAs crystal are shown in Fig. 2

8. Covalent bonding
Covalent bonding of the Silicon atom Covalent bonding of the GaAs crystal
Figure 2

9. Free electrons
•Covalent bond will result in a stronger bond between the valence electrons and their
parents atom
•However, it is still possible for the valence electrons to absorb sufficient kinetic
energy (from external such heat from surrounding medium) to break the covalent
bond - cause ‘free’ state
•Free state refer to any electron that has separated from the fixed lattice structure
•At room temperature there are approximately 1.5x1010 free carriers in 1 cm3 of
intrinsic Silicon material (15 billion electrons in a space smaller than small sugar
cube)
•The free electrons is a material due only to external cause reffered as intrinsic
carriers.

10. • Interesting difference between semiconductors and conductors is their
reaction to the application of heat.
• For conductors, the resistance increase with an increase in heat – due to
number of carriers in a conductor do not increase significantly with
temperature but their vibration pattern
• Semiconductor materials however exhibit an increased level of conductivity
with the application of heat.
• As temperature rises, an increasing number of valence electrons absorb
sufficient thermal energy to break covalent bond and contribute to the
number of free carriers.

11. Energy levels
• Within the atomic structure of every isolated atom there are specific energy levels
associate with each shell and orbiting electron.
• The farther an electron is from the nucleus, the higher is the energy state, and any
electron that has left its parent atom has a higher energy state than any electron in
the atomic structure.
• Only specific energy levels can exist for the electrons in the atomic structure of an
isolated atom.
• The result is a series of gaps between allowed energy levels where carriers are not
permitted.

12. Energy levels
Figure 3. : Energy levels (a)
discrete levels in isolated
atomic structures
Figure 3 (b) :Conduction and
valence bands of an insulator,
a semiconductor and a
conductor

13. • An electron in the valence band of Silicon must absorb more
energy than one in the valence band of Germanium to become a
free carrier.
• Similarly an electron in the valence band of GaAs must gain more
energy than one in Silicon or Ge to enter the conduction band.
• This difference in energy gap requirements reveals the sensitivity
of each type of semiconductor to changes in temperature.
• This sensitivity to changes in energy level can have positive and
negative effects.
• For transistor networks, where stability is a high priority, this
sensitivity to temperature or light can be a detrimental factor.

14. • The wider the energy gap, the greater is the possiblility of energy
being released in the form of visible or invisible light waves
• For conductors, the overlapping of valence and conduction bands
essentially results in all the additional energy picked up by the
electrons being dissipated in the form heat.
• Similarly, for Ge and Si since the energy gap is small, most of the
electrons that pick up sufficient energy to leave the valence band
end up in conduction band and the energy is dissipated in the
form of heat.
• However for GaAs the gap is sufficintly large to result in
significant light radiation.

15. • As the atoms of a material are brought closer together to form the crystal lattice structure,
there is an interaction between atoms
• The electrons of a particular shell of an atom having slightly different energy levels from
electrons in the same orbit of an adjoining atom.
• The result is an expansion of the fixed, discrete energy levels of the valence electrons (Fig.3a)
to bands as shown in Fig. 3b.
• The valence electrons in a Silicon material can have varying energy levels as long as they fall
within the band of Fig. 3b
• Fig. 3b clearly reveals that there is a minimum energy level associated with electrons in the
conduction band and a max energy level of electrons bound to the valence shell of the atom
• Between the two is an energy gap that the electron in the valence band must overcome to
become free carrier.
• That energy gap is different for Ge, Si and GaAs; Ge has the smallest gap and GaAs has the
largert gap.

16. Energy bands in Insulators, Conductors and Semiconductors

17. •

18. Semiconductor Material Classification
 A semiconductor is a material that has intermediate conductivity between a conductor and an insulator
 Semi = Half
 Conductor = Material that can carry electricity
S/C Material
Intrinsic
(Pure Si & Ge)
Extrinsic
(doped with impurities)
N-type
-doped with pentavalents
impurities
-donor impurities
-e majority carrier
-h minority carrier
P-Type
-doped with trivalent
impurities
-Acceptor impurities
-h majority carrier
-e minority carrier

19. • The ability to produce semiconductor material of very high purity is
important.
• Adding one part of impurity (of proper type) per million in a wafer of Silicon
material can change the material from relatively poor conductor to a good
conductor of electricity.
• The ability to change the characteristics of a material through this process is
called doping.
Doping

20. Doping
• Doping process
– Adding impurities into pure semiconductor material (Si @ Ge)
• Why need doping process?
– To produce two types of semiconductor material
• N-type
• P-type
• Higher dopant concentration = more carriers (electrons or holes)

21. N-Type
• To produce N-type semiconductor
• The addition of pentavalent (5) or donor impurities contributes free electrons & greatly increasing
the conductivity of the intrinsic semiconductor.
+
Si
Antimony
Arsenic
Phosphorus

22. P-Type
• To produce P-type semiconductor
– The addition of trivalent (3) or acceptor impurities creates deficiencies of valence electrons,
called "holes"
Boron
Aluminium
Gallium
+
Therefore, creates a hole in the structure and it is considered the
MAJORITY carriers and the electrons are the MINORITY carriers.
Si

23. P-N Junction
• When p-type and n-types materials are placed in contact with
each other, the junction behaves very differently than either type
of material alone.
• The open circles on the left side of the junction above represent
"holes" which can act like positive charge carriers.
• The solid circles on the right of the junction represent the
available electrons from the n-type dopant.

24. P-N Junction
• Depletion region
– Electron and holes diffuses across the p-n junction creating depletion region

25. Depletion Region Process
• Before formation of P-N junction
– There is a gap between P and N material

26. Depletion Region Process
• When a p-n junction is formed, some of the free electrons in the n-region
diffuse across the junction and combine with holes to form negative ions. In
doing so they leave behind positive ions at the donor impurity sites.

27. Depletion Region Process
• Filling a hole makes a negative ion and leaves behind a positive ion on the n-
side. A space charge builds up, creating a depletion region which inhibits any
further electron transfer unless it is helped by putting a bias or supply on the
junction.

28. Depletion Region Process

29. Doped semiconductor material application: DIODE
• A diode is a circuit component, which is used to allow current flow through a
circuit in one way and blocking current from opposite direction.

30. How diode is formed?
• The combination of P and N doped semiconductor materials

31. • Formed by principle of doping semiconductor materials (such as
silicon) to create P and N-type semiconductors.
• When a P-type material and an N-type material are placed in close
contact, a PN junction is formed.
• This separation of charges develops a potential across the
depletion region, preventing further diffusion of carriers across
the junction.
• This potential, known as the potential barrier, is about 0.7V in a
typical silicon and 0.3V in a germanium p-n junction
How diode is formed?

32. Activity
1) The term 'covalent bonding' refers to:
(a) the introduction of an impurity
(b) the sharing of valence electrons
(c) the generation of surplus electrons
2) How many electrons are present in the outer valence shell of a silicon
atom?
(a) 1
(b) 2
(c) 4
3) In its pure state, silicon has the properties of:
(a) a conductor
(b) an insulator
(c) a semiconductor
4) In a semiconductor diode, the depletion region is removed when:
(a) the diode is in its forward conducting state
(b) the diode is in its reverse non-conducting state
(c) there is no potential difference between the anode and cathode
5) In a semiconductor, are the valence shells filled, empty, or partially filled?
6) Are electrons in the valence band of a semiconductor are in the bonding or anti bonding state?
7) As one electron is promoted from the valence band to the conduction band, a is formed in the valence band.
8) As the temperature increases, (more, less) electrons can be promoted to the conduction band?
9) Both and are considered charge carriers.
10) Group elements are used as dopants to produce n-type semiconductors, because they have than the
original Group 4 material.
11) Group elements are used as dopants to produce p-type semiconductors, because they have than the
original Group 4 material.
12) A diode contains both and regions.
13) For current to flow through a diode, the positive terminal of the power supply must be connected to the material.