physcsi

1. Semiconductor Fundamentals
Dr. M. Yousuf Soomro
Semiconductors Fundamentals

2. Course Outline
To understand the semiconductor materials that are
suitable for electronic devices
To study the properties of materials for electronic
devices
Semiconductor devices are fabricated using
specific materials has the desired physical
properties

3. Solid State Materials
Metals (conductor)
Insulators
Semiconductors
Superconductors

4. Metals
Materials with zero bandgap are metals
A metal has a partially filled conduction band, so there is
no energy gap between filled and unfilled regions.
A significant number of electrons can be excited by heat
into empty energy levels and move easily throughout the
material, allowing the material to conduct electricity

5. Insulators
Materials with an energy gap larger than 3 eV
An insulator possesses a considerable energy gap
between the valence band and the conduction band
It is difficult to excite electrons from the valence band to
the conduction band. As a result an insulator does not
conduct electricity

6. Semiconductor
Special class of materials having conductivity b/w that of
a good conductor and that of an insulator
A material with electrical resistivity lying in the range of
10-2 – 109 Ω.cm
Material whose energy gap for electronic excitations lies
between zero and about 4 electron volts (eV).
a small number of electrons from the valence band can be
promoted to the conduction band by an energy input (e.g.
thermal energy from heat)

7. Semiconductor
Classification of Semiconductor materials
oElemental semiconductor
oCompound semiconductor
oNarrow band-gap semiconductor
oWide band-gap semiconductor
oOxide semiconductor
oMagnetic semiconductor
oOrganic semiconductor
oLow dimension semiconductor

8. Semiconductor
Elemental semiconductor
Silicon (Si)
Germanium (Ge)
o These are important group IV elemental
semiconductors
o All of them have diamond crystal structure
Boron (B)
o It belongs to group III
o It has rhombohedral crystal structure
Phosphorus (P)
o It belongs to group V
Sulphur (S)
Selenium (Se)
Tellurium (Te)
o These belong to group VI

10. Semiconductor
 Elemental semiconductor
Currently silicon is the most important semiconductor material
used in electronic devices
Advantages of Si over other semiconductors are:
A relative ease of passivating the surface by oxidizing in a
controlled manner forming a layer of stable native oxide that
substantially reduces the surface recombination velocity
Its hardness that large wafers to be handled safely without
damaging it
It is thermally stable up to 11000C that allows high-
temperature processes like diffusion, oxidation, and annealing
It is relatively low cost due to established processes

11. Semiconductor
 Elemental semiconductor
 limitations of silicon
Its energy band-gap is 1.12eV
It is a direct semiconductor that limits the application in
optoelectronics
It has relatively low carrier mobility as compared to other
semiconductor such as gallium arsenide GaAs

12. Semiconductor
Compound Semiconductors
They are usually formed from
o III-V group
o II-VI
o IV-VI
III-V group semiconductors are GaAs, GaP, GaN, A1As, InSb,
InAs, InP etc
In general, these crystallized materials

13. Semiconductor
Compound Semiconductors
GaAs, InAs, InP, InSb have direct energy band-gaps and high
carrier mobilities
Common applications of these semiconductors:
 used to design a variety of optoelectronic devices for
1. detection and generation of electromagnetic radiation
2. in high-speed electronic devices
The energy band-gaps of these compounds are useful for
optoelectronic applications
The energy bandgap ranges from 0.17eV for InSb to 3.44eV for
GaN

14. Semiconductor
Compound Semiconductors
II-VI compound semiconductor
II-VI compound semiconductor such as Zn and compounds with
oxygen O, S, Se
These cover a wide range of electronic and optical properties
due to the wide variations in their energy bandgap
These are typically n-type as grown, except ZnTe, which is p-
type
All the II-VI compound semiconductors have direct energy
bandgaps

15. Semiconductor
Compound Semiconductors
IV-VI compound semiconductor
PbS, PbSe, and PbTe
characterized by narrow energy gaps, high carrier mobilities, and
high dielectric constants
The unique feature of the direct energy gap in these compounds
is that its energy band-gap increases with increasing
temperature, which means the energy gap has a positive
temperature coefficient
Main applications of these compounds are in light emitting
devices and detectors in the infrared spectral region

16. Semiconductor
Narrow Band-gap Semiconductor
 InSb
 InAs
 PbSe
 PbTe
 PbS
They have the energy band-gap below about 0.5eV
they are direct semiconductor materials
extensively employed in:
infrared optoelectronic device applications as detectors and
diode lasers

17. Semiconductor
Wide Band-gap Semiconductor
 SiC
 II-V nitrides
high thermal conductivity
high saturation electron drift velocity
high breakdown electric field
superior chemical stability
physical stability
It has wide band-gap that enables detection and emission of light
in short-wavelength region likes blue and ultraviolet

18. Semiconductor
Oxide Semiconductor
 Cu2O
 Bi2O
 ZnO
These are also referred as semiconductor ceramics
They are used in electronic devices and sensors

19. Semiconductor
Magnetic Semiconductor
Semiconductor compound that contains magnetic ions such as
Cr, Mn, Fe, Co, Ni, may exhibit magnetic properties
Some oxides such as FeO and NiO exhibit antiferromagnetic
properties and oxide such as europium oxide EuO is
ferromagnetic properties
The semiconductor exhibits large magneto-optical effect that can
be used to design optical modulators

20. Semiconductor
Organic Semiconductor
 Anthracene CH14H10
 Polyacetylene (CH)n
 advantages of organic semiconductors
1. Diversity
2. relative ease of changing their properties to specific
application

21. Semiconductor
Organic Semiconductor
One of the promising applications of organi semiconductors is in
less iexpensive light emitting diode, covering whole the
spectrum of colors
The main advantages of organic materials in such applications
include
I. low operating voltages
II. color tunability
III. relative simplicity of device fabrication