PHYS 423
Physics of Semiconductor Devices
2022/2023 Academic Year
Course outline
I. Semiconductor materials
• Energy bands, Intrinsic & extrinsic semiconductors, recombination
II. Carrier transport
• Conductivity, mobility, Diffusion processes, and drift
• III. Wafer preparation & Fabrication
• Epitaxial deposition, oxidation, masking and photoprocessing,
impurity introduction and redistribution, chemical vapour
deposition, metallization
IV. PN Junctions & Devices
• Fabrication processes of p-n junctions, metal-semiconductor
junction diodes, Junction and Metal-Oxide-Semiconductor Field
Effect Transistors, Bipolar Junction Transistors (BJTs).
V. Special purpose Diodes
• Light Emitting Diodes, Charge transfer devices, semiconductor
lasers, solar cell, Negative resistance devices e.t.c
PHYS 423 Dr. D. Choge
Lecture One
References
PHYS 423 Dr. D. Choge
Lecture One
References
PHYS 423 Dr. D. Choge
Lecture One
Lecture One
PHYS 423 Dr. D. Choge
Lecture One
Solid-State Electronic Materials
• In studying solid state electronic devices we are
interested primarily in the electrical behavior of
solids.
• The transport of charge through a metal or a
semiconductor depends not only on the properties of
the electron but also on the arrangement of atoms in
the solid.
PHYS 423 Dr. D. Choge
Lecture One
Semiconductor Materials
• Semiconductors are a group of materials having
electrical conductivities intermediate between metals
and insulators
• The conductivity of these materials can be varied
over orders of magnitude by changes in
temperature, optical excitation, and impurity
content
• Generally, electronic materials fall into three categories
(WRT resistivity):
– Insulators
 > 105 -cm (diamond  = 1016 )-electrons tightly bound
– Semiconductors 10-3 <  < 105 -cm(some electrons free to carry current)
– Conductors
 < 10-3 -cm (copper  = 10-6 ) many electrons available
PHYS 423 Dr. D. Choge
Lecture One
Semiconductor Materials
• Semiconductor materials are found in column IV
and neighboring columns of the periodic table
•
PHYS 423 Dr. D. Choge
➢ Column IV semiconductors,
silicon and germanium, are
called
elemental
semiconductors because they
are composed of single species
of atoms
➢ Compounds of column III and
column V atoms, as well as
certain combinations from II
and VI, and from IV, make up
the compound semiconductors.
Lecture One
Compound Semiconductors
➢ The two-element (binary)
III-V compounds such as
GaN, GaP, and GaAs are
common in light-emitting
diodes (LEDs).
➢ Three-element
(ternary)
compounds such as GaAsP
and
four-element
(quaternary)
compounds
such as InGaAsP can be
grown to provide added
flexibility
in
choosing
materials properties
PHYS 423 Dr. D. Choge
Lecture One
Electronic Bands
• One of the most important characteristics of a
semiconductor, which distinguishes it from
metals and insulators, is its energy band gap.
• This property, determines among other things
the wavelengths of light that can be absorbed or
emitted by the semiconductor.
PHYS 423 Dr. D. Choge
Lecture One
Electronic Bands
• Bandgap is an energy range in a solid where no electron
states can exist. It refers to the energy difference between
the top of the valence band and the bottom of the
conduction band in insulators and semiconductors
Semiconductor
PHYS 423 Dr. D. Choge
Bandgap
Energy EG (eV)
Carbon (diamond)
5.47
Silicon
1.12
Germanium
0.66
Tin
0.082
Gallium arsenide
1.42
Gallium nitride
3.49
Indium phosphide
1.35
Boron nitride
7.50
Silicon carbide
3.26
Cadmium selenide
1.70
Lecture One
Conductors
• In a conductor, the outer electrons of the atoms
are loosely bound and free to move through the
material.
• The outer-most band in an atom is the valence
band.
• When atoms form a solid, electrons may be
elevated into a higher energy level called the
conduction band.
• In conductors, we find that the conduction and
valence bands overlap and there is absence of
forbidden zone
PHYS 423 Dr. D. Choge
Lecture One
Conductors
Fig: Energy band structure of a conductor
PHYS 423 Dr. D. Choge
Lecture One
Semiconductors
•
•
•
These are materials having conductivities falling
between those of metals (conductors) and insulators.
Here, the forbidden zone is very small. With such a
small gap, a small thermal or electrical excitation can
elevate electrons into the conduction zone.
Applied electric fields can elevate many electrons to
the conduction band.
PHYS 423 Dr. D. Choge
Lecture One
Insulators
•
Here, the outermost electrons of their atoms are so
tightly bound that there are no free electrons that can
freely move throughout the material
PHYS 423 Dr. D. Choge
Lecture One
Band structure in Semiconductors
•
•
•
•
•
Transition of electrons in the semiconductors may
also be used to categorize them either as direct
bandgap semiconductors or indirect bandgap
semiconductors.
The electron energy levels are arranged in bands.
These bands can either be conducting or forbidden.
Pauli’s Exclusion Principle: no two electrons can
simultaneously occupy the same quantum state.
This principle applies to the class of particles called
fermions and includes electrons, protons, and
neutrons.
The energy separation between the filled and empty
bands is called the energy gap, Eg.
PHYS 423 Dr. D. Choge
Lecture One
Band structure in Semiconductors
•
Wave-particle duality: an electron has a wavelength
associated with it, called the de- Broglie wavelength
that accounts for its wavelike behavior
•
The momentum of an electron is expressed as
• Free electron has only kinetic energy given by
PHYS 423 Dr. D. Choge
Lecture One
Band structure in Semiconductors
Substituting for p, the expression becomes
PHYS 423 Dr. D. Choge
Lecture One
Band structure in Semiconductors
The relationship between E and k is called a parabolic
energy vs. k relationship
Each point on the parabola is an
Electron state corresponding to
Possibility of an electron with E
P moving with v=p/m
PHYS 423 Dr. D. Choge
Lecture One
Band structure in Semiconductors
•
•
In some semiconductors, when an electron makes a
transition from the maximum point of the valence band
to the minimum point of the conduction band, it may
only require change in energy.
Such semiconductors are called direct band-gap
semiconductor and includes GaAs, InP, InAs, InSb and
GaN among others
PHYS 423 Dr. D. Choge
Lecture One
Band structure in Semiconductors
•
•
Semiconductor absorbs light with photon energy hv Eg
exciting an electron from VB to CB state
Light with photon energy hv Eg is not absorbed and
passes through the material.
PHYS 423 Dr. D. Choge
Lecture One
Band structure in Semiconductors
•
•
•
Some semiconductors do not only require energy
change greater than band gap energy Eg, but also some
momentum change for electron transition from the top
point of valence band to the bottom point of the
conduction band.
These semiconductors are called indirect band-gap
semiconductors.
Examples include:
• Silicon, Germanium
PHYS 423 Dr. D. Choge
Lecture One
REVIEW QUESTIONS
1. What are elemental semiconductors? Give examples
2. Define compound semiconductors and give some examples
3. Define binary, ternary and quaternary semiconductors
and give examples in each case
4. On the basis of band theory explain how the crystalline
solids are classified into conductors, semiconductors and
insulators.
5. Use energy band diagrams to differentiate between direct
bandgap and indirect bandgap semiconductors and give
examples in each case
5. What is electron wavevector? How does it relate to the
electron momentum?
PHYS 423 Dr. D. Choge
Lecture One