1. SOLIDS AND SEMICONDUCTOR
DEVICES
Electrical conductivity
Energy bands in solids
Band structure and conductivity
Semiconductors
Intrinsic semiconductors
Doped semiconductors
n-type materials
p-type materials
Diodes and transistors
p-n junction
depletion region
forward biased p-n junction
reverse biased p-n junction diode
2.
3. ELECTRICAL CONDUCTIVITY
R = ρL/A, R = resistance, L = length, A = cross section area; resistivity at 20 o
C in order of conductivity:
superconductors, conductors, semiconductors, insulators
superconductors: certain materials have zero resistivity at very low
temperature.
conductors: material capable of carrying electric current, i.e. material
which has “mobile charge carriers” (e.g. electrons, ions,..) e.g. metals,
liquids with ions (water, molten ionic compounds), plasma.
semiconductors: materials with conductivity between that of conductors
and insulators; e.g. germanium Ge, silicon Si, GaAs, GaP, InP.
insulators: materials with no or very few free charge carriers; e.g. quartz,
most covalent and ionic solids, plastics
4. ENERGY BANDS IN SOLIDS:
In solid materials, electron energy levels form bands of
allowed energies, separated by forbidden bands
valence band = outermost (highest) band filled with
electrons (“filled” = all states occupied)
conduction band = next highest band to valence band
(empty or partly filled)
“gap” = energy difference between valence and conduction
bands, = width of the forbidden band
Note:
electrons in a completely filled band cannot move, since
all states occupied (Pauli principle); only way to move
would be to “jump” into next higher band - needs energy;
electrons in partly filled band can move, since there are
5. Classification of solids into three
according to their band structure:
types,
Insulators: gap = forbidden region between
highest filled band (valence band) and lowest
empty or partly filled band (conduction band) is
very wide, about 3 to 6 eV;
semiconductors: gap is small - about 0.1 to 1 eV;
conductors: valence band only partially filled, or (if
it is filled), the next allowed empty band overlaps
with it
7. What Is a Semiconductor?
•Many materials, such as most metals, allow electrical current
to flow through them
•These are known as conductors
•Materials that do not allow electrical current to flow through
them are called insulators
•Pure silicon, the base material of most transistors, is
considered a semiconductor because its conductivity can be
modulated by the introduction of impurities
8. Semiconductors
A material whose properties are such that it is not quite a
conductor, not quite an insulator
Some common semiconductors
elemental
• Si - Silicon (most common)
• Ge - Germanium
compound
• GaAs - Gallium arsenide
• GaP - Gallium phosphide
• AlAs - Aluminum arsenide
• AlP - Aluminum phosphide
• InP - Indium Phosphide
9. Crystalline Solids
In a crystalline solid, the periodic
arrangement of atoms is repeated over the
entire crystal
Silicon crystal has a diamond lattice
10. Crystalline Nature of Silicon
Silicon
as utilized in integrated circuits is
crystalline in nature
As
with all crystalline material, silicon
consists of a repeating basic unit structure
called a unit cell
For
silicon, the unit cell consists of an atom
surrounded by four equidistant nearest
neighbors which lie at the corners of the
tetrahedron
11. What’s so special about Silicon?
Cheap and abundant
Amazing mechanical,
chemical and electronic
properties
The material is very
well-known to mankind
SiO2: sand, glass
Si is column IV of the periodic table
Similar to the carbon (C) and the
germanium (Ge)
Has 3s² and 3p² valence electrons
12. Semiconductor Crystalline Structure
Silicon atoms have 4 electrons
in outer shell
inner
electrons are very
closely bound to atom
These electrons are shared
with neighbor atoms on both
sides to “fill” the shell
resulting structure is very
stable
electrons are fairly tightly
bound no “loose” electrons
at
room temperature, if
battery applied, very little
electric current flows
13. Conduction in Crystal Lattices
Semiconductors (Si and Ge) have 4 electrons in their
outer shell
2 in the s subshell
2 in the p subshell
As the distance between atoms decreases the discrete
subshells spread out into bands
As the distance decreases further, the bands overlap and
then separate
the subshell model doesn’t hold anymore, and the
electrons can be thought of as being part of the crystal,
not part of the atom
4 possible electrons in the lower band ( valence band)
4 possible electrons in the upper band ( conduction
band)
15. Nature of Intrinsic Silicon
Silicon
that is free of doping impurities is
called intrinsic
Silicon
has a valence of 4 and forms
covalent
bonds
with
neighboring silicon atoms
four
other
16. Insulators, Semiconductors, and Metals
The separation of the valence and conduction bands
determines the electrical properties of the material
Insulators have a large energy gap
electrons can’t jump from valence to conduction bands
no current flows
Conductors (metals) have a very small (or nonexistent) energy
gap
electrons easily jump to conduction bands due to thermal
excitation
current flows easily
Semiconductors have a moderate energy gap
only a few electrons can jump to the conduction band
• leaving “holes”
only a little current can flow
18. Hole - Electron Pairs
Sometimes thermal energy is enough to cause an electron to
jump from the valence band to the conduction band
produces a hole - electron pair
Electrons also “fall” back out of the conduction band into the
valence band, combining with a hole
pair elimination
hole
pair creation
electron
19. Improving Conduction by Doping
To
make semiconductors better conductors,
add impurities (dopants) to contribute extra
electrons or extra holes
elements with 5 outer electrons contribute an
extra electron to the lattice (donor dopant)
elements with 3 outer electrons accept an
electron from the silicon (acceptor dopant)
20. Improving Conduction by Doping (cont.)
Phosphorus and arsenic are
donor dopants
if phosphorus is introduced
into the silicon lattice,
there is an extra electron
“free” to move around and
contribute
to
electric
current, very loosely bound
to atom and can easily
jump to conduction band
produces n type silicon
sometimes use + symbol
to indicate heavier doping,
so n+ silicon
phosphorus
becomes
positive ion after giving up
electron
21. Improving Conduction by Doping (cont.)
Boron has 3 electrons
in its outer shell, so it
contributes a hole if it
displaces a silicon
atom
boron is an acceptor
dopant
yields p type silicon
Boronbecomes negative
ion after accepting an
electron
22. Diffusion of Dopants
It is also possible to
introduce dopants into
silicon by heating them so
they diffuse into the silicon
no new silicon is added
high
heat
causes
diffusion
Can
be
done
with
constant concentration in
atmosphere
close
to straight line
concentration gradient
Or with constant number
of atoms per unit area
predeposition
bell-shaped gradient
Diffusion
causes
spreading of doped areas
top
side
23. Hole and Electron Concentrations
To produce reasonable levels of conduction doesn’t require
much doping
silicon has about 5 x 1022 atoms/cm3
typical dopant levels are about 1015 atoms/cm3
In undoped (intrinsic) silicon, the number of holes and number
of free electrons is equal, and their product equals a constant
actually, ni increases with increasing temperature
np = ni2
This equation holds true for doped silicon as well, so increasing
the number of free electrons decreases the number of holes
24. INTRINSIC (PURE) SILICON
At 0 Kelvin Silicon density
is 5x10²³ particles/cm³
Silicon has 4 valence
electrons, it covalently bonds
with four adjacent atoms in
the crystal lattice
Higher temperatures
create free charge carriers.
A “hole” is created in the
absence of an electron.
At 23ºC there are 10¹º
particles/cm³ of free
carriers
25. DOPING
There are two types of doping
N-type and P-type.
The N in N-type stands for
negative.
A column V ion is inserted.
The extra valence electron is free
to move about the lattice
The P in P-type stands for
positive.
A column III ion is inserted.
Electrons from the surrounding
Silicon move to fill the “hole.”
26. Energy-band Diagram
A very important concept in the study of
semiconductors is the energy-band diagram
It is used to represent the range of energy a
valence electron can have
For semiconductors the electrons can have any
one value of a continuous range of energy levels
while they occupy the valence shell of the atom
That band of energy levels is called the valence
band
Within the same valence shell, but at a slightly
higher energy level, is yet another band of
continuously variable, allowed energy levels
This is the conduction band
27. Band Gap
Between
the valence and the conduction
band is a range of energy levels where there
are no allowed states for an electron
This is the band gap E G
In silicon at room temperature [in electron
.
volts]: E G = 11 eV
Electron volt is an atomic measurement
unit, 1 eV energy is necessary to decrease of
the potential of the electron with 1 V.
1eV = 1.602 × 10 −19 joule
28. Impurities
Silicon crystal in pure form is
good insulator - all electrons
are bonded to silicon atom
Replacement of Si atoms can
alter electrical properties of
semiconductor
Group number - indicates
number of electrons in
valence level (Si - Group IV)
29. Impurities
Replace Si atom in crystal with Group V atom
substitution of 5 electrons for 4 electrons in outer shell
extra electron not needed for crystal bonding structure
• can move to other areas of semiconductor
• current flows more easily - resistivity decreases
• many extra electrons --> “donor” or n-type material
Replace Si atom with Group III atom
substitution of 3 electrons for 4 electrons
extra electron now needed for crystal bonding structure
• “hole” created (missing electron)
• hole can move to other areas of semiconductor if
electrons continually fill holes
• again, current flows more easily - resistivity decreases
• electrons needed --> “acceptor” or p-type material
31. n-type material
Donor (n-type) impurities:
Dopant with 5 valence electrons (e.g. P, As, Sb)
4 electrons used for covalent bonds with surrounding Si
atoms, one electron “left over”; left over electron is only
loosely bound⇒ only small amount of energy needed
to lift it into conduction band (0.05 eV in Si)
⇒ “n-type semiconductor”, has conduction electrons, no
holes (apart from the few intrinsic holes)
32. p-type material
Acceptor (p-type) impurities:
dopant with 3 valence electrons (e.g. B, Al, Ga, In) ⇒ only 3 of the
4 covalent bonds filled ⇒ vacancy in the fourth covalent bond ⇒
hole.
“p-type semiconductor”, has mobile holes, very few mobile
electrons (only the intrinsic ones).
33. P-N Junction
Also known as a diode
One of the basics of semiconductor technology -
Created by placing n-type and p-type material in close
contact
Diffusion - mobile charges (holes) in p-type combine
with mobile charges (electrons) in n-type
34. P-N Junction
Region of charges left behind (dopants fixed in crystal lattice)
Group III in p-type (one less proton than Si- negative charge)
Group IV in n-type (one more proton than Si - positive charge)
Region is totally depleted of mobile charges - “depletion
region”
Electric field forms due to fixed charges in the depletion region
Depletion region has high resistance due to lack of mobile charges
35.
36. p-n JUNCTION:
p-n junction = semiconductor in which impurity
changes abruptly from p-type to n-type ;
“diffusion” = movement due to difference in
concentration, from higher to lower concentration;
• in absence of electric field across the junction, holes
“diffuse” towards and across boundary into n-type and
capture electrons;
• electrons diffuse across boundary, fall into holes
(“recombination of majority carriers”); ⇒ formation of a
“depletion region”(= region without free charge carriers)
around the boundary;
• charged ions are left behind (cannot move):
negative ions left on p-side ⇒ net negative charge on p-side of
the junction;
positive ions left on n-side ⇒ net positive charge on n-side of the
junction
⇒ electric field across junction which prevents further diffusion.