Transcript 10.Doping of Semiconductors

Introduction to
Semiconductor
Material and Devices
The Valence Band
The valence band is the band made up
of the occupied molecular orbitals and
is lower in energy than the so-called
conduction band. It is generally
completely full in semi-conductors
When heated, electrons from this band
jump out of the band across the band
gap and into the conduction band,
making the material conductive.
Conduction Band
• The conduction band is the band of
orbitals that are high in energy and
are generally empty. In reference to
conductivity in semiconductors, it is
the band that accepts the electrons
from the valence band.
Silicon Lattice
• Silicon atoms form covalent bonds and
can crystallize into a regular lattice. The
illustration below is a simplified sketch;
the actual crystal structure of silicon is a
diamond lattice.
• This crystal is called an intrinsic
semiconductor and can conduct a small
amount of current. Pure silicon is not
capable to conduct current very well.
Silicon Crystal
Intrinsic Semiconductor
• A silicon crystal is different from an insulator
because at any temperature above absolute
zero temperature, there is a finite probability
that an electron in the lattice will be knocked
loose from its position, leaving behind an
electron deficiency called a "hole".
• If a voltage is applied, then both the electron
and the hole can contribute to a small current
flow.
Intrinsic Semiconductor
• The conductivity of a semiconductor can
be modeled in terms of the band theory
of solids. The band model of a
semiconductor suggests that at ordinary
temperatures there is a finite possibility
that electrons can reach the conduction
band and contribute to electrical
conduction.
The term intrinsic here distinguishes between
the properties of pure "intrinsic" silicon and the
dramatically different properties of doped ntype or p-type semiconductors.
Valence Electrons
• The electrons in the outermost shell of an
atom are called valence electrons; they dictate
the nature of the chemical reactions of the
atom and largely determine the electrical
nature of solid matter.
• The electrical properties of matter are
pictured in terms of how much energy it takes
to free a valence electron.
Different valence shells
Germanium and Silicon
• In solid state electronics, either pure silicon or
germanium may be used as the intrinsic semiconductor
which forms the starting point for fabrication. Each has
four valence electrons, but germanium at a given
temperature have more free electrons and a higher
conductivity. Silicon is by far the more widely used
semiconductor for electronics, partly because it can be
used at much higher temperatures than germanium.
• Germanium is temperature sensitive and Silicon is
temperature efficient.
• Silicon is most widely used material for fabrication
because of its characteristics.
Silicon and Germanium atoms
Doping
• The property of semiconductors that makes
them most useful for constructing electronic
devices is that their conductivity may easily be
modified by introducing impurities into their
crystal. The process of adding controlled
impurities to a semiconductor is known as
doping. The amount of impurity, or dopant,
added to an intrinsic (pure) semiconductor
varies its level of conductivity. Doped
semiconductors are often referred to as extrinsic
semiconductor.
The Doping of Semiconductors
The addition of a small percentage of
foreign atoms in the regular crystal
Lattice of pure silicon or germanium
produces dramatic changes in their
electrical properties, producing:
n-type and
p-type semiconductors.
Pentavalent impurities:
Impurity atoms with 5
valence electrons produce ntype semiconductors by
contributing extra electrons.
Trivalent impurities:
Impurity atoms with 3
valence electrons produce ptype semiconductors by
producing a "hole" or an
electron deficiency.
P- and N- Type Semiconductors
N-Type Semiconductor
The addition of pentavalent
impurities such as antimony,
arsenic or phosphorous
contributes free electrons,
greatly increasing the
conductivity of the intrinsic
semiconductor.
Phosphorous may be added
by diffusion of phosphine
gas.
P-Type Semiconductor
The addition of trivalent
impurities such as boron,
aluminum or gallium to an
intrinsic semiconductor
creates deficiencies of
valence electrons, called
"holes". It is typical to use
B2H6 diborane gas to diffuse
boron into the silicon
material.
Bands for Doped Semiconductors
• The application of band theory to n-type and
p-type semiconductors shows that extra levels
have been added by the impurities. In n-type
material there are electron energy levels near
the top of the band gap so that they can be
easily excited into the conduction band. In ptype material, extra holes in the band gap
allow excitation of valence band electrons,
leaving mobile holes in the valence band.
• Doping is the process of add impurities to
intrinsic semiconductors to alter their
properties. Normally Trivalent and Pentavalent
elements are used to dope Silicon and
Germanium. When a intrinsic semiconductor
is doped with Trivalent impurity it becomes a
P-Type semiconductors. The P stands for
Positive, which means the semiconductor is
rich in holes or Positive charged ions.
• When we dope intrinsic material with
Pentavalent impurities we get N-Type
semiconductor, where N stands for
Negative. N-type semiconductors have
Negative charged ions or in other words
have excess electrons.
N-Type Doping
• Now lets see what will happen when we pop
in a pentavalent element into the lattice. As
you can see the image (Figure : N-type), we
have doped the silicon lattice with
Phosphorous, a pentavalent element. Now
pentavalent element has five electrons, so it
shares a electron with each of the four
neighboring silicon atoms, hence four atoms
are tied up with the silicon atoms in the
lattice.
• This leaves an electron extra. This excess
electron is free to move and is
responsible conduction.
• Hence N-type (Negative Type) extrinsic
semiconductor (silicon in this case) is
made by doping the semiconductor with
pentavalent element.
P-Type Doping
P-Type
• To create a P-type semiconductor, all we
must do is to pop in a trivalent element into
the lattice. A trivalent element has three
electrons in its valence shell. It shares three
electrons with three neighboring silicon
atoms in the lattice, the fourth silicon atom
demands an electron but the trivalent atom
has no more electron to share.
P-Type
• This creates a void in lattice which we call it
has hole. Since the electron is deficient, the
hole readily accepts an electron, this makes it
a P-type (Positive type) extrinsic
semiconductor.
• As you can see at image (Figure: P-type), we
have doped in boron (trivalent element) in
silicon lattice. This has created a hole making
the semiconductor a P-type material.
• The case is no different in Germanium. It
behaves same as silicon how ever some
properties do differ which makes
germanium based devices used in
certain application and silicon based
devices used in other applications.
Fermi Level
• Fermi Level is the energy level at which an
average of 50% of the available quantum
states are filled by an electron. It was named
after the famous physicist Enrico Fermi, who
had a significant hand in developing the atom
bomb. The Fermi Level relates the probable
location of electrons in a band diagram.
Fermi Level
• If you are looking at a band diagram of a
substance (usually a semiconductor) the Fermi
Level tells you where the average electron is.
For metals the Fermi Level lies in the
conduction band and for insulators the Fermi
Level lies in the valence band and for
semiconductors the Fermi Level lies in the
band gap.
Fermi Level
• Semiconductors are unique because the
Fermi Level lies in the band gap which
cannot contain electrons. This, however,
doesn't prevent the statistical location of
the Fermi Level lying in the band gap.
Fermi level
• Energy ^
| Conduction Band
|
| ----------------------|
| Band
| - - - - - - - - Fermi Level
| Gap
|
|
| ----------------------|OOOOOOOOOOOO
| Valence Band \
|
\ electrons
|------------------> x direction
Fermi Level
• Note how the valence band is full of electrons
and there are relatively few electrons in the
conduction band, placing the Fermi Level right
in the band gap. It is worth mentioning that
when a piece of semiconductor (or any
substance) is at equilibrium, with no net
current or applied field, then the Fermi Level
will be flat.