Fundamental aspects of

semiconductor physics

Introduction
Semiconductors are materials whose electronic properties are intermediate
between those of Metals and Insulators.
The interesting feature about semiconductors is that they are bipolar and current
is transported by two charge carriers of opposite sign.
Silicon and Germanium are elemental semiconductors and they have four
valence electrons which are distributed among the outermost S and p orbital's.
These form four covalent bonds of equal angular separation leading to a
tetrahedral arrangement of atoms in space results tetrahedron shape, resulting
crystal structure is known as Diamond cubic crystal structure

Semiconductors are mainly two types

1. Intrinsic (Pure) Semiconductors
2. Extrinsic (Impure) Semiconductors

Intrinsic Semiconductor
A Semiconductor which does not have any kind of impurities,
behaves as an Insulator at 0k and behaves as a Conductor at higher
temperature is known as Intrinsic Semiconductor or Pure
Semiconductors.
Germanium and Silicon (4th group elements) are the best examples
of intrinsic semiconductors and they possess diamond cubic
crystalline structure.

Extrinsic Semiconductors
The Extrinsic Semiconductors are those in which impurities of
large quantity are present. Usually, the impurities can be either 3 rd
group elements or 5th group elements.
Based on the impurities present in the Extrinsic Semiconductors,
they are classified into two categories.
1. N-type semiconductors
2. P-type semiconductors

N - type Semiconductors
When any pentavalent element such as Phosphorous, Arsenic or
Antimony is added to the intrinsic Semiconductor , four electrons
are involved in covalent bonding with four neighboring pure
Semiconductor atoms.
The fifth electron is weakly bound to the parent atom. And even for
lesser thermal energy it is released Leaving the parent atom
positively ionized.
The Intrinsic Semiconductors doped with pentavalent impurities are
called N-type Semiconductors.
The energy level of fifth electron is called donor level.
The donor level is close to the bottom of the conduction band most
of the donor level electrons are excited in to the conduction band at
room temperature and become the Majority charge carriers.
Hence in N-type Semiconductors electrons are Majority carriers and
holes are Minority carriers.

N-type
Semiconductor
Free electron
Si

Si

Si

Si

Impure atom
(Donor)

Conduction band
Ec
Ec
E
Donor levels
Electron
energy

Ev
Valence band

Distance

Ed
Eg

P-type semiconductors
When a trivalent elements such as Al, Ga or Indium have three
electrons in their outer most orbits , added to the intrinsic
semiconductor all the three electrons of Indium are engaged in
covalent bonding with the three neighboring Si atoms.
Indium needs one more electron to complete its bond. this
electron maybe supplied by Silicon , there by creating a vacant
electron site or hole on the semiconductor atom.
Indium accepts one extra electron, the energy level of this
impurity atom is called acceptor level and this acceptor level lies
just above the valence band.
These type of trivalent impurities are called acceptor impurities
and the semiconductors doped the acceptor impurities are called
P-type semiconductors.

Hole

Co-Valent
bonds

Si

Si

In

Si
Impure atom
(acceptor)

Si

Conduction band

Ec

Ec
E
Eg

Electron
energy

Acceptor levels

Ev
Valence band

temperature

Ea

 Even at relatively low temperatures, these acceptor atoms get

ionized taking electrons from valence band and thus giving rise to
holes in valence band for conduction.
Due to ionization of acceptor atoms only holes and no electrons
are created.
Thus holes are more in number than electrons and hence holes are
majority carriers and electrons are minority carriers in P-type
semiconductors.

P-N junction

A diode is formed by putting a N-type and P-type of

semiconductor together

Migration of holes from P to N

And electrons from N to P causes
a formation of depletion layer
P type

Anode

- - ++
- - ++
- - ++
- - ++

N type

Cathode

This gives rise to barrier potential(E)

preventing further migration of
holes and electrons

Process of emitting Light

n-type & p-type semiconductors are
combined in one device.
With the application of a voltage
between the p-side and the n-side, free
electrons from the n-type side go to the ptype side through the junction.
When an electron meets a hole, it
recombines and thus releases its energy
by emitting a photon.
The energy of photon is the energy of
the bandgap .
The photons are released with bandgaps
corresponding to near infrared, visible, or
near ultraviolet light .

Forward-biased pn junction
Electrons
injected

Holes
injected

Optical radiation
Optical radiation is part of the electromagnetic spectrum. It is
subdivided into ultraviolet radiation (UV), the spectrum of
light visible for man (VIS) and infrared radiation (IR).
It ranges between wavelengths of 100 nm to 1 mm.

Electromagnetic waves in this range obey the laws of optics-they can be focused and refracted with lenses

Electromagnetic Spectrum

Luminescence
The phenomenon of emission of optical radiation by
the recombination of injected carriers is called
luminescence.
The p-n junction diode exhibiting this phenomenon is
referred to as light emitting diode.

Characteristics of optical
radiation

Spectral characteristics
Geometrical characteristics
Electrical characteristics
Mechanical characteristics

Optical radiation measurement

quantities
There is a set of five radiation measurement quantities: total
flux, intensity, radiance, exitance and irradiance.
The first four of these quantities are characteristics of radiation
emanating from a radiation source, whereas the fifth,
irradiance, is characteristic of radiation incident upon a
surface.

Solid angle
The geometrical configurations of intensity and radiance make
use of the 3-dimensional concept of a solid angle (), which is
analogous to the 2-dimensional plane angle ().

Total flux
When radiation sources such as the common incandescent
lamps or general lighting service lamps are measured, total
flux is used to indicate the total flux output from the lamp into
all directions.
Specialized equipment, such as integrating spheres or
goniophotometers, is required to collect the radiation output
into all directions from the lamp.

Intensity
First,
the direction from the source (d) in which the intensity is
to be defined must be indicated.
Then the intensity of the source in this specified direction is
defined as the ratio of the flux leaving the source in that
particular direction and propagating into an element of solid
angle containing that specified direction, divided by the size of
that element of solid angle.
The radiant intensity is defined as

Radiance
A radiation source does not emit the same flux into all
directions.
Therefore we have a quantity called radiance that specifies the
flux emitted by a radiation surface from a specified area on the
surface and in a specified direction from the surface, and into a
specified solid angle containing the given direction d.

Irradiance and Exitance

The quantity irradiance is a measure of the amount of radiation
incident upon a surface.
Exitance is very similar to irradiance except that the direction
of flow of the flux is reversedit is leaving the surface.

Spectral characteristics
The emission, reflection and absorption properties of the
sources, detectors and other materials used for optical
radiation measurement are all dependent upon the wavelength
of the radiation under consideration.
The output spectral distributions of incandescent lamps are
particularly suitable for use as standards because the spectral
distribution is a smooth and continuous function of the
wavelength.
This reduces the errors and uncertainties associated with
measurements of their spectral distributions in narrow
wavelength bandwidths.

Geometrical characteristics
The construction of the bulb and the filament characteristics of
these lamps can be somewhat adjusted to facilitate their
application to the different geometrical quantities.
The lamps used for total flux in integrating spheres are usually
designed with spherical bulbs and circular or distributed
filament shapes to provide a uniform spatial output for a
uniform illumination of the sphere walls.
The lamps used for intensity and illuminance are designed
with planar filaments that enables the distance between the
lamp and detector to be determined reproducibly and
accurately.
The bulb of the lamp is also shaped, such as a triangular shape,
to reduce the inter-reflections inside the bulb.
spectral irradiance lamps are usually of the tungsten-halogen
design.

Electrical characteristics
The filament of the incandescent lamp is heated by passing an
electrical current through it. For measurement standard lamps,
this is usually a direct current.
The same polarity of the electrical current must be used each
time the lamp is operated.
To avoid thermal shock to the filament, this current should be
applied gradually, over times on the order of a minute or more.
The electrical quantities must be measured accurately since the
radiant output of an incandescent lamp depends strongly upon
the electrical power applied to the lamp.
Since the electrical power causes a change in temperature, the
spectral distribution of the radiant output changes as well as
the absolute amount of output.

Mechanical characteristics
Incandescent lamps are sensitive to vibration and shock.
In addition to breakage of the glass envelope or the electrical
feed-throughs, the filament structure is particularly fragile.
The electrical and mechanical properties of a lamp change
rapidly when a lamp is first used.

Thank you