Silicon in Semiconductor devices

Electronic Material and
Their Applications
Term Paper
Topic : Silicon In Semiconductor Devices
Ashish Singh
B10007

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Electronic Material and Their Applications
Silicon as element
Silicon is the most abundant electropositive element in The Earth’s crust. It’s a
metalloid with a marked metallic lustre and very brittle .Silicon is group IV element.
Silicon is the eighth most common element in the universe by mass, but very rarely
occurs as the pure free element in nature. It is most widely distributed in dusts,
sands, planetoids, and planets as various forms of silicon dioxide (silica) or silicates.
Over 90% of the Earth's crust is composed of silicate minerals, making silicon the
second most abundant element in the Earth's crust (about 28% by mass) after
oxygen.
Silicon is denoted by the symbol Si. Its atomic number is 14 and atomic weight is
28.08. It is a tetravalent element and contains four electrons in its outermost orbit.It
is less reactive than its chemical analog carbon, the non-metal directly above it in the
periodic table, but more reactive than germanium.
Physical properties
Silicon is a solid at room temperature, with relatively high melting and boiling points
of 1414 and 3265 degrees Celsius respectively. It has a greater density in a liquid
state than a solid stateIn its crystalline form, pure silicon has a gray color and a
metallic lustre.
Chemical Properties
Silicon is a relatively inactive element at room temperature. It does not combine with oxygen
or most other elements. Water, steam, and most acids have very little affect on the element.
At higher temperatures, however, silicon becomes much more reactive. In the molten
(melted) state, for example, it combines with oxygen, nitrogen, sulphur, phosphorus, and
other elements. It also forms a number of alloys very easily in the molten state.
Isotopes
There are three naturally occurring isotopes of silicon: silicon-28, silicon-29, and
silicon-30. Isotopes are two or more forms of an element. Isotopes differ from each
other according to their mass number. The number written to the right of the

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element's name is the mass number. The mass number represents the number of
protons plus neutrons in the nucleus of an atom of the element. The number of
protons determines the element, but the number of neutrons in the atom of any one
element can vary. Each variation is an isotope.
Five radioactive isotopes of silicon are known also. A radioactive isotope is one that
breaks apart and gives off some form of radiation. Radioactive isotopes are
produced when very small particles are fired at atoms. These particles stick in the
atoms and make them radioactive.
Figure 1 Pure Silicon
Silicon –a giant covalent Structure
Silicon is a non-metal, and has a giant covalent structure exactly the same as carbon
in diamond - hence the high melting point. You have to break strong covalent bonds
in order to melt it. The element silicon would be expected to form 4 covalent bond(s)
in order to obey the octet rule. Si is a nonmetal in group 4A, and therefore has 4
valence electrons. In order to obey the octet rule, it needs to gain 4 electrons. It can
do this by forming 4 single covalent bonds.

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Figure 2 arrangement of Silicon atoms in unit Cell
Silicon crystallizes in the same pattern as diamond, in a structure which Ashcroft and
Mermin call "two interpenetrating face-centered cubic" primitive lattices. The lines
between silicon atoms in the lattice illustration indicate nearest-neighbor bonds. The
cube side for silicon is 0.543 nm. Germanium has the same diamond structure with a
cell dimension of .566 nm
Semiconductor
A semiconductor is a material which has electrical conductivity between that of a
conductor such as copper and an insulator such as glass. The conductivity of a
semiconductor increases with increasing temperature, behavior opposite to that of a
metal. Current conduction in a semiconductor occurs via free electrons and "holes",
collectively known as charge carriers.
Conduction in Semiconductors

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The most important parameters for conduction in semiconductor material
 the band gap
 the number of free carriers (electrons or holes) available for conduction; and
 the "generation" and recombination of free carriers (electrons or holes) in
response to light shining on the material.
Figure 3 semiconductor energy band
Current is the rate of flow of charge , we shall be able calculate currents flowing in
real devices since we know the number of charge carriers. There are two current
mechanisms which cause charges to move in semiconductors. The two mechanisms
are drift and diffusion.
Drift Current
Figure 4 crystal lattice where the free electrons move randomly in Brownian
motion
Free electrons collide with the stationary atoms and get deflected in a different
direction. The average distance travelled between two collisions is called the 'mean
free path'. In absence of any electric field, no net movement in any direction takes
place as shown by solid arrows.

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With application of the electric field, electrons get accelerated in opposite direction of
the electric field and hence, although the motion is still random, the net movement
takes place as indicated by arrow from left to right. This movement is called 'drift'.
Figure 5 (a) Drift of electrons (b) Drift of holes
Diffusion Current
Figure 6 Diffusion of holes in Brownian motion
Concentration of carriers (holes) is much high on the left side of the surface YY′.
Since the holes are in Brownian motion, they cross over the surface YY′ randomly
from left to right and right to left. But as the concentration of holes on the left side is
much higher than that on the right side, the average number holes going from left to
right is more than number of holes going from right to left of the surface YY′. This
constitutes a net flow of holes (carriers) from left to right, i.e. a net current flow due to
flow of holes from left to right. This is called diffusion current because it results
from diffusion of carriers. Thus, diffusion current is due to flow of charge carriers
from higher concentration to lower concentration region.
The diffusion current is proportional to the concentration gradient

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P –type and N-type Semiconductors
The addition of a small percentage of foreign atoms in the regular crystal lattice of
silicon or germanium produces dramatic changes in their electrical properties,
producing n-type and p-type semiconductors.
Figure 7 P -and N - Type Semiconductor
P-Type
The addition of trivalent impurities such as boron, aluminum or gallium to an intrinsic
semiconductor creates deficiencies of valence electrons,called "holes".
N-Type
The addition of pentavalent impurities such as antimony, arsenic or phosphorous
contributes free electrons, greatly increasing the conductivity of the semiconductor.

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Semiconductor Devices
Diodes
The diode is a device made from a single p–n junction. At the junction of a p-type
and an n-type semiconductor there forms a region called the depletion zone or
region which blocks current conduction from the n-type region to the p-type region,
but allows current to conduct from the p-type region to the n-type region. Thus, when
the device is forward biased, with the p-side at higher electric potential, the diode
conducts current easily; but the current is very small when the diode is reverse
biased.
Exposing a semiconductor to light can generate electron–hole pairs, which increases
the number of free carriers and its conductivity. Diodes optimized to take advantage
of this phenomenon are known as photodiodes. Compound semiconductor diodes
can also be used to generate light, as in light-emitting diodes and laser diodes.
Silicon Diodes
Silicon rectifier diodes, are used in many applications from high voltage, high current
power supplies, where they rectify the incoming mains (line) voltage and must pass
all of the current required by whatever circuit they are supplying; this may be several
tens of Amperes or more.

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Carrying such currents requires a large junction area so that the forward resistance
of the diode is kept as low as possible. Even so the diode is likely to get quite warm.
The black resin case helps dissipate the heat.
The resistance to current in the reverse direction (when the diode is "off") must be
high, and the insulation offered by the depletion layer between the P and N layers
extremely good to avoid the possibility of "reverse breakdown", where the insulation
of the diode fails due to the high reverse voltage across the junction.
Silicon diodes are made in many different forms with widely differing parameters.
They vary in current carrying ability from milli-amps to tens of amps, some will have
reverse breakdown voltages of thousands of volts; others use their junction
capacitance to act as tuning devices in radio and TV tuners.
Silicon Zener Diode
Figure 8 Zener Diode
Zener diodes are special silicon diodes which have a relatively low, defined
breakdown voltage, called the Zener voltage.At low reverse voltages a Zener diode
behaves in a simi-lar manner to an ordinary silicon diode, that is, it passes only a
very small leakage current. If, however, the reverse bias is increased until it reaches
the breakdown region, then a small reverse voltage increase causes a considerable
increase in leakage current; the reverse current is then called the Zener current.
The characteris-tics of a Zener diode operating under reverse breakdown conditions
are similar to those of a struck glow dis-charge tube. Because of this, Zener diodes
can be used in a similar way, i. e. as stabilizers, limiters, ripple reduc-tion elements,
reference voltage sources, and also as DC coupling elements with a constant
voltage drop.

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Tunnel Diode
A tunnel diode or Esaki diode is a type of semiconductor diode that is capable of
very fast operation, well into the microwave frequency region, by using the quantum
mechanical effect called tunneling.
These diodes have a heavily doped p–n junction only some 10 nm (100 Å) wide. The
heavy doping results in a broken bandgap, where conduction band electron states
on the n-side are more or less aligned with valence band hole states on the p-side.
Tunnel diodes are usually made from germanium, but can also be made in gallium
arsenide and silicon materials. They are used in frequency converters and
detectors.[4]
They have negative differential resistance in part of their operating
range, and therefore are also used as oscillators, amplifiers, and in switching circuits
using hysteresis
Silicon Solar cells
The "photovoltaic effect" is the basic physical process through which a PV cell
converts sunlight into electricity. Sunlight is composed of photons, or particles of
solar energy. These photons contain various amounts of energy corresponding to
the different wavelengths of the solar spectrum. When photons strike a PV cell, they
may be reflected or absorbed, or they may pass right through. Only the absorbed
photons generate electricity. When this happens, the energy of the photon is
transferred to an electron in an atom of the cell (which is actually a semiconductor).
With its newfound energy, the electron is able to escape from its normal position
associated with that atom to become part of the current in an electrical circuit. By
leaving this position, the electron causes a "hole" to form. Special electrical
properties of the PV cell—a built-in electric field—provide the voltage needed to drive
the current through an external load (such as a light bulb).

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To induce the electric field within a PV cell, two separate semiconductors are
sandwiched together. The "p" and "n" types of semiconductors correspond to
"positive" and "negative" because of their abundance of holes or electrons (the extra
electrons make an "n" type because an electron actually has a negative charge).
Although both materials are electrically neutral, n-type silicon has excess electrons
and p-type silicon has excess holes. Sandwiching these together creates a p/n
junction at their interface, thereby creating an electric field.
When the p-type and n-type semiconductors are sandwiched together, the excess
electrons in the n-type material flow to the p-type, and the holes thereby vacated
during this process flow to the n-type. (The concept of a hole moving is somewhat
like looking at a bubble in a liquid. Although it's the liquid that is actually moving, it's
easier to describe the motion of the bubble as it moves in the opposite direction.)
Through this electron and hole flow, the two semiconductors act as a battery,
creating an electric field at the surface where they meet (known as the "junction"). It's
this field that causes the electrons to jump from the semiconductor out toward the
surface and make them available for the electrical circuit. At this same time, the
holes move in the opposite direction, toward the positive surface, where they await
incoming electrons.

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Silicon controlled rectifier(Thyristor)
A silicon-controlled rectifier (or semiconductor-controlled rectifier) is a four-
layer solid state current controlling device. The name "silicon controlled rectifier" or
SCR is General Electric's trade name for a type of thyristor.
Figure 9 SCR schematic diagram
SCRs are unidirectional devices (i.e. can conduct current only in one direction) as
opposed to TRIACs which are bidirectional (i.e. current can flow through them in
either direction). SCRs can be triggered normally only by currents going into the gate
as opposed to TRIACs which can be triggered normally by either a positive or a
negative current applied to its gate electrode.
This device is generally used in switching applications. In the normal "off" state, the
device restricts current to the leakage current. When the gate-to-cathode voltage
exceeds a certain threshold, the device turns "on" and conducts current. The device
will remain in the "on" state even after gate current is removed so long as current
through the device remains above the holding current. Once current falls below the
holding current for an appropriate period of time, the device will switch "off". If the
gate is pulsed and the current through the device is below the latching current, the
device will remain in the "off" state.
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals
and electrical power. It is composed of semiconductor material with at least three

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terminals for connection to an external circuit. A voltage or current applied to one
pair of the transistor's terminals changes the current through another pair of
terminals. The transistor is the fundamental building block of modern electronic
devices, and is ubiquitous in modern electronic systems.
Bipolar Junction Transistor
Figure 10 BJT silicon
Figure 11 BJT N-P-N Transistor
A bipolar junction transistor (BJT or bipolar transistor) is a type of transistor that
relies on the contact of two types of semiconductor for its operation. BJTs can be
used as amplifiers, switches, or in oscillators. BJTs can be found either as individual
discrete components, or in large numbers as parts of integrated circuits.
Bipolar transistors are so named because their operation involves both electrons and
holes. These two kinds of charge carriers are characteristic of the two kinds of doped

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semiconductor material. In contrast, unipolar transistors such as the field-effect
transistors have only one kind of charge carrier.
A Bipolar Junction Transistor is made up of a piece of silicon that has been treated to
create a 'channel' in the silicon. First, an undoped (untreated) piece of silicon is
selected. (Undoped silicon acts as an insulator, preventing the flow of current.) Next,
a process is used to create an N-type channel in the silicon. This is silicon that has
an overabundance of negative charge carriers (electrons). A second channel is
created in the first, this time with P-type silicon. P-type silicon carries current
because of the overabundance of positive charge carriers ('holes', they're called).
The process has created three regions in the silicon, an N-type region, a P-type
region, and another N-type region. The base terminal is connected to the P-type
region, while the collector and emitter are connected to the opposing N-type regions.
Darlington Transistor
Figure 12 Darlington Pair diagram
The Darlington transistor (often called a Darlington pair) is a compound structure
consisting of two bipolar transistors (either integrated or separated devices)
connected in such a way that the current amplified by the first transistor is amplified
further by the second one.[1]
This configuration gives a much higher common/emitter
current gain than each transistor taken separately and, in the case of integrated
devices, can take less space than two individual transistors because they can use a
shared collector. Integrated Darlington pairs come packaged singly in transistor-like
packages or as an array of devices (usually eight) in an integrated circuit.

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Silicon Quantum Dot LED
A quantum dot display is a type of display technology used in flat panel displays as
an electronic visual display. Quantum dots (QD) or semiconductor nanocrystals are a
form of light emitting technology and consist of nano-scale crystals that can provide
an alternative for applications such as display technology.
Quantum-dot-based LEDs are characterized by pure and saturated emission colors
with narrow bandwidth, and their emission wavelength is easily tuned by changing
the size of the quantum dots. Moreover, QD-LED combine the color purity and
durability of QDs with efficiency, flexibility, and low processing cost of organic light-
emitting devices. QD-LED structure can be tuned over the entire visible wavelength
range from 460 nm (blue) to 650 nm (red).
The structure of QD-LED is similar to basic design of OLED. The major difference is
that the light emitting centers are cadmium selenide (CdSe) nanocrystals, or
quantum dots. A layer of cadmium-selenium quantum dots is sandwiched between
layers of electron-transporting and hole-transporting organic materials. An applied
electric field causes electrons and holes to move into the quantum dot layer, where
they are captured in the quantum dot and recombine, emitting photons. The
spectrum of photon emission is narrow, characterized by its full width at half the
maximum value.
Bringing electrons and holes together in small regions for efficient recombination to
emit photons without escaping or dissipating was one of the major challenges. To
address this problem, a thin emissive layer sandwiched between a hole-transporter
layer (HTL) and an electron-transport layer (ETL). By making an emissive layer in
single layer of quantum dots, electrons and holes may be transferred directly from
the surfaces of the ETL and HTL., and resulting high recombination efficiency.
Both ETL and HTL consist of organic materials. It is well known that most organic
electroluminescent materials are in favor of injection and transport of holes rather
than electrons. Thus, the electron-hole recombination generally occurs near the
cathode, which could lead to the quenching of the exciton produced. In order to
prevent the produced excitons or holes from approaching cathode, a hole-blocking

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layer plays dual roles in blocking holes moving towards the cathode and transporting
the electrons to the emitting layer, QD layer. Tris-Aluminium (Alq3), bathocuproine
(BCP), and TAZ are most commonly used hole-blocking materials. These materials
can be used as both electron-transporting layer and hole blocking layer.
The array of quantum dots is manufactured by self-assembly in process known as
spin-casting; a solution of quantum dots in an organic material is poured into a
substrate, which is then set spinning to spread the solution evenly.
Quantum dot LEDs often rely on toxic heavy metals such as cadmium to emit light.
Now, researchers report new colors of LEDs that use silicon quantum dots.
Silicon could be a less toxic alternative, but scientists have struggled to fabricate
silicon-based devices that glow in colors across the visible spectrum. Current silicon
LEDs emit only red and near-infrared light.

Silicon in Semiconductor devices

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