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Defence Research and Development Organisation
SSPL Timarpur , Lucknow Road, Delhi
Project Title: Semiconductor and it’s contacts with metal.
Submitted By:
Pujit Gandhi
Sri Venkateswara College
(University of Delhi)

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Acknowledgement
I would like to express my special thanks of gratitude
to my mentor Mr.S.Sitharaman as well as my
principal Dr.P.Hemalatha Reddy who gave me the
golden opportunity to do this wonderful project on the
topic- Semiconductor and it’s contacts with metal,
which also helped me in doing a lot of research and i
came to know about so many new things I am really
thankful to them.
Secondly I would also like to thank my department
teachers, family and friends who helped me a lot in
researching on this project within the limited time frame.
.
-Pujit Gandhi
Sri Venkateswara College
(University of Delhi)

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Contents
1.About 4
2.Introduction 6
3.Properties 8
4.Silicon and Germanium 10
5.Band theory of solids 11
6.Metal-Semiconductor contacts 12
7.Basic Characteristics 13
8.Barrier height 16
9.Effect of biasing 17
10.Schottky Barrier 21
11.Schottky Diode 23
12.I-V characteristics 26
13.Ohmic contact 34
14.Tunneling Process 36
15.MESFET 37
16.Bibliography 38

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About
Defence Research and Development Organisation
(DRDO)
is an agency of the Republic of
India, responsible for the development of technology for
use by the military, headquartered in New Delhi, India. It
was formed in 1958 by the merger of the Technical
Development Establishment and the Directorate of
Technical Development and Production with the Defence
Science Organisation. It is under the administrative
control of the Ministry of Defence, Government of India.
With a network of 52 laboratories, which are engaged in
developing defence technologies covering various fields,
like aeronautics, armaments, electronics, land combat
engineering, life sciences, materials, missiles, and naval
systems, DRDO is India's largest and most diverse
research organisation.
The organisation includes around 5,000 scientists
belonging to the Defence Research & Development
Service (DRDS) and about 25,000 other scientific,
technical and supporting personnel.

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Solid State Physics Laboratory (SSPL) is a
laboratory under the Defence Research & Development
Organization (DRDO). Located in Delhi its primary
function is research in the field of Solid State
Materials, Devices and Sub-systems. Their activities
include development of semi-conductor materials, solid
state devices, electronic components/sub-systems and
investigation of solid state materials/devices for
futuristic defence applications.

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Introduction
A semiconductor material has an electrical conductivity value
falling between that of a conductor, such as copper, and
an insulator, such as glass. Semiconductors are the foundation of
modern electronics. Semiconducting materials exist in two types
- elemental materials and compound materials. The modern
understanding of the properties of a semiconductor relies
on quantum physics to explain the movement
of electrons and holes in a crystal lattice.
The unique arrangement of the crystal lattice makes silicon and
germanium the most commonly used elements in the
preparation of semiconducting materials. An increased
knowledge of semiconductor materials and fabrication processes
has made possible continuing increases in the complexity and
speed of microprocessors and memory devices.
The electrical conductivity of a semiconductor material
increases with increasing temperature, which is behaviour
opposite to that of a metal. Semiconductor devices can display
a range of useful properties such as passing current more easily
in one direction than the other, showing variable resistance, and
sensitivity to light or heat. Because the electrical properties of a
semiconductor material can be modified by controlled addition
of impurities, or by the application of electrical fields or light,
devices made from semiconductors can be used for
amplification, switching, and energy conversion.

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Current conduction in a semiconductor occurs through the
movement of free electrons and "holes", collectively known as
charge carriers.
Adding impurity atoms to a semiconducting material, known as
"doping", greatly increases the number of charge carriers
within it. When a doped semiconductor contains mostly free
holes it is called "p-type", and when it contains mostly free
electrons it is known as "n-type".
The semiconductor materials used in electronic
devicesaredoped under precise conditions to control the
concentrationand regions of p- and n-type dopants. A single
semiconductor crystal can have many p- and n-type regions;
the p–n junctions between these regions are responsible for the
useful electronic behaviour.

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Properties
Variable conductivity
Semiconductors in their natural state are poor conductors
because a current requires the flow of electrons, and
semiconductors have their valence bands filled. There are
several developed techniques that allows semiconducting
materials to behave like conducting materials, such
as doping or gating. These modifications have two outcomes: n-
type and p-type.
Heterojunctions
Heterojunctions occur when two differently doped
semiconducting materials are joined together. For example, a
configuration could consist of p-doped and n-doped germanium.
This results in an exchange of electrons and holes between the
differently doped semiconducting materials. The n-doped
germanium would have an excess of electrons, and the p-doped
germanium would have an excess of holes.
Excited Electrons
A difference in electric potential on a semiconducting material
would cause it to leave thermal equilibrium and create a non-
equilibrium situation. This introduces electrons and holes to the
system. Such disruptions can occur as a result of a temperature
difference or photons, which can enter the system and create
electrons and holes. The process that creates and annihilates
electrons and holes are called generation and recombination.[1]

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Light emission
In certain semiconductors, excited electrons can relax by
emitting light instead of producing heat. These semiconductors
are used in the construction of light emitting diodes and
fluorescent quantum dots.
Thermal energy conversion
Semiconductors have large thermoelectric power factors making
them useful in thermoelectric generators, as well as
high thermoelectric figures of merit making them useful
in thermoelectric coolers.
Silicon crystals are the most common
Semi-conducting materials
used in microelectronics and
photovoltaics.

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Silicon and Germanium
Solid state electronics arises from the unique
properties of silicon and germanium, each of which
has 4 valence electrons and which form crystal lattice
in which substituted atoms(dopants) can dramatically
change the electrical properties.
In solid state electronics, either pure silicon
orgermanium may be used as the intrinsic
semiconductor which forms the starting point for
fabrication. Each has four valence electrons, but
germanium will 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

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Band Theory of Solids
A useful way to visualize the difference between
conductors,insulators and semiconductors is to plot the available
energies for electrons in the materials. Instead of having discrete
energies as in the case of free atoms, the available energy states
form bands. Crucial to the conduction process is whether or not
there are electrons in the conduction band. In insulators the
electrons in the valence band are separated by a large gap from the
conduction band, in conductors like metals the valence band
overlaps the conduction band, and in semiconductors there is a
small enough gap between the valence and conduction bands that
thermal or other excitations can bridge the gap. With such a small
gap, the presence of a small percentage of a doping material can
increase conductivity dramatically.
An important parameter in the band theory is the Fermi level, the
top of the available electron energy levels at low temperatures. The
position of the Fermi level with the relation to the conduction band
is a crucial factor in determining electrical properties.

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Metal-Semiconductor Contacts
In solid-state physics, a metal–semiconductor (M–S) junction is a type
of junction in which a metal comes in close contact with
a semiconductor material.
It is the oldest practical semiconductor device. M–S junctions can either
be rectifying or non-rectifying. The rectifying metal–semiconductor
junction forms a Schottky barrier, making a device known as a Schottky
diode, while the non-rectifying junction is called an Ohmic contact. (In
contrast, a rectifying semiconductor–semiconductor junction, the most
common semiconductor device today, is known as a p–n junction.)
Metal–semiconductor junctions are crucial to the operation of all
semiconductor devices. Usually an ohmic contact is desired, so that
electrical charge can be conducted easily between the active region of
a transistor and the external circuitry. Occasionally however a Schottky
barrier is useful, as in Schottky diodes, Schottky transistors, and metal–
semiconductor field effect transistors.
The first practical semiconductor device was the metal-semiconductor
contact in the form of a point contact rectifier, that is , a metallic whisker
pressed against a semiconductor .The device found many applications
beginning in 1904 . In 1938, Schottky , suggested that the rectifying
behaviour could arise from a potential barrier as a result of the stable
space charges in the semiconductor.
Metal semiconductor contacts can also be non-rectifying , that is the
contact has negligible resistance regardless of the polarity of the applied
voltage. This type of contact is called an ohmic contact. All
semiconductor devices as well as integrated circuits need ohmic contact
to make connections to other devices in an electronic system.

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`Basic Characteristics
The characteristics of point contact rectifiers were not reproducible
from one device to another. They have been largely replaced by
metal-semiconductor contacts fabricated by planar processes. To
fabricate a device, a window is opened in an oxide layer, and metal
layer is deposited in a vacuum system. The metal layer covering the
window is subsequently defined by a lithographic step.
The following figure shows the perspective view of a metal-
semiconductor contact fabricated by the planar process .
Before discussing the behaviour of a metal-semiconductor boundary, it
is first necessary to introduce the concept of the work function. The
work function of a material is the energy required to remove an electron
from the level of the chemical potential and give it enough energy to
escape to infinity and arrive there with zero energy.
Albert Einstein first proposed the concept of the work function in his
work on the photoelectric effect in metals. It was for this work, rather
than for his work on relativity, that Einstein was awarded the Nobel
prize in 1921.

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The energy band diagram of an isolated metal adjacent to an isolated n-
type semiconductor is as shown. Note that metal work function “qϕm” is
generally different from the semiconductor work function “qϕs” .Work
function is defined as the energy difference between the Fermi level and
the vacuum level. Also shown is the electron affinity “q ”, which is the
energy difference between the conduction band edge and the vaccum
level in the semiconductor.
When the metal makes the intimate contact with the semiconductor,
 The Fermi levels in the two materials must be equal at thermal
equilibrium.
 In addition , the vacuum level must be continuous.
These two requirements determine a unique energy band diagram for
the ideal metal-semiconductor contact .

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For this ideal case , the barrier height “qϕBn” ,is simply the
difference between the metal work function and the semiconductor
electron affinity.
qϕBn = qϕm - q (1)
Similarly, for the case of an ideal contact between a metal and a p-
type semiconductor , the barrier height is given by:
qϕBp = Eg- (qϕm - q )
where Eg is the bandgap of the semiconductor. Therefore , for a
given semiconductor and for any metal , the sum of the barrier
heights on n-type and p-type substrates is expected to be equal to
the bandgap.
q(ϕBn + ϕBp ) = Eg
On the semiconductor side,”Vbi” is the built in potential that is seen
by electrons in the conduction band trying to move into the metal.
Vbi = ϕBn - Vn
Where “qVn” is the distance between the bottom of the conduction
band and the Fermi level.

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Variation of barrier height
with metal work function.
Figure shows the variation of barrier height for n-type silicon and
n- type gallium arsenide. As per equation “1”, the barrier height
qϕBn increases with increasing qϕm .
However the dependence isn’t as strong as predicted .This is
because in practically Schottky diodes , the disruption of crystal
lattice at semiconductor surface produces large amount of surface
energy states located in forbidden gap. These energy states can act
as donors or acceptors that influence the final determination of
barrier height .

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Effect of biasing on energy
band diagrams
Consider n-type semiconductor ,when the bias voltage is zero, the
band diagram is under thermal equilibrium condition .The Fermi
levels of both materials are equal .
If we apply positive voltage to the metal wrt to the n-type
semiconductor ,the semiconductor to metal barrier height decreases.
This is forward bias. When forward bias is applied,electrons can
move easily into the metal because barrier has been reduced by
voltage “Vf”, which is referred to as forward bias voltage.

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In reverse bias situation, the barrier has been increased by voltage
“Vr” as shown.It’s more difficult for the electrons to flow from
semiconductor into the metal.
We have similar results for a p-tyoe semiconductor, however the
polarities must be reversed.

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Charge and electric–field distribution
in a metal-semiconductor contact
The charge and electric field distributions for a metal-
semiconductor contact are shown. The metal is assumed to be a
perfect conductor , the charge transferred to it from the
semiconductor exist in a very narrow region at the metal surface
.The extent of the space charge in the semiconductor is W, ie
ρs = qND for x<W
ρs = 0 for x>W
Thus , the charge distribution is identical to that of a one sided
abrupt p+-n junction.
The magnitude of a electric field is decreasing linearly with distance
.The maximum electric field Ɛm is located at the interface .The
electric field distribution is then given by:
|Ɛ(𝐱)| =
𝐪𝐍𝐝(𝐖−𝐱)
Ɛ𝐬
= Ɛ(𝐦) -
𝐪𝐍𝐃(𝐱)
Ɛ𝐬

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𝐂 = |
𝐝𝐐𝐬𝐜
𝐝𝐕
| = √
𝒒 Ɛ𝐬 𝐍𝐝
𝟐(𝐕𝐛𝐢 – 𝐕)
=
Ɛ𝐬
𝑾
F/cm2
Ɛ(𝐦) =
𝐪𝐍𝐝𝐖
Ɛ𝐬
Where Ɛ𝐬 is the dielectric permittivity of the semiconductor.The
voltage across the space charge region , which is represented by the
area under the field curve given by:
Vbi – V =
Ɛ(m)W
2
=
qNd
2Ɛs
W2
The depletion-layer width W is given as:
W = √2Ɛs(Vbi – V)/𝑞𝑁𝑑
The space charge density Qsc , in the semiconductor is given as:
Qsc= qNDW =√2𝑞 Ɛs Nd(Vbi – V) C/cm2
Where the voltage V is equal to +Vf for forward bias and –Vr for
reverse bias .The depletion layer capacitance C per unit area can be
calculated as:

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The Schottky Barrier
A Schottky barrier refers to a metal-semiconductor contact having
a large barrier height and a low doping concentration that is less
than the density of states in the conduction band or valence band.
The current transport in a Schottky barrier is mainly due to majority
carriers, in contrast to p-n junction ,where current transport is
mainly due to minority carriers. For the Schottky diodes operated at
moderate temperature (300K),the dominate current mechanism is
thermionic emission of majority carriers from the semiconductor
over the potential barrier into the metal.
At thermal equilibrium ,the current density is balanced by the
two equal and opposite flow of carriers ,thus there is zero net
current. Electrons in the semiconductor tend to flow into the
metal and there is an opposing balanced flow of electrons from
metal into the semiconductor. These current components are
proportional to the density of electrons at the boundry.
the semiconductor surface an e-
can be thermionically emitted into
the metal if it’s energy is above the barrier height.here
semiconductor work function qϕs is replaced by qϕBn ,and
nth =Nc exp(−qϕBn
𝑘𝑇
)
where Nc is the density of states in the conduction band. At thermal
equilibrium we have :

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|Jm→s| = |Js→m| ∞ nth
|Jm→s| = |Js→m| = C1Nc exp(−qϕBn
𝑘𝑇
)
where Jm→s is the current from the metal to the semiconductor,
Js→m ,is the current from semiconductor to the metal and C1 is the
proportionality constant.
When a forward bias Vf is applied to the contact , the electrostatic
potential difference across the barrier is reduced , and the electron
density at the surface increases to:
nth =Nc exp(−q(ϕBn−Vf)
𝑘𝑇
)
The current Js→m that results from the electron flow out of the
semiconductor is therefore altered by the same factor. The flux of
electrons from metal to the semiconductor however remain the same
because the barrier ϕBn remains at it’s equilibrium value. The net
current under forward bias is:
J= Js→m − Jm→s
=C1actNc exp(−q(ϕBn−Vf)
𝑘𝑇
) − C1Nexp(−q(ϕBn)
𝑘𝑇
)
=C1Nc exp(−q(ϕBn)
𝑘𝑇
) exp(q(Vf)
𝑘𝑇
−1)
Using the same argument for reverse bias condition , the expression
for net current is identical to above expression except that Vf is
replaced by -Vr..
The coefficient C1Nc is found to be equal to A*T2
,where A* is
called the effective Richardson constant (in units of A/K2
-cm2
) ,and
T is the absolute temperature. The value of *A depend on the
effective mass and are equal to 110 and 32 for n- and p-type silicon,
respectively , and 8 and 74 for n- and p-type gallium arsenide
respectively.

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The current-voltage characteristic of a metal-semiconductor contact
under thermionic emission condition is then :
J= Js[ exp(qV
𝑘𝑇
) - 1]
Js = A*T2
exp(−q(ϕBn)
𝑘𝑇
)
Where Js is the saturation current density and applied voltage V is
positive for forward bias and negative for reverse bias .
In addition to the majority carrier (electron) current, a minority –
carrier (hole) current also exist in a metal n-type semiconductor
contact because of hole injection from the metal to the
semiconductor. The hole injection is the same as in a p+-n
junction.The current density is given by:
Jp= Jpo [ exp(qV
𝑘𝑇
) - 1]
Jpo =
𝑞𝐷 𝑝(𝑛 𝑖
2)
𝐿 𝑝 𝑁 𝐷
Under normal operating conditions , the minority –carrier current is
orders of magnitude smaller that the majority carrier current.
Therefore a Schottky diode is a unipolar device .
Expeimental forward I-V
characteristics of two schottky
diodes are as shown .By
extrapolating the forward I-V
curve at V=0, we can find Js.
From equation for Js, we can
obtain barrier height .

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The Schottky Diode
The Schottky diode (named after German physicist Walter H.
Schottky); also known as hot carrier diode is a semiconductor diode
with a low forward voltage drop and a very fast switching action.
When forward current flows through a solid state diode there is a
small voltage drop across its terminals. A silicon diode has a typical
voltage drop of 0.6 to 0.7 volts, while a Schottky diode has a
voltage drop of 0.15 to 0.45 volts. This lower voltage drop can be
used to give higher switching speeds and better system efficiency.
Various Schottky barrier diodes: Small signal RF devices (left),
medium and high power Schottky rectifying diodes (middle and
right).
Type Passive
Invented Walter H. Schottky
Pin
configuration
anode and cathode

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Silicon Schottky diodes are used in power applications because of
their low forward voltage drop, which allows lower power loss than
ordinary silicon PN junction diodes. These Schottky diodes are used
in many applications because they offer a number of advantages:
Low turn-on voltage
Fast recovery time
Low junction capacitance
Schottky diodes high current density and low forward voltage drop
means less wasted power compared with ordinary PN junction
diodes. This efficiency increase also allows smaller diode heat sinks
and less cooling.

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Schottky diode IV characteristic
The IV characteristic is generally that shown below. In the forward
direction the current rises exponentially, having a knee or turn on
voltage of around 0.2 V. In the reverse direction, there is a greater level
of reverse current than that experienced using a more conventional PN
junction diode.
Key specification parameters
In view of the particular properties of the Schottky diode there are
several parameters that are of key importance when determining the
operation of one of these diodes against the more normal PN junction
diodes.
 Forward voltage drop: In view of the low forward voltage drop
across the diode, this is a parameter that is of particular concern.
As can be seen from the Schottky diode IV characteristic, the
voltage across the diode varies according to the current being
carried. Accordingly any specification given provides the forward
voltage drop for a given current. Typically the turn-on voltage is
assumed to be around 0.2 V.
 Reverse breakdown: Schottky diodes do not have a high
breakdown voltage. If these figures are exceeded then there is a
possibility the diode will enter reverse breakdown. The upper limit
for reverse breakdown is not high when compared to normal PN
junction diodes. Maximum figures, even for rectifier diodes only
reach around 100 V.

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 Capacitance: The capacitance parameter is one of great
importance for small signal RF applications. Normally the
junctions areas of Schottky diodes are small and therefore the
capacitance is small. Typical values of a few pico-farads are
normal. As the capacitance is dependent upon any depletion areas,
etc, the capacitance must be specified at a given voltage.
 Reverse recovery time: This parameter is important when a diode
is used in a switching application. It is the time taken to switch the
diode from its forward conducting or 'ON' state to the reverse
'OFF' state. The charge that flows within this time is referred to as
the 'reverse recovery charge'. The time for this parameter for a
Schottky diode is normally measured in nanoseconds, ns. Some
exhibit times of 100 ps. In fact what little recovery time is required
mainly arises from the capacitance rather than the majority carrier
recombination. As a result there is very little reverse current
overshoot when switching from the forward conducting state to the
reverse blocking state.
 Working temperature: The maximum working temperature of the
junction is normally limited to between 125 to 175°C. This is less
than that which can be sued with ordinary silicon diodes. Care
should be taken to ensure heatsinking of power diodes does not
allow this figure to be exceeded.
 Reverse leakage current: The reverse leakage parameter can be
an issue with Schottky diodes. It is found that increasing
temperature significantly increases the reverse leakage current
parameter. Typically for every 25°C increase in the diode junction
temperature there is an increase in reverse current of an order of
magnitude for the same level of reverse bias.

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Schottky diode characteristics summary
The Schottky diode is used in many applications as a result of its
characteristics that differ appreciable from several aspects of the more
widely used standard PN junction diode.
COMPARISON OF CHARACTERISTICS OF SCHOTTKY DIODE AND PN
DIODE
CHARACTERISTIC SCHOTTKY
DIODE
PN JUNCTION DIODE
Forward current
mechanism
Majority carrier
transport.
Due to diffusion currents, i.e.
minority carrier transport.
Reverse current Results from
majority carriers
that overcome the
barrier. This is
less temperature
dependent than
for standard PN
junction.
Results from the minority carriers
diffusing through the depletion
layer. It has a strong temperature
dependence.
Turn on voltage Small - around
0.2 V.
Comparatively large - around 0.7
V.
Switching speed Fast - as a result
of the use of
majority carriers
because no
recombination is
required.
Limited by the recombination
time of the injected minority
carriers.

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Typical Schottky diode characteristics
To give some idea of the characteristics to be expected from Schottky
diodes a couple of real examples are provided below. These summarise
the main specifications and give an idea of their performance.
 1N5711 Schottky barrier switching diode This diode is described
as an ultra-fast switching diode with high reverse breakdown, low
forward drop voltage and a guard ring for junction protection.
TYPICAL 1N5711 CHARACTERISTICS / SPECIFICATIONS
CHARACTERISTIC TYPICAL
VALUE
UNIT DETAILS
Max DC Blocking Voltage, Vr 70 V
Max forward continuous current, Ifm 15 mA
Reverse breakdown voltage, V(BR)R 70 V @ reverse current of 10µA
Reverse leakage current, IR 200 µA At VR=50V
Forward voltage drop, VF 0.41
1.00
V at IF = 1.0 mA
IF=15mA
Junction capacitance, Cj 2.0 pF VR = 0V, f=1MHz
Reverse recovery time, trr 1 nS

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 1N5828 Schottky barrier power rectifier diode This diode is
described as a Schottky diode, stud type, i.e. for power
rectification.
TYPICAL 1N5258 SHOTTKY DIODE CHARACTERISTICS / SPECIFICATIONS
CHARACTERISTIC TYPICAL
VALUE
UNIT DETAILS
Maximum recurrent peak reverse
voltage
40 V
Maxim DC blocking voltage 40 V
Average forward current, IF (AV) 15 A T = 100°C
Peak forward surge current, IFSM 500 A
Maximum instantaneous forward
voltage, VF
0.5 V At IFM = 15A and Tj = 25°C
Maximum instantaneous reverse
current at rated blocking voltage, IR
10
250
mA Tj= 25°C
Tj = 125°C

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Guard Ring and its significance
A guard ring is traditionally used to protect high impedance nodes in a
circuit from surface leakage currents. The guard ring is a ring of copper
driven by a low impedance source to the same voltage as the high
impedance node.
The use of a guard ring in the fabrication of the diode has an effect on its
performance in both forward and reverse directions. Both forward and
reverse characteristics show a better level of performance.
However the main advantage of incorporating a guard ring into the
structure is to improve the reverse breakdown characteristic. There is
around a 4 : 1 difference in breakdown voltage between the two - the
guard ring providing a distinct improvement in reverse breakdown.
Some small signal diodes without a guard ring may have a reverse
breakdown of only 5 to 10 V.
Schottky diode rectifier structure showing with guard ring
One of the problems with the simple deposited metal diode is that
breakdown effects are noticed around the edge of the metallised area.
This arises from the high electric fields that are present around the edge
of the plate.
To overcome these problems a guard ring of P+ semiconductor
fabricated using a diffusion process is used. It operates by driving this
region into avalanche breakdown before the Schottky junction is
damaged by large levels of reverse current flow during transient events.
This form of Schottky diode structure is used particularly in rectifier
diodes where the voltages may be high and breakdown is more of a
problem.

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Applications
The Schottky barrier diodes are widely used in the electronics industry finding many uses
as diode rectifier. Its unique properties enable it to be used in a number of applications
where other diodes would not be able to provide the same level of performance. In
particular it is used in areas including:
 RF mixer and detector diode: The Schottky diode has come into its own for
radio frequency applications because of its high switching speed and high
frequency capability. In addition to this their low turn on voltage ,high frequency
capability and low capacitance make them ideal as RF detectors.
 Power rectifier: Schottky barrier diodes are also used in high power applications,
as rectifiers. Their high current density and low forward voltage drop mean that
less power is wasted than if ordinary PN junction diodes were used. This increase
in efficiency means that less heat has to be dissipated.
 Power OR circuits: Schottky diodes can be used in applications where a load is
driven by two separate power supplies. One example may be a mains power
supply and a battery supply. In these instances it is necessary that the power from
one supply does not enter the other. This can be achieved using diodes.
 Solar cell applications: Solar cells are typically connected to rechargeable
batteries, often lead acid batteries because power may be required 24 hours a day
and the Sun is not always available. Solar cells do not like the reverse charge
applied and therefore a diode is required in series with the solar cells. Any voltage
drop will result in a reduction in efficiency and therefore a low voltage drop diode
is needed.
 Clamp diode : Schottky barrier diodes may also be used as a clamp diode in a
transistor circuit to speed the operation when used as a switch. In these chips the
diodes are inserted between the collector and base of the driver transistor to act as
a clamp. To produce a low or logic "0" output the transistor is driven h on, and in
this situation the base collector junction in the diode is forward biased. When the
Schottky diode is present this takes most of the current and allows the turn off
time of the transistor to be greatly reduced, thereby improving the speed of the
circuit.
An NPN transistor with Schottky diode clamp

33. 33 | P a g e
Disadvantage
 The main disadvantage of a schottky diode is that it has a relatively
high reverse current.Because of its metal semiconductor junction,
it is more suspectible to leaking current when voltage is connected
in reverse.
 Also, schottky diodes tend to have low maximum reverse voltages.
They tend to have a maximum value of 50V or less. This means
schottky diodes cannot withstand much reverse voltage without
breaking down and conducting large amounts of current. And even
before reaching this maximum reverse value, it will still leak small
amounts of current.

34. 34 | P a g e
The Ohmic Contact
An ohmic contact is a non-rectifying junction: an electrical
junction between two conductors that has a linear current–
voltage (I-V) curve as with Ohm's law. Low resistance ohmic
contacts are used to allow charge to flow easily in both directions
between the two conductors, without blocking due to rectification or
excess power dissipation due to voltage thresholds.
Low-resistance, stable ohmic contacts to semiconductors are critical
for the performance and reliability of semiconductor devices, and
their preparation and characterization are major efforts in circuit
fabrication.
Poorly prepared junctions to semiconductors can easily show
rectifying behaviour by causing depletion of the semiconductor near
the junction, rendering the device useless by blocking the flow of
charge between those devices and the external circuitry.
Ohmic contacts to semiconductors are typically constructed by
depositing thin metal films of a carefully chosen composition,
possibly followed by annealing to alter the semiconductor–metal
bond.

35. 35 | P a g e
Fabrication of metal–semiconductor ohmic contacts
Both ohmic contacts and Schottky barriers are dependent on the Schottky barrier
height, which sets the threshold for the excess energy an electron requires to pass
from the semiconductor to the metal. For the junction to admit electrons easily in both
directions (ohmic contact), the barrier height must be small. To form an excellent
ohmic contact (low resistance), the barrier height should be small everywhere. The
dependence of contact resistance on the details of the interfacial chemistry is what
makes the reproducible fabrication of ohmic contacts such a manufacturing challenge.
The fabrication of ohmic contacts is a much-studied part of materials engineering .
The reproducible, reliable fabrication of contacts relies on extreme cleanliness of the
semiconductor surface. Often the contact region is heavily doped to ensure the type of
contact wanted. As a rule, ohmic contacts on semiconductors form more easily when
the semiconductor is highly doped nearby the junction; a high doping narrows
the depletion region at the interface and allow electrons to flow in both directions
easily at any bias by tunneling through the barrier.
The fundamental steps in contact fabrication are:
Semiconductor surface cleaning: Surface cleaning may be performed by sputter-
etching, chemical etching, reactive gas etching or ion milling. For example, the native
oxide of silicon may be removed with an hydrofluoric acid dip, while GaAs is more
typically cleaned by a bromine-methanol dip.
Contact metal deposition: After cleaning, metals are deposited via sputter
deposition, evaporation or chemical vapor deposition (CVD). Sputtering is a faster
and more convenient method of metal deposition than evaporation but the ion
bombardment from the plasma may induce surface states or even invert the charge
carrier type at the surface
Patterning : Patterning of contacts is accomplished with standard photolithographic
methods such as lift-off, where contact metal is deposited through holes in a
photoresist layer that is later dissolved away.
Annealing: Annealing of contacts is useful for relieving stress as well as for inducing
any desirable reactions between the metal and the semiconductor.

36. 36 | P a g e
Tunneling Process
Tunneling process have a positive barrier at the metal-
semiconductor interface, but also have a high enough doping in the
semiconductor that there is only a thin barrier separating the metal
from the semiconductor. If the width of the depletion region at the
metal-semiconductor interface is very thin, on the order of 3 nm or
less, carriers can readily tunnel across such barrier. The required
doping density for such contact is 1019
cm-3
or higher.
For the metal-semiconductor with low doping concentration , the
thermionic –emission current dominates the current transport.
For contacts with high doping concentration , the barrier width
becomes very narrow , and the tunneling current becomes dominate.
The tunneling current is proportional to the tunneling probability
We usually think of electron as a particle. But when a large number
of electrons hit a thin barrier, quantum mechanics predicts that the
electrons will behave more like a wave. Part of the electrons still
bounces back, but strangely enough, a certain number of electrons
do pass through the barrier .This is analogy to water waves
tunneling through a dike.This strange effect is called electron
tunneling

37. 37 | P a g e
MESFET
The metal-semiconductor field-effect transistor(MESFET) was
proposed in 1966.The MESFET has three metal-semiconductor
contacts – one Schottky barrier for the gate electrode and two ohmic
contacts for the source and the drain electrodes. A perspective view
of a MESFET is shown:
The basic device parameters include L(gate length), Z(gate width) and
‘a’ the thickness of the epitaxial layer. Most MESFET’s are made on n-
type III-V compound semiconductors , such as gallium arsenide ,
because of their high electrode mobilities , which help to minimise
series resistances , and because of their high saturation velocities ,which
result in the increase in the cutoff frequency.
Practical MESFETs are fabricated by using epitaxial layers on semi
insulating substrates to minimise parasitic capacitances .The Ohmic
contacts are labelled –source and drain and the Schottky barrier is
labelled as gate. A MESFET is often described in terms of the gate
dimensions .If the gate length (L) is 0.5um and the gate width (Z) is
300um , the device is referred to as a 0.5×300um device. A microwave –
or-millimeterwave device typically has a gate length 0.1-1.0um.The
thickness ‘a’ of the epitaxial layer is typically one-third one-fifth of the
gate length. The spacing between the electrodes is one to four times of
the gate length. The current handling capability of a MESFET is directly
proportional to the gate width Z because of the cross-sectional area
available for channel current is proportional to Z.

38. 38 | P a g e
Principles Of Operation
Let us consider section under the gate. The source is grounded and the gate
and drain voltage are measured with respect to the source.Under normal
operation ,the gate voltage is zero or reverse biased and drain voltage is
zero or forward biased ie.
VG ≤ 0 and VD ≥ 0
Since the channel is n-type material, the device is referred to as an n-
channel MESFET. The resistance of the channel is given by:
𝑅 = 𝜌
L
𝐴
=
L
𝑞𝑢 𝑛 𝑁 𝐷 𝐴
=
L
𝑞𝑢 𝑛 𝑁 𝐷 𝑍(𝑎−𝑊)
Where ND is the donor concentration , A is the cross sectional area for
current flow and equal Z(a-W), and W is the width of the depletion region
of the Schottky barrier .
When no gate voltage is applied and VD is small, a small drain current ID
flows in the channel which is equal to VD/R. Thus current varies linearly
with drain voltage. The voltage increases from zero at source to VD at
drain. Thus Schottky barrier becomes reverse biased as we proceed from
source to drain. As VD increases, W increases and the cross sectional area
for current to flow decreases. The Channel resistance R also increases.
Thus the current increases at slower rate.
As drain voltage is further increased , the depletion region touches the
semi-insulating substrate. This happens when a=W at the drain. We can
obtain corresponding value of drain voltage , called the saturation voltage ,
VDsat .
VDsat =
q𝑁 𝐷 𝑎2
2Ɛs
- Vbi ( for Vg=0, V= -VDsat)
At this drain voltage , the source and the drain are pinched off or
completely separated by a reverse-biased depletion region .The location
P is called the pinch –off point . At this point , a large drain current
called the saturation current IDsat can flow across the depletion region
Beyond pinch-off point ,as VD is increased further , the depletion region
near the drain will expand and point P will move toward the source .
However, the voltage at point P remains the same , VDsat. Thus , the

39. 39 | P a g e
number of electrons per unit time arriving from the source to point P,
and hence the current flowing in the channel remain the same. Thus for
drain voltages larger than VDsat ,the current remains essentially at the
value IDsat.
When a gate voltage is applied to reverse bias gate contact , the
depletion –layer width W increases. For a small VD ,the channel again
acts as a resistor nut it’s resistance is higher because the cross sectional
area available for current flow is decreased . An initial current is smaller
for VG = -1V than for VG =0 .When VD is increased to a certain value ,
the depletion region again touches the semi-insulating substrate given
by:
VDsat =
q𝑁 𝐷 𝑎2
2Ɛs
- Vbi -VG
For an n-channel MESFET , the gate voltage is negative wrt to the
source . Hence the gate voltage reduces the drain voltage by an amount
VG .

40. 40 | P a g e
I-V Characteristics
We now consider MESFET before the onset of pinch-off. The drain
voltage variation along the channel is shown. The voltage drop
across section dy of the channel is given by:
Where we have replaced L by dy. The depletion –layer width at
distance y from the source is given by:
The drain current ID is constant , independent of y. We can rewrite
first equation as:
Also , dV is given as:
Substituting dV and integrating from y=0 to y=L yields

41. 41 | P a g e
Or,
Where,
And,
The voltage Vp is called the pinch-off voltage that is the total
voltage (VD+VG+Vbi) at which W2=a.

42. 42 | P a g e
The I-V characteristics of a MESFET are as shown .The curves
shown are calculated for 0<VD<VDSat , Beyond VDSat , the current is
taken to be constant .Note there are 3 different regions in the curve.
When Vd is small,the cross section area of the channel is
independent of Vd and I-V characteristics are linear or ohmic .We
refer to this region of operation as the linear region .
For Vd>Vdsat, the current saturates at Idsat. We refer to this region
as the saturation region. As the drain voltage further increased,
avalanche breakdown of the gate to channel diode occurs and the
drain current suddenly increases. This region is the breakdown
region.

43. 43 | P a g e
Bibliography
 www.wikipedia.org
 Physics of Semiconductor Devices
(S.M.Sze, 2nd
edition)
 hyperphysics.phy-astr.gsu.edu