semiconductors and metal contacts

Semiconductor and metal contacts 1
ESSENCE OF SEMICONDUCTORS
A semiconductor can be considered a material having a conductivity
ranging between that of an insulator and a metal. A crucial property of
semiconductors is the band gap; a range of forbidden energies within the
electronic structure of the material. Semiconductors typically have band
gaps ranging between 1 and 4 eV, whilst insulators have larger band
gaps, often greater than 5 eV. The thermal energy available at room
temperature, 300 K, is approximately 25 meV and is thus considerably
smaller than the energy required to promote an electron across the band
gap. This means that there are a small number of carriers present at
room temperature, due to the high energy tail of the Boltzmann-like
thermal energy distribution. It is the ability to control the number of
charge carriers that makes semiconductors of great technological
importance.
Semiconducting materials are very sensitive to impurities in the crystal
lattice as these can have a dramatic effect on the number of mobile
charge carriers present. The controlled addition of these impurities is
known as DOPING and allows the tuning of the electronic properties, an
important requirement for technological applications. The properties of a
pure semiconductor are called INTRINSIC, whilst those resulting from
the introduction of dopants are called EXTRINSIC. This introduction of
dopants results in the creation of new, intra-band, energy levels and the
generation of either negative (electrons) or positive (holes) charge
carriers.

CRYSTAL STRUCTURE AND ITS
REPRESENTATION

Different semiconductor materials differ in their properties. Thus, in
comparison with silicon, compound semiconductors have both advantages
and disadvantages.
For example ,Gallium Arsenide (GaAs) has six times higher electron
mobility than silicon, which allows faster operation; wider band gap,
which allows operation of power devices at higher temperatures, and
gives lower thermal noise to low power devices at room temperature.
Conversely, Silicon is robust, cheap, and easy to process, whereas
GaAs is brittle and expensive.
Depending upon the chemical and physical properties of the
semiconductor materials, we use them in different circumstances n
devices.

Band Theory of SOLIDS
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.

Basics of PN JUNCTION
A PN Junction Diode is one of the simplest Semiconductor Devices around,
and which has the characteristic of passing current in only one direction
only. However, unlike a resistor, a diode does not behave linearly with
respect to the applied voltage as the diode has an exponential current-
voltage ( I-V ) relationship and therefore we can not described its
operation by simply using an equation such as Ohm’s law.
Ideal Diode equation is-
If a suitable positive voltage (forward bias) is applied between the two
ends of the PN junction, it can supply free electrons and holes with the
extra energy they require to cross the junction as the width of the
depletion layer around the PN junction is decreased.
By applying a negative voltage (reverse bias) results in the free charges
being pulled away from the junction resulting in the depletion layer width
being increased. This has the effect of increasing or decreasing the
effective resistance of the junction itself allowing or blocking current flow
through the diode.
Then the depletion layer widens and narrows due to the differences in the
electrical properties on the two sides of the PN junction resulting in
physical changes taking place.

BACKGROUND
Whenever a metal and a semiconductor are in intimate contact, there
exists a potential barrier between the two that prevents most charge
carriers (electrons or holes) from passing from one to the other. Only a
small number of carriers have enough energy to get over the barrier and
cross to the other material. When a bias is applied to the junction, it can
have one of two effects: it can make the barrier appear lower from the
semiconductor side, or it can make it appear higher. The bias
does not change the barrier height from the metal side.
The result of this is a Schottky Barrier (rectifying contact), where the
junction conducts for one bias polarity, but not the other. Almost all
metal-semiconductor junctions will exhibit some of this rectifying
behaviour.
Schottky Contacts make good diodes, and can even be used to make a
kind of transistor, but for getting signals into and out of a semiconductor
device, we generally want a contact that is Ohmic. Ohmic contacts
conduct the same for both polarities. (They obey Ohm's Law).
There are two ways to make a metal-semiconductor contact look ohmic
enough to get signals into and out of a semiconductor (or doing the
opposite makes a good Schottky contact).
1. Lower the barrier height
The barrier height is a property of the materials we use. We try to
use materials whose barrier height is small.
Annealing can create an alloy between the semiconductor and the
metal at the junction, which can also lower the barrier height.
2. Make the barrier very narrow
One very interesting property of very tiny particles like electrons
and holes is that they can "tunnel" through barriers that they don't
have enough energy to just pass over. The probability of tunnelling
becomes high for extremely thin barriers (in the tens of
nanometres).
We make the barrier very narrow by doping it very heavily
(1019 dopant atoms/cm3 or more).

SCHOTTKY CONTACT
A Schottky barrier refers to a metal-semiconductor contact having a large
barrier height (i.e. and low doping concentration that is less
than the density of states in the conduction band or valence band. The
potential barrier between the metal and the semiconductor can be
identified on an energy band diagram. To construct such a diagram we
first consider the energy band diagram of the metal and the
semiconductor, and align them using the same vacuum level as shown
in Fig. 1 (a). As the metal and semiconductor are brought together, the
Fermi energies of the two materials must be equal at thermal
equilibrium Fig. 1 (b).
Figure 1: Energy band diagram of a metal adjacent to n-type semiconductor under thermal noneqilibrium
condition (a), metal-semiconductor contact in thermal equilibrium (b).

The barrier height is defined as the potential difference between the
Fermi energy of the metal and the band edge where the majority carrier
reside. From Fig. 1 one finds that for n-type semiconductors the barrier
height is obtained from
(1)
where is the work function of the metal and is the electron affinity.
For p-type material, the barrier height is given by the difference between
the valence band edge and the Fermi energy in the metal,
(2)
A metal-semiconductor junction will therefore form a barrier for electrons
and holes if the Fermi energy of the metal is located between the
conduction and the valence band edge.
In addition, we define the work function difference as the difference
between the work function of the metal and that of the semiconductor.
For n-type material it reads
(3)
similarly, for p-type material
(4)
The work function difference energy becomes
(5)

Here are a pictorial representations showing schottky (rectifying) contact
 N type semiconductor making a contact with metal
(a) (b)
(c) (d)

(e) (f)
 P type semiconductor making a contact with metal
(a) (b)

(c) (d)
(e) (f)
I-V Characteristics

OHMIC CONTACTS
When a metal and an n-type semiconductor are joined and ΦM < ΦS,
electrons will flow from the Fermi energy level in the metal into the
semiconductor conduction band to lower their energy. This will cause the
chemical potential of the semiconductor to move up into equilibrium with
that of the metal. It will also deform the semiconductor bands, so that
they curve upwards away from the metal. This situation is depicted
below-

This type of contact yields a linear relationship between the voltage
applied and the current that flows across the junction. It is therefore
called an Ohmic contact, because it obeys Ohm's law. This type of
contact is also described as metallization, and is used to supply electric
current into semiconductor devices.

SCHOTTKY DIODE
The Schottky diode or Schottky Barrier diode is an electronics component
that is widely used for radio frequency, RF applications as a mixer or
detector diode.
The diode is also used in power applications as a rectifier, again because
of its low forward voltage drop leading to lower levels of power loss
compared to ordinary PN junction diodes.
Although normally called the Schottky diode these days, named after
Schottky, it is also sometimes referred to as the surface barrier diode, hot
carrier diode or even hot electron diode.
Discover & introduction
Despite the fact that Schottky barrier diodes have many applications in
today's high tech electronics scene, it is actually one of the oldest
semiconductor devices in existence. As a metal-semiconductor devices, its
applications can be traced back to before 1900 where crystal detectors, or
cat's whisker detectors were all effectively Schottky barrier diodes.
Circuit symbol
Schottky diode symbol
Advantages
Schottky diodes are used in many applications where other types of diode
will not perform as well.
 Low turn on voltage: The turn on voltage for the diode is between
0.2 and 0.3 volts for a silicon diode against 0.6 to 0.7 volts for a
standard silicon diode. This makes it have very much the same turn
on voltage as a germanium diode.

 Fast recovery time: The fast recovery time because of the small
amount of stored charge means that it can be used for high speed
switching applications.
 Low junction capacitance: In view of the very small active area,
often as a result of using a wire point contact onto the silicon, the
capacitance levels are very small.
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 view of this Schottky barrier
diodes are used in many high performance diode ring mixers. In
addition to this their low turn on voltage and 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, and smaller heat sinks
may be able to be incorporated in the design.
 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. As in other applications, the low voltage drop of
the Schottky diode is particularly useful, and as a result they are
the favoured form of diode in this application.

Basic Schottky diode structure
The Schottky barrier diode can be manufactured in a variety of forms. The
most simple is the point contact diode where a metal wire is pressed
against a clean semiconductor surface. This was how the early Cat's
Whisker detectors were made, and they were found to be very unreliable,
requiring frequent repositioning of the wire to ensure satisfactory
operation. In fact the diode that is formed may either be a Schottky
barrier diode or a standard PN junction dependent upon the way in which
the wire and semiconductor meet and the resulting forming process.
Point contact Schottky diode structure
Although some diodes still use this very simple format, any diode
requiring a long term reliability needs to be fabricated in a more reliable
way.
Vacuum depositedSchottky diode structure
Although point contact diodes were manufactured many years later, these
diodes were also unreliable and they were subsequently replaced by a
fabrication technique in which metal was vacuum deposited.
Deposited metal Schottky barrier diode structure
This format for a Schottky diode is very basic and is more diagrammatic
than actually practical. However it does show the basic metal-on-
semiconductor format that is key to its operation.

Schottkydiode structure 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. Leakage effects are also noticed.
To overcome these problems a guard ring of P+ semiconductor fabricated
using a diffusion process is used along with an oxide layer around the
edge. In some instances metallic silicides may be used in place of the
metal.
The guard ring in this form of Schottky diode structure operates by
driving this region into avalanche breakdown before the Schottky junction
is damaged by large levels of reverse current flow during transient
events.
Schottky diode rectifier structure showing with guard ring
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.
Schottkydiode characteristics
The Schottky diode is what is called a majority carrier device. This gives it
tremendous advantages in terms of speed because it does not rely on
holes or electrons recombining when they enter the opposite type of
region as in the case of a conventional diode. By making the devices small
the normal RC type time constants can be reduced, making these diodes
an order of magnitude faster than the conventional PN diodes. This factor

is the prime reason why they are so popular in radio frequency
applications.
The diode also has a much higher current density than an ordinary PN
junction. This means that forward voltage drops are lower making the
diode ideal for use in power rectification applications.
Its main drawback is found in the level of its reverse current which is
relatively high. Reverse leakage current increases with temperature,leads
to thermal instability. While higher reverse voltages are achievable, they
would be accompanied by higher forward voltage drops,comparable to
other types; such a schottky diode would have no advantage unless
great switching speed is required. For many uses this may not be a
problem, but it is a factor which is worth watching when using it in more
exacting applications.
Schottky diode IV characteristic
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. Guard ring
decreases the electric field at the junction periphery,thereby increasing
breakdown voltage,it also increases the junction area and reduces the
depletion width region,contributing to increase in excess capacitance.
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.

Key specification parameters
 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.
 Capacitance: Normally the junctions areas of Schottky diodes are
small and therefore the capacitance is small. Typical values of a few
picofarads 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, Tj 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.

COMPARISON OF CHARACTERISTICS OF SCHOTTKY DIODE AND PN DIODE
CHARACTERISTIC SCHOTTKY DIODE PN JUNCTION DIODE
Forw ard 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.
Sw itching 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.
The Schottky diode finds many uses as a high voltage or power rectifier.
The Schottky diode rectifier has many advantages over other types of
diode and as a such can be utilised to advantage.
The Schottky diode has been used as a rectifier for many years in the
power supply industry where its use is essential to many designs.

Advantages of using a Schottky diode rectifier
The Schottky diode rectifier offers many advantages in power rectifier and
power supply circuits. There are a number of aspects of the Schottky
diode rectifier that makes them ideal components in many power supply
applications:
 Low forward voltage drop: The low forward voltage drop
offered by Scottky diode power rectifiers is a significant advantage
in many applications. It reduces the power losses normally incurred
in the rectifier and other diodes used within the power supply. With
standard silicon diodes offering the main alternative, their turn on
voltage is around 0.6 to 0.7 volts. With Schottky diode rectifiers
having a turn on voltage of around 0.2 to 0.3 volts, there is a
significant power saving to be gained. However it is necessary to
remember that there will also be losses introduced by the resistance
of the material, and the voltage drop across the diode will increase
with current. The losses of the Schottky diode rectifier will be less
than that of the equivalent silicon rectifier.
 Fast switching speeds: The very fast switch speeds of the
Schottky diode rectifier mean that this diode lends itself to use in
switching regulator circuits.
Schottkydiode rectifierdesign considerations
Schottky diode rectifiers offer many advantages, but when they are used,
there are a number of design considerations to account for. These should
be acknowledged in the circuit design being undertaken.
Some of the points to be taken into account include the following:
 High reverse leakage current: Schottky diode rectifiers have a
much higher reverse leakage current than standard PN junction
silicon diodes. Although this may not be a problem in some designs
it may have an impact on others.
 Limited junction temperature: The maximum junction
temperature of a Schottky diode rectifier is normally limited to the

range 125°C to 175°C but check the manufacturers ratings for the
given component. This compares to temperatures of around 200°C
for silicon diode rectifiers.
 Limited reverse voltage: As a result of its structure, Schottky
diode rectifiers have a limited reverse voltage capability. The
maximum figures are normally around 100 volts. If devices were
manufactured with figures above this, it would be found that the
forward voltages would rise and be equal to or greater than their
equivalent silicon diodes for reasonable levels of current.

PHOTODIODE
A photodiode is a semiconductor device that converts light into current.
The current is generated when photons are absorbed in the photodiode. A
small amount of current is also produced when no light is present.
Photodiodes may contain optical filters, built-in lenses, and may have
large or small surface areas. Photodiodes usually have a slower response
time as their surface area increases. The common, traditional solar
cell used to generate electric solar power is a large area photodiode.
Photodiodes are similar to regular semiconductor diodes except that they
may be either exposed (to detect vacuum UV or X-rays) or packaged with
a window or optical fiber connection to allow light to reach the sensitive
part of the device. Many diodes designed for use specifically as a
photodiode use a PIN junction rather than a p–n junction, to increase the
speed of response. A photodiode is designed to operate in reverse bias.
Electronic symbol
PRINCIPAL OF OPERATION
A photodiode is a p–n junction or PIN structure. When a photon of
sufficient energy strikes the diode, it creates an electron-hole pair. This
mechanism is also known as the inner photoelectric effect. If the
absorption occurs in the junction's depletion region, or one diffusion
length away from it, these carriers are swept from the junction by the
built-in electric field of the depletion region. Thus holes move toward
the anode, and electrons toward the cathode, and a photocurrent is
produced. The total current through the photodiode is the sum of the dark
current (current that is generated in the absence of light) and the
photocurrent, so the dark current must be minimized to maximize the
sensitivity of the device.
Photovoltaic mode
When used in zero bias or photovoltaic mode, the flow of photocurrent out
of the device is restricted and a voltage builds up. This mode exploits
the photovoltaic effect, which is the basis for solar cells – a traditional
solar cell is just a large area photodiode.

Photoconductive mode
In this mode the diode is often reverse biased (with the cathode driven
positive with respect to the anode). This reduces the response time
because the additional reverse bias increases the width of the depletion
layer, which decreases the junction's capacitance. The reverse bias also
increases the dark current without much change in the photocurrent. For
a given spectral distribution, the photocurrent is linearly proportional to
the illuminance (and to the irradiance).
Although this mode is faster, the photoconductive mode tends to exhibit
more electronic noise. The leakage current of a good PIN diode is so low
(<1 nA) that the Johnson–Nyquist noise of the load resistance in a typical
circuit often dominates.
Other modes of operation
Avalanche photodiodes have a similar structure to regular photodiodes,
but they are operated with much higher reverse bias. This allows
each photo-generated carrier to be multiplied by avalanche breakdown,
resulting in internal gain within the photodiode, which increases the
effective responsivity of the device.

PHOTOTRANSISTOR
A phototransistor is a light-sensitive transistor. A common type of
phototransistor, called a photobipolar transistor, is in essence a bipolar
transistor encased in a transparent case so that light can reach the base–
collector junction. It was invented by Dr. John N. Shive (more famous for
his wave machine) at Bell Labs in 1948, but it wasn't announced until
1950.The electrons that are generated by photons in the base–collector
junction are injected into the base, and this photodiode current is
amplified by the transistor's current gain β (or hfe). If the emitter is left
unconnected, the phototransistor becomes a photodiode. While
phototransistors have a higher responsivity for light they are not able to
detect low levels of light any better than photodiodes. Phototransistors
also have significantly longer response times. Field-effect
phototransistors, also known as photoFETs, are light-sensitive field-effect
transistors. Unlike photobipolar transistors, photoFETs control drain-
source current by creating a gate voltage.
Electronic symbol

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