Unit4 special semiconductor devices class4

Unit 4 Special Semiconductor
Devices
Dr.R.Senthilkumar
Assistant Professor
Department of Electronics and Communication Engineering
Institute of Road and Transport Technology
Erode,Tamilnadu, INDIA

Outline
● Metal Semiconductor Junction
MESFET
FINFET
PINFET
CNTFET
DUAL Gate MOSFET

Outline
● Schotty Barrier Diode
● Zener Diode
● Varactor Diode
● Tunnel Diode
● LASER diode
● LDR and GalleiumArsenide
Device

Zener Diode
A Zener diode is a type of diode
that allows current to flow in the
conventional manner - from its
anode to its cathode i.e. when the
anode is positive with respect to
the cathode. When the voltage
across the terminals is reversed and
the potential reaches the Zener
voltage (or "knee"), the junction will
break down and current will flow in
the reverse direction - a desired
characteristic. This effect is known
as the Zener effect.

Zener Diode
Practical Zener Diode
Zener Diode Symbol

Zener Diode
V-I Characteristics of Zener Diode

Zener Diode
V-I Characteristics of Zener Diode –
Working Principle:
When the diode is connected in
forward bias, this diode acts as a
normal diode but when the reverse
bias voltage is greater than zener
voltage, a sharp breakdown takes
place. In the V-I characteristics above
Vz is the zener voltage. It is also the
knee voltage because at this point the
current increases very rapidly.

Zener Diode
Zener Diode Applications:
Regulator in power supplies,
stabilizers, UPS

Schotty Barrier
Diode
The Schottky diode (named
after the German physicist
Walter H. Schottky), also known
as Schottky barrier diode or hot-
carrier diode, is a
semiconductor diode formed by
the junction of a semiconductor
with a metal.

Schotty Barrier
Diode
Schotty
Barrier Diode
Symbol
Schotty
Barrier Diode

Schotty Barrier
Diode
What is a Schottky Diode?
A Schottky diode is also known as a hot carrier
diode; it is a semiconductor diode with a very
fast switching action, but a low forward voltage
drop. When a current flows through the diode
there is a small voltage drop across the diode
terminals. In a normal diode, the voltage drop is
between 0.6 to 1.7 volts, while in a Schottky
diode the voltage drop normally ranges between
0.15 and 0.45volts. This lower voltage drop
provides higher switching speed and better
system efficiency. In Schottky diode, a
semiconductor–metal junction is formed
between a semiconductor and a metal, thus
creating a Schottky barrier. The N-type
semiconductor acts as a cathode and the metal
side acts as the anode of the diode.

Schotty Barrier
Diode
Schottky Diode Applications:
It is widely used in different applications like a
mixer, in radio frequency applications, and as a
rectifier in power applications. It’s a low voltage
diode.

1
Varactor Diode
What is a Varactor Diode?
The varactor diode was named because of
the variable reactor or variable reactance
or variable capacitor or variable
capacitance property of these diodes. A
varactor diode is considered as a special
type of diode that is widely used in the
electronics industry and is used in various
Electronics applications.
Varactor
Diode Symbol
Varactor Diode
Sample example

In general, it looks like a normal PN-
junction diode in which one
terminal is termed as the cathode
and the other terminal is termed as
anode. Here, varactor diode
consists of two lines at one end
(cathode end of normal diode) that
indicates the capacitor symbol.
The varactor diode
symbol consists of the
capacitor symbol at one
end of the diode that
represents the variable
capacitor characteristics
of the varactor diodes.

Working Principle
Let us consider the capacitor that consists of two
plates separated by an insulating dielectric as
shown in the figure .1
The capacitance of an electrical capacitor is directly
proportional to the area of the plates, as the area of
the plates increases the capacitance of the
capacitor increases. Consider the reverse biased
mode of the diode in Figure.2, in which P-type
region and N-type region are able to conduct and
thus can be treated as two plates. The depletion
region between the P-type and N-type regions
can be considered as insulating dielectric. Thus, it is
exactly similar to the capacitor shown in Figure.1
Figure.1
Figure.3

The size of the depletion region of
diode changes with change in reverse
bias. If the varactor diode reverse
voltage is increased, then the depletion
region size increases. Similarly, if the
varactor diode reverse voltage is
decreased, then the depletion region
size decreases or narrows.
Figure.3

Varactor diode is also a
semiconductor microwave solid-
state device, it is frequently used in
applications where variable
capacitance is desired which can be
achieved by controlling voltage.
Applications:

Light Dependent
Resistor (LDR)
Sample 1 LDR
Sample 2 LDR
Symbol LDR
A photoresistor (acronymed LDR for
Light Decreasing Resistance, or light-
dependent resistor, or photo-conductive
cell) is an active component that
decreases resistance with respect to
receiving luminosity (light) on the
component's sensitive surface

Working Principle
● Note:
In the dark, a photoresistor can have
a resistance as high as several
megaohms (MΩ), while in the light, a
photoresistor can have a resistance
as low as a few hundred ohms.
If incident light on a photoresistor
exceeds a certain frequency, photons
absorbed by the semiconductor give
bound electrons enough energy to jump
into the conduction band. The resulting
free electrons (and their hole partners)
conduct electricity, thereby lowering
resistance.

Applications A photoresistor can be applied
in light-sensitive detector
circuits and light-activated and
dark-activated switching
circuits acting as a resistance
semiconductor

LASER Diode
A laser diode, injection laser
diode, or diode laser is a
semiconductor device similar to
a light-emitting diode in which a
diode pumped directly with
electrical current can create
lasing conditions at the diode's
junction. Laser diodes can
directly convert electrical
energy into light.
Sample 3 LASER Diode
Sample 2 LASER DiodeSample 1 LASER Diode
LASER Diode Symbol

LASER Diode Principle
Driven by voltage, the doped p-n-transition allows for recombination of an electron
with a hole. Due to the drop of the electron from a higher energy level to a lower one,
radiation, in the form of an emitted photon is generated. This is spontaneous
emission. Stimulated emission can be produced when the process is continued and
further generate light with the same phase, coherence and wavelength.
Partially reflecting mirror is used on either side of the diode so that the photons
released from spontaneous emission are trapped in the p-n junction until their
concentration reaches a threshold value. These trapped photons stimulate the
excited electrons to recombine with holes even before their recombination time. This
results in the release of more photons that are in exact phase with the initial photons
and so the output gets amplified. Once the photon concentration goes above a
threshold, they escape from the partially reflecting mirrors, resulting in a bright
monochromatic coherent light.

Unit4 special semiconductor devices class4

High-power laser diodes are used in industrial applications such as heat treating,
cladding, seam welding and for pumping other lasers, such as diode-pumped solid-
state lasers.
LASER Diode Applications

Gallium Arsenide Device
Gallium arsenide is a semiconducting material composed of equal
amounts of the elements gallium and arsenic. It is a member of a group
of semiconductors commonly referred to as the III–V, the constituents
of which are to be found in groups III and V of the periodic table.

Gallium Arsenide Applications
Gallium arsenide is used in the manufacture of devices such
as
1. microwave frequency integrated circuits,
2. monolithic microwave integrated circuits,
3. infrared light-emitting diodes,
4. laser diodes,
5. solar cells and
6. optical windows.

Gallium Arsenide Advantages
1. gallium arsenide transistors function at frequencies
2. insensitive to overheating
3. tend to create less noise
4. since it has direct band gap, it can absorb and emit
light efficiently compared to silicon which has indirect
band gap,

Tunnel Diode
A tunnel diode or Esaki diode is a type of
semiconductor diode that has effectively "negative
resistance" due to the quantum mechanical effect
called tunneling.

Tunneling Effect
Tunneling is known as a direct flow of electrons
across the small depletion region from n-side
conduction band into the p-side valence band

Symbol of Tunnel Diode and Tunnel Diode
Samples
Sample 1 Sample 2 Symbol of Tunnel Diode

The p-type and n-type semiconductor is heavily doped in a tunnel diode
due to a greater number of impurities. Heavy doping results in a narrow
depletion region. When compared to a normal p-n junction diode,
tunnel diode has a narrow depletion width. Therefore, when small
amount of voltage is applied, it produces enough electric current in the
tunnel diode.

V-I
Characteristics
of Tunnel Diode

Small voltage Bias <
Built-in voltage of
depletion layer.
Figure shows
Energy band
diagram of Tunnel
Diode
Tunnelling
Phenomenon

Increase voltage Bias further
Maximum tunnel current flows
when the energy level of n-
side conduction band and the
energy level of a p-side
valence band becomes equal.

A further increase in the applied
voltage will cause a slight
misalignment of the conduction band
and valence band. Still there will be
an overlap between conduction band
and valence band. The electrons
move from conduction band to
valence band of p region. Therefore,
this causes small current to flow.
Hence, tunnel current starts
decreasing.

The tunneling current will be zero
when applied voltage is increased
more to the maximum. At this
voltage levels, the valence band
and the conduction band does not
overlap. This makes tunnel diode
to operate same as a PN junction
diode.

Tunnel Diode Applications
 Due to the tunneling mechanism, it is used as an ultra high speed
switch.
 The switching time is of the order of nanoseconds or even
picoseconds.
 Due to the triple valued feature of its curve from current, it is used as a
logic memory storage device.
 Due to extremely small capacitance, inductance and negative
resistance, it is used as a microwave oscillator at a frequency of about
10 GHz.
 Due to its negative resistance, it is used as a relaxation oscillator
circuit.

Tunnel Diode
Advantages
 Low cost
 Low noise
 Ease of operation
 High speed
 Low power
Tunnel Diode
Disadvantages
 Being a two-terminal device,
it provides no isolation
between output and input
circuits.
 The voltage range, which can
be operated properly in 1 volt
or below.

Metal Semiconductor Junction Devices
MESFET
A MESFET (metal–semiconductor field-effect transistor)
is a field-effect transistor semiconductor device similar
to a JFET with a Schottky (metal-semiconductor)
junction instead of a p-n junction for a gate.

MESFET
Figure shows Depletion MESFET and
Enhancement MESFET Symbol
The D-MESFET is
normally ‘ON’ and its
threshold voltage, Vtdep,
is negative. The E-MES
FET is normally ‘OFF’
and its threshold Vtenh is
positive. The threshold
voltage is determined by
the channel thickness, ‘a’,
and concentration density
of the implanted impurity,
ND.

MESFET
Figure shows MESFET Structure
The MESFET has a thin n-type active
region which is used to join the two ohmic
contacts . A thin metal Schottky barrier
gate is used to separate the highly doped
drain and source terminals.

MESFET
GaAs MESFETs are similar to silicon MOSFETs. The major difference is the presence of a
Schottky diode at the gate region which separates two thin n-type active regions, that is, source
and drain, connected by ohmic contacts. It should be noted that both D type and E type MESFETs,
that is, ‘ON’ and ‘OFF’ devices, operate by the depletion of an existing doped channel. This can be
compared with silicon MOS devices where the E [Enhancement] mode transistor functions by
inverting the region below the gate to produce a channel, while the D [depletion] mode device
operates by doping the region under the gate slightly in order shift the threshold to a normally ‘ON’
condition.

MESFET Applications
The MESFET is a high performance form of
field effect transistor that is used mainly for
exacting microwave applications both as a
low noise signal amplifier and in higher power
RF circuits.

A fin field-effect transistor is a multigate device or multiple
gate field-effect transistor (MuGFET) refers to a MOSFET
(metal–oxide–semiconductor field-effect transistor) which
incorporates more than one gate into a single device. The
multiple gates may be controlled by a single gate electrode,
wherein the multiple gate surfaces act electrically as a
single gate, or by independent gate electrodes.
FINFET
Figures Show the symbol of
P-channel FINFET and N-
channel FINFET

A fin field-effect transistor is a multigate device or multiple
gate field-effect transistor (MuGFET) refers to a MOSFET
(metal–oxide–semiconductor field-effect transistor) which
incorporates more than one gate into a single device. The
multiple gates may be controlled by a single gate electrode,
wherein the multiple gate surfaces act electrically as a
single gate, or by independent gate electrodes.
FINFET
Figure Shows the structure of
FINFET

FINFET Applications
FinFET Technology New multi-gate or tri-gate structures, also known as Fin Field
Effect Transistors (FinFETs), have been adopted for the high-volume production
of CMOS integrated circuits beginning at the 22nm technology generation.
FINFET Advantages
The FinFET devices have significantly faster switching times and higher current
density than planar CMOS technology.

The term “PINFET” (p-intrinsic-n, field-effect transistor)
indicates the integration of a PIN photodiode and a discrete,
high-performance transimpedance amplifier stage.
A monolithically integrated planar structure for an
InP/InGaAs optoelectronic circuit consisting of a pin
photodiode and a heterojunction field-effect transistor.
(i.e.) PIN-FET integrated receiver formed by the combination
of a PIN photodiode and a Field Effect Transistor (FET) and
packaged in single housing.
PINFET
Figures show
samples of PINFET

PINFET
Figure shows
PINFET Structure

PINFET
Advantage:
PINFET provides an excellent solution for optical receiver
systems that require both high sensitivity and wide
dynamic range.
Applications:
Applications include telecommunications line-terminating
equipment or repeaters and optical sensor systems.
The receiver package offers high reliability

A carbon nanotube field-effect
transistor (CNTFET) refers to a
field-effect transistor that utilizes
a single carbon nanotube or an
array of carbon nanotubes as the
channel material instead of bulk
silicon in the traditional MOSFET
structure.
CNTFET
P-type and N-type CNTFET Symbol

CNTFET

The CNTFET consists of carbon
nanotubes deposited on a silicon
oxide substrate pre-patterned with
chromium/gold source and drain
contacts
CNTFET

CNTFET
Carbon Nanotube

Advantages:
CNTFET-based devices offer high mobility for near-ballistic transport, high carrier velocity for
fast switching, as well as better electrostatic control due to the quasi one-dimensional structure
of CNTs.
Applications:
Logic gates
OPAMP
VLSI devices
CNTFET

Dual Gate MOSFETs are a
form of MOSFET with
two gates - they can be
used to provide additional
isolation between drain
& gate as shown in Figure.
Symbols of P-channel & N-
channel E-MOSFET & D-
MOSFET
Dual Gate MOSFET

Dual Gate MOSFET
The different gates
control different
sections of the
channel which are
in series with each
other.
The Figure shows
the structure of
Dual Gate MOSFET

Dual Gate MOSFETs Advantages:
Due to the presence of second gate, it avoids the feedback capacitance effect and
thereby it acts as a stable amplifier.
Dual Gate MOSFETs Applications:
1. mixers for RF applications
2. RF amplifier
Dual Gate MOSFET

Unit4 special semiconductor devices class4

Unit4 special semiconductor devices class4

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