In electronics, short-channel effects occur in MOSFETs in which the channel length is comparable to the depletion layer widths of the source and drain junctions. These effects include, in particular, drain-induced barrier lowering, velocity saturation, Quantum confinement and hot carrier degradation
Here are the all short channel effects that you require.It consist of:-
Drain Induced Barrier Lowering
Hot electron Effect
Impact Ionization
Surface Scattering
Velocity saturation
This presentation discusses the Lambda based design rules for drawing the layouts. The spacing between ltwo layers, extent if of overlap, minimum dimensions of each layer etc are decided by the lambda based design rules. the separation between metal and poly, poly and diffusion , width of metal etc
Microelectronic technology
This report briefly discusses the need for Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs), their structure and principle of operation. Then it details the fabrication and characterization of the MOSFETs fabricated at the microelectronic lab at University of Malaya
shows the simulation and analysis of a MOSFET device using the MOSFet tool. Several powerful analytic features of this tool are demonstrated, including the following:
calculation of Id-Vg curves
potential contour plots along the device at equilibrium and at the final applied bias
electron density contour plots along the device at equilibrium and at the final applied bias
spatial doping profile along the device
1D spatial potential profile along the device
SHORT-CHANNEL EFFECTS
A MOSFET is considered to be short when the channel length ‘L’ is the same order of magnitude as the depletion-layer widths (xdD, xdS). The potential distribution in the channel now depends upon both, transverse field Ex, due to gate bias and also on the longitudinal field Ey, due to drain bias When the Gate channel length <<1 m, short channel effect becomes important .
This leads to many
undesirable effects in MOSFET.
The short-channel effects are attributed to two physical phenomena:
A) The limitation imposed on electron drift characteristics in the channel,
B) The modification of the threshold voltage due to the shortening channel length.
In particular five different short-channel effects can be distinguished:
1. Drain-induced barrier lowering and “Punch through”
2. Surface scattering
3. Velocity saturation
4. Impact ionization
5. Hot electrons
Please read the following IEEE Spectrum articles and answer the quest.pdffasttrackcomputersol
Please read the following IEEE Spectrum articles and answer the questions given. You may
want to use illustrations in your answer to the questions, and mark them up accordingly as part of
answering the questions. If you take illustrations from some source (including the IEEE
Spectrum articles) please make sure this is properly cited.
http://spectrum.ieee.org/semiconductors/nanotechnology/the-next-highperformance-transistor-
could-be-made-from-lateral-nanowires Describe a FINFET and how it works. How is it different
than the planar MOSFET described in the first 5 slides of the TFET lecture? Is the FINFET a
quantum device? Give reasons why or why not. How is the nanowire device described here
different than the FINFET? Why is this difference an advantage for the nanowire device? They
one problem with the nanowire device is capacitive coupling. What is this and explain why it is a
problem with the nanowire device?
Solution
1)
The FinFET technology promises to provide the deliver superior levels of scalability needed to
ensure that the current progress with increased levels of integration within integrated circuits can
be maintained.
The FinFET offers many advantages in terms of IC processing that mean that it has been adopted
as a major way forwards for incorporation within IC technology.
FinFET technology has been born as a result of the relentless increase in the levels of
integration. The basic tenet of Moore\'s law has held true for many years from the earliest years
of integrated circuit technology. Essentially it states that the number of transistors on a given
area of silicon doubles every two years.
Some of the landmark chips of the relatively early integrated circuit era had a low transistor
count even though they were advanced for the time. The 6800 microprocessor for example had
just 5000 transistors. Todays have many orders of magnitude more.
basically what is finfet??
FinFET technology takes its name from the fact that the FET structure used looks like a set of
fins when viewed.
The main characteristic of the FinFET is that it has a conducting channel wrapped by a thin
silicon \"fin\" from which it gains its name. The thickness of the fin determines the effective
channel length of the device.
In terms of its structure, it typically has a vertical fin on a substrate which runs between a larger
drain and source area. This protrudes vertically above the substrate as a fin.
The gate orientation is at right angles to the vertical fin. And to traverse from one side of the fin
to the other it wraps over the fin, enabling it to interface with three side of the fin or channel.
This form of gate structure provides improved electrical control over the channel conduction and
it helps reduce leakage current levels and overcomes some other short-channel effects..
The term FinFET is used somewhat generically. Sometimes it is used to describe any fin-based,
multigate transistor architecture regardless of number of gates.
Due to the increased emphas.
In electronics, short-channel effects occur in MOSFETs in which the channel length is comparable to the depletion layer widths of the source and drain junctions. These effects include, in particular, drain-induced barrier lowering, velocity saturation, Quantum confinement and hot carrier degradation
Here are the all short channel effects that you require.It consist of:-
Drain Induced Barrier Lowering
Hot electron Effect
Impact Ionization
Surface Scattering
Velocity saturation
This presentation discusses the Lambda based design rules for drawing the layouts. The spacing between ltwo layers, extent if of overlap, minimum dimensions of each layer etc are decided by the lambda based design rules. the separation between metal and poly, poly and diffusion , width of metal etc
Microelectronic technology
This report briefly discusses the need for Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs), their structure and principle of operation. Then it details the fabrication and characterization of the MOSFETs fabricated at the microelectronic lab at University of Malaya
shows the simulation and analysis of a MOSFET device using the MOSFet tool. Several powerful analytic features of this tool are demonstrated, including the following:
calculation of Id-Vg curves
potential contour plots along the device at equilibrium and at the final applied bias
electron density contour plots along the device at equilibrium and at the final applied bias
spatial doping profile along the device
1D spatial potential profile along the device
SHORT-CHANNEL EFFECTS
A MOSFET is considered to be short when the channel length ‘L’ is the same order of magnitude as the depletion-layer widths (xdD, xdS). The potential distribution in the channel now depends upon both, transverse field Ex, due to gate bias and also on the longitudinal field Ey, due to drain bias When the Gate channel length <<1 m, short channel effect becomes important .
This leads to many
undesirable effects in MOSFET.
The short-channel effects are attributed to two physical phenomena:
A) The limitation imposed on electron drift characteristics in the channel,
B) The modification of the threshold voltage due to the shortening channel length.
In particular five different short-channel effects can be distinguished:
1. Drain-induced barrier lowering and “Punch through”
2. Surface scattering
3. Velocity saturation
4. Impact ionization
5. Hot electrons
Please read the following IEEE Spectrum articles and answer the quest.pdffasttrackcomputersol
Please read the following IEEE Spectrum articles and answer the questions given. You may
want to use illustrations in your answer to the questions, and mark them up accordingly as part of
answering the questions. If you take illustrations from some source (including the IEEE
Spectrum articles) please make sure this is properly cited.
http://spectrum.ieee.org/semiconductors/nanotechnology/the-next-highperformance-transistor-
could-be-made-from-lateral-nanowires Describe a FINFET and how it works. How is it different
than the planar MOSFET described in the first 5 slides of the TFET lecture? Is the FINFET a
quantum device? Give reasons why or why not. How is the nanowire device described here
different than the FINFET? Why is this difference an advantage for the nanowire device? They
one problem with the nanowire device is capacitive coupling. What is this and explain why it is a
problem with the nanowire device?
Solution
1)
The FinFET technology promises to provide the deliver superior levels of scalability needed to
ensure that the current progress with increased levels of integration within integrated circuits can
be maintained.
The FinFET offers many advantages in terms of IC processing that mean that it has been adopted
as a major way forwards for incorporation within IC technology.
FinFET technology has been born as a result of the relentless increase in the levels of
integration. The basic tenet of Moore\'s law has held true for many years from the earliest years
of integrated circuit technology. Essentially it states that the number of transistors on a given
area of silicon doubles every two years.
Some of the landmark chips of the relatively early integrated circuit era had a low transistor
count even though they were advanced for the time. The 6800 microprocessor for example had
just 5000 transistors. Todays have many orders of magnitude more.
basically what is finfet??
FinFET technology takes its name from the fact that the FET structure used looks like a set of
fins when viewed.
The main characteristic of the FinFET is that it has a conducting channel wrapped by a thin
silicon \"fin\" from which it gains its name. The thickness of the fin determines the effective
channel length of the device.
In terms of its structure, it typically has a vertical fin on a substrate which runs between a larger
drain and source area. This protrudes vertically above the substrate as a fin.
The gate orientation is at right angles to the vertical fin. And to traverse from one side of the fin
to the other it wraps over the fin, enabling it to interface with three side of the fin or channel.
This form of gate structure provides improved electrical control over the channel conduction and
it helps reduce leakage current levels and overcomes some other short-channel effects..
The term FinFET is used somewhat generically. Sometimes it is used to describe any fin-based,
multigate transistor architecture regardless of number of gates.
Due to the increased emphas.
Structural and Electrical Analysis of Various MOSFET DesignsIJERA Editor
Invention of Transistor is the foundation of electronics industry. Metal Oxide Semiconductor Field Effect Transistor (MOSFET) has been the key to the development of nano electronics technology. This paper offers a brief review of some of the most popular MOSFET structure designs. The scaling down of planar bulk MOSFET proposed by the Moore’s Law has been saturated due to short channel effects and DIBL. Due to this alternative approaches has been considered to overcome the problems at lower node technology. SOI and FinFET technologies are promising candidates in this area.
REVIEW PAPER ON NEW TECHNOLOGY BASED NANOSCALE TRANSISTORmsejjournal
Owing to the fact that MOSFETs can be effortlessly assimilated into ICs, they have become the heart of the
growing semiconductor industry. The need to procure low power dissipation, high operating speed and
small size requires the scaling down of these devices. This fully serves the Moore’s Law. But scaling down
comes with its own drawbacks which can be substantiated as the Short Channel Effect. The working of the
device deteriorates owing to SCE. In this paper, the problems of device downsizing as well as how the use
of SED based devices prove to be a better solution to device downsizing has been presented. As such the
study of Short Channel effects as well as the issues associated with a nanoMOSFET is provided. The study
of the properties of several Quantum dot materials and how to choose the best material depending on the
observation of clear Coulomb blockade is done. Specifically, a study of a graphene single electron
transistor is reviewed. Also a theoretical explanation to a model designed to tune the movement of
electrons with the help of a quantum wire has been presented.
REVIEW PAPER ON NEW TECHNOLOGY BASED NANOSCALE TRANSISTORmsejjournal
Owing to the fact that MOSFETs can be effortlessly assimilated into ICs, they have become the heart of the
growing semiconductor industry. The need to procure low power dissipation, high operating speed and
small size requires the scaling down of these devices. This fully serves the Moore’s Law. But scaling down
comes with its own drawbacks which can be substantiated as the Short Channel Effect. The working of the
device deteriorates owing to SCE. In this paper, the problems of device downsizing as well as how the use
of SED based devices prove to be a better solution to device downsizing has been presented. As such the
study of Short Channel effects as well as the issues associated with a nanoMOSFET is provided. The study
of the properties of several Quantum dot materials and how to choose the best material depending on the
observation of clear Coulomb blockade is done. Specifically, a study of a graphene single electron
transistor is reviewed. Also a theoretical explanation to a model designed to tune the movement of
electrons with the help of a quantum wire has been presented.
Multiple gate field effect transistors foreSAT Journals
Abstract
This is a review paper on the topic of multiple gate field effect transistors: MuGFETs, or FinFETs, as they are called. First, the motivation behind multiple gate FETs is presented. This is followed by looking at the evolution of FinFET technologies; the main flavors (variants) of Multigate FETs; and their advantages/disadvantages. The physics and technology of these devices is briefly discussed. Results are then presented which show the performance figures of merit of FinFETs, and their strengths and weaknesses. Finally, a perspective on the future of the FinFET technology is presented. Keywords: CMOS scaling, Double gate MOSFET, FinFET, Multiple gate FET, Multigate FET
International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.
5 Levels of Leadership for New Sales MangersGeorge Ferko
Talk delivered by George J. Ferko V on the five levels of leadership in his journey through the sales management.
Core material of this talk is from John Maxwell's "The 5 Levels of Leadership" and is protected under copyright law.
1. MAT355 Junctionless Transistor - GJ Ferko 1
___________________________________
* Email: gjf210@lehigh.edu
Phone: 610.597.8007
The Junctionless Transistor
By George J. Ferko V*
05/10/10
1. Summary
The operation of the modern conventional bulk transistor is dependent on the junctions
that exist between the source electrode, channel region, and drain electrode. These junctions are
formed as a result of the channel being doped to have an opposite polarity in comparison to the
source and the drain. The channel itself is actually formed by a bias that is the result of a gate
electrode. If you ask any materials scientist or electrical engineer they will tell you that the
modern transistor is on its way out. As the size of the transistor is shrinking so is the channel
region. As a result the channel region demands a higher doping concentration to avoid short-
channel effects. Unfortunately this concentration is too high for the devices to properly operate.
An immense amount of research into the successor of the current transistor design has been
going on for nearly 30 years out of a necessity to fill the void the modern bulk transistor is about
to leave behind. The focus of this body of work is to discuss the junctionless silicon nanowire
transistor and to explain why it may be the most feasible option for the next generation of
transistors. This report will touch upon some of the history that has led today’s researchers to the
junctionless transistor as well as the theory, processing, operation, and future work that is
involved with said transistor.
2. History
The concept of the junctionless transistor is far from new to the world of solid-state
electronics. In fact the notion of a junctionless transistor of uniform doping between the gate,
channel, and drain precedes that of the point-contact and the bipolar junction transistor of Bell
Telephone Laboratories from the late 1940’s [1]. In 1928 Julius Edgar Lilienfeld filed a patent
in the United States entitled “Device for Controlling Electric Current.” Lilienfeld recognized the
limitations of electron transport through evacuated spaces as the driving force behind the rapidly
expanding electronics industry. He proposed what would later be recognized as the field-effect
transistor as a solution. The Lilienfeld transistor, pictured in figure 1, is very similar to modern
metal-oxide-semiconductor field effect transistor (MOSFET) [2,3]. In this device a thin
semiconductor film is deposited over a thin insulating layer which itself is deposited over the
metal electrodes. Lilienfeld theorized that when a gate voltage was applied to the thin
semiconductor region between the electrodes the charge carriers would become completely
depleted allowing for modulation of the current through the device. This device is actually a
resistor in which the current through the resistor is controlled by applying a gate voltage that
essentially drives the resistance to infinity [4]. Lilienfeld knew that the semiconductor layer had
to be extremely thin; in his patent he stated that for the transistor to work “this layer is extremely
thin [then] the field is permitted to penetrate the entire volume thereof and thus will change the
conductivity throughout the entire cross-section….” He even recognized a need for what could
be considered nanotechnology by choosing the method of depositing a colloidal copper
suspension and then sulphurizing it to produce the thin semiconductor layer [2]. Unfortunately
2. MAT355 Junctionless Transistor - GJ Ferko 2
for Lilienfeld the ability to produce a
layer thin enough for his device was
beyond the technology of the time
and he was never able to produce a
working transistor. Lilienfelds
concept went fairly unnoticed until
William Shockley attempted to file a
patent in the 1940’s that named the
field-effect concept as the basis for
controlling current in his solid-state
amplifier. Much to Shockley’s
dismay the patent was shot down
when it was discovered that the
concept had previously been patented
by Lilienfeld [1].
Today Lilienfelds
contribution to solid-state electronics
has been recognized. In an address
to the American Institute of Physics
in 1988 John Bardeen, one of Shockley’s coworkers at Bell Telephone Labs during the creation
of the first solid-state transistors, acknowledged Lilienfeld for his contributions to the
semiconductor field. He said, “Lilienfeld had the basic concept of controlling the flow of current
in a semiconductor to make an amplifying device. It took many years of theory development and
material technology to make his dream a reality [1].” Although Bardeen may have spoken to
soon because it wasn’t until 20 years after this address that Lilienfelds junctionless concept was
actually realized [4].
The research that has been done on silicon-on-insulator transistor technology has also
played a large role in the development of the junctionless transistor. In 1984 T. Sekigawa and Y.
Hayashi published the first paper on a multi-gate transistor. They discovered that the short-
channel effects being caused by the decreasing size of transistor channels could be minimized by
adding an additional gate electrode thus producing more electrostatic control. They named the
transistor type XMOS because when it was cross sectioned it had the appearance of the Greek
letter Ξ (Xi) [5,6,7]. Their discovery came long before short-channel effects actually became a
problem in commercially available transistors, however, it paved the way for future discoveries
that helped to develop the multi-gate transistor as a solution to the approaching end-of-the-road
for CMOS devices. From then on the Silicon-on-insulator (SOI) MOSFET became a major
competitor among the transistor designs that were studied to extend Moore’s Law for years to
come. A multitude of other designs and research followed. Some of these designs are shown in
figure 2 and further descriptions of them are available in the literature [5,8]. The schematics
shown to the right in the figure are the orientations that have been studied in recent years in order
to bring the CMOS device down to the nanometer scale.
This year (2010) a team lead by Jean-Pierre Colinge at the Ireland based Tyndall National
Institute announced the first working junctionless nanowire transistor [7]. By building on the
idea of the multi-gate SOI MOSFET this team was able to produce a heavily doped short-
channel device that comes close to full free carrier depletion in the channel. They have made
this transistor on a silicon nanowire that has equal dopant concentrations between the channel
Figure 1: Diagram of the Lilienfeld concept for a solid-
state transistor [2].
3. MAT355 Junctionless Transistor - GJ Ferko 3
and the source and drain extensions [4,9,10]. In the literature the team at Tyndall often mentions
the scientists from history who’s contributions to electronics have made their work possible
[4,6,5]. Despite this modesty, it is clear from the number of news articles and attention of many
peer review journals that the junctionless transistor is building up steam [4,9,21].
3. Theory
The motivation for the concept of the junctionless transistor was to find a way to
inexpensively produce a practical transistor that doesn’t face some of the problems that the
conventional inversion-mode devices do. The design of the silicon nanowire, or nano ribbon,
junctionless transistor is shown in figure 3 [4]. The junctionless transistor is an accumulation-
mode device. The literature suggests that on the nano scale accumulation-mode devices have
some advantage over the inversion-mode devices. Larger scale accumulation-mode devices have
given accumulation-mode the reputation of facing many process variability issues, however; in
short-channel construction the inversion-mode has been found to have bigger issues then the
accumulation-mode devices. As inversion-mode devices shrink it becomes difficult to maintain
suitable current drive at a reasonable threshold voltage because of the necessity of reducing
doping concentration when the transistor is made on the nanometer scale. The accumulation-
mode devices have been found to perform better with these issues [11]. The junctionless
nanowire transistor is basically an accumulation-mode device with equal doping between the
channel and the source and drain.
As mentioned above, conventional short-channel devices require that large dopant
concentration gradients occur over a distance of a few nanometers. The super-sharp source and
drain junctions are not stable as diffusion may occur during operation of the device. Even
Figure 2: Different multi-gate FET configurations for the SOI transistor meant to aid in the
continuously decreasing size of the transistor [5,8].
4. MAT355 Junctionless Transistor - GJ Ferko 4
forming such devices requires extremely
costly thermal processing in device
fabrication [4,10,12]. The junctionless
transistor clearly solves this short-channel
issue because no dopant concentration
gradient exists. The dopant concentration in
the nanowire used does require a very high
doping concentration, which in turn requires
that the cross-section of the wire be very
small to ensure that it can be fully depleted
of free carriers.
Another problem with the
conventional transistor is its low theoretical
subthreshold slope limit of 60 mV/decade at
room temperature. The subthreshold slope is
a measure of the switching capability of the MOSFET over time. It is assumed that with any
FET the switching ability will decrease over time. Subthreshold slope is taken as the slope of the
drain current-gate voltage curve in subthreshold operation (below the threshold voltage). This
value of 60 mV/decade is no longer sufficient, especially when we consider the ever decreasing
supply voltages that the semiconductor industry is demanding. The literature suggests that the
junctionless transistor can obtain sub-60 mV/decade subthreshold slopes with a drain voltage as
low as 1.75 V, which is about half the value needed for standard inversion mode devices [13].
This is just a speculation, however, which was based on an experiment involving the comparison
of impact ionization between junctionless and junctioned transistors. When the impact
ionization occurs over a larger section of the drain extension the drain voltage necessary to
produce a low subthreshold slope is less.
As the MOSFET is scaled down so is the effective oxide thickness of the gate insulator.
This has caused a problem with the vertical electric field in the conventional MOSFET because
as the effective oxide thickness decreases the vertical electric field in the channel increases. This
causes an increase in free carrier scattering, which has led to a decrease in mobility thus lowering
the current drive of the transistor. It has been found that the junctionless accumulation-mode
transistor poses yet another advantage to the inversion-mode junctioned transistor. The
inversion-mode junctioned transistor’s peak electron concentration occurs in the part of the
channel that has the highest electric field. The opposite is true in the junctionless transistor. The
peak electron concentration in the junctionless transistor occurs in the region with the lowest
electric field. As a result, the current drive in the junctionless transistor will be much better then
in junctioned inversion-mode transistors as transistors reach the nanoscale [14]. The quantum-
mechanical confinement that causes this down turn in electron concentration for inversion-mode
transistors is discussed further in the literature [15]. An interesting result of this study is that
inversion-mode transistors did not exhibit the same reduction in electron concentration when
they had the gate all around structure, shown in figure 2. This would lead many to think that the
gate all around structure shows more promise than other multigate structures, however; it seems
that the same quantum confinement issue does not apply to a junctionless transistor and other
multi-gate structures are being used with high functionality.
The current drive in the junctionless transistor has the upper hand on the inversion-mode
transistor for yet another reason. In inversion-mode transistors the electrons are confined to a
Figure 3: Diagram of the concept for the
silicon nano ribbon junctionless transistor [4].
5. MAT355 Junctionless Transistor - GJ Ferko 5
surface channel. This is not so in the junctionless transistor. The junctionless transistor allows
the drain current to flow through the entire cross-section of the nanoribbon. This means that the
conduction path is primarily located in the middle of the nanoribbon which reduces the scattering
effects that occur near interfaces and would be experienced by normal transistors. This can be
seen in figure 4 which shows the electron concentration in an n-type junctionless transistor at
gate voltages ranging from pinch-off voltage to flat-band conditions [4].
The silicon nanowire has faced other competitors for nanowire transistors, most notably
the carbon nano tube. In past research it was found that the quantum confinement to the center
of the silicon nanowire resulted in a necessary increase in the gate voltage needed to switch the
transistor. However, the silicon nanowire compensates for this effect by not facing a reduction
in its density of states. In contrast, the carbon nanotube does see a reduction in density of states
and its electrons are confined to the surface of the channel. In one particular study by A. Marchi,
et al. the carbon nano tube was found to outperform the silicon nanowire because of the lower
gate voltage required to switch transistors where electrons are confined to the surface of a
channel. It should be noted that this study was performed on wires ranging from 45 nm to 65 nm
in diameter [16]. When the silicon nano ribbon has a thickness of 10 nm the negative effect of
quantum confinement in the center of the channel is lost and the silicon nanowire outperforms
the carbon nano tube [17,18]. Even before the junctionless transistor was successfully made
these effects were predicted. A. Afzalian, et al. have used non-equilibrium Green’s function
(NEGF) modeling to predict how the junctionless transistor will work. The specifics of this type
Figure 4: Plots that were formed as the result of a simulation carried out to reveal the electron
concentration in the junctionless transistor [4].
6. MAT355 Junctionless Transistor - GJ Ferko 6
of modeling are far beyond the scope of this paper, and the understanding of the author writing
this paper, but in depth description of the NEGF are available in the literature [17,19].
4. Fabrication
The silicon nanowires, or nano-
ribbons are produced from conventional
SOI wafers using electron-beam
lithography. At Tyndall a JEOL 6000FS
lithography tool is used to define
nanowires about 30 nm wide and 10 nm
thick. In e-beam lithography an electron
beam of very small spot size is used to
damage the surface of a material and
create very small structures. This method
is already widely used in the
semiconductor and nanotechnology
industries. The system is located in a 100
class clean room [20]. After growing a
gate oxide of 10 nm the nanowires are
uniformly doped by ion implantation.
The n-type devices are doped with arsenic
and the p-type devices are doped with
BF2. The process is controlled to yield
uniform doping concentrations ranging
from 2 × 1019
to 5 × 1019
atoms per
centimeter cubed. The high doping is
required to create a large current drive
and good source drain contact resistance.
The gate is formed by depositing a 50 nm
thick layer of amorphous silicon using low pressure chemical vapor deposition at a temperature
of 550 ⁰C. To obtain reasonable values for the threshold voltage the gate in the n-type device is
doped to be p-type and vice versa. After heavy gate doping the specimens are annealed in an
ambient nitrogen atmosphere at 900 ⁰C for 30 minutes to make the amorphous silicon
polycrystalline and to activate the dopants. The gate electrodes are then patterned and etched in
a reactive-ion etch reactor. Finally a protective SiO2 layer is deposited and holes are etched in it
to make way for the electrical contacts which are formed by a TiW-metallization process.
Transmission electron micrographs of the completed device are shown in figure 5 [4]. As stated
above, the thinness and narrowness of the silicon ribbon is essential to ensuring that full
depletion occurs in the channel when a bias is applied. As electron-beam lithography continues
to advance it may become possible to consistently make even smaller devices which would only
make the Si nanowire junctionless transistor even more effective as long as suitable current drive
can be maintained. This process also clearly eliminates the need for ultrafast annealing making it
much cheaper and simpler then the production of junctioned transistors.
Figure 5: Transmission electron micrographs of
the silicon nanoribbon devices sharing the same
gate [1].
7. MAT355 Junctionless Transistor - GJ Ferko 7
Operation:
The successful
fabrication of the junctionless
nanowire transistor has meant
that the junctionless transistor
can now be experimentally
tested against the regular
MOSFET. Finally modeling and
estimations are no longer
necessary to prove the potential
of junctionless transistors. In a
recent study by JP Colinge, et
al., the junctionless transistor’s
current-voltage characteristics
were compared to those of a
junctioned tri-gate FET. The
graph of drain current versus
gate voltage that was produced is
shown in figure 6. In this study
the drain voltage was ± 1 V and
the devices had a width of 30 nm and a length of 1 μm. The junctionless transistors are entitled
“gated resistors” in the figure. By simply looking at the figure it can be seen that the properties
of the junctionless transistor and those of the regular MOSFET are almost exactly the same.
This establishes the theories that the accumulation-mode junctionless transistor can perform as
well as the inversion-mode transistor. Essentially the channel can be electrostatically depleted
with the same gate voltage of a regular MOSFET. The subthreshold slope can also be
determined from this graph. Previously it was stated that the theoretical minimum subthreshold
slope for the regular transistor is about 60 mV/decade. The best junctioned SOI transistors have
reached values of 63 mV/decade. The junctionless transistor that produced these gate voltage-
Figure 6: Drain current-gate voltage characteristics for a
junctionless transistor and a junctioned transistor [4].
a) b)
Figure 7: Output characteristics of the junctionless transistor described by a plot of drain current
versus drain voltage for a) a p-doped transistor and b) an n-doped transistor [4].
8. MAT355 Junctionless Transistor - GJ Ferko 8
drain current characteristics had a subthreshold slope of 64 mV/decade. In addition to this
exceptional performance the subthreshold slope of the transistor only changed a few percent
when it was heated from room temperature to 475 K [1]. Shown in figure 7 are the output I-V
characteristics for the junctionless transistor. The nanowire used for this particular output had a
width of 20 nm and a length of 1 μm. To anyone who has opened an electronic properties text
book or been involved with the electronics industry these curves should look familiar. They
show exactly the kind of behavior that would be expected of a traditional MOSFET device.
Although the operational characteristics of the junctionless transistor do not exceed those
of regular MOSFETs the fact that such similar characteristics have been produced in the first
published experimental study goes to show that the junctionless transistor is a feasible option for
the nanoscale transistor design. Academics aren’t the only people that take this success
seriously; it appears as though commercial industry has taken an interest as well. Tyndall CEO,
Professor Roger Whatmore, has been quoted as saying, “We are very excited by the outstanding
results that Jean-Pierre has achieved. We are beginning to talk about these results with some of
the world's leading semiconductor companies and are receiving a lot of interest in further
development and possible licensing of the technology [21].”
Future:
Whether or not the junctionless transistor continues to gain popularity most certainly
depends on its ability to continue shrinking in size. L. Ansari, et al. have already begun
modeling a junctionless transistor made from a silicon nanowire with a radius of 0.52 nm and a
gate length of 3.1 nm. They have simulated this atomic scale device and produced some
predicted current-voltage output characteristics that look very promising. Though in the present
time such transistors cannot be constructed the study is still significant to the future abilities of
the junctionless transistor. The team found that the short-channel effect of tunneling across the
gate voltage barrier can be avoided at atomic scales as long as the gate length is greater the twice
the radius of the nanowire. It was also found that at such small scales the gate-all-around
structure is needed in order to produce enough electrostatic control to shut off the gate. Perhaps
the most interesting result produced is that the location of the dopant or dopant atoms in the
channel region can change the band structure of the nanowire significantly. This means that as
the gate length shrinks the doping concentration must be increased to ensure a semi-uniform
doping in the gated nanowire. The researchers concluded that the junctionless nanowire
transistor would still continue to work even on the atomic scale [22].
The junctionless nanowire is certainly paving a narrow road to success in the
semiconductor industry. It is almost inconceivable that the path that has lead to the junctionless
transistor began more than 80 years ago. Now it would seem that having an equally doped
source, drain, and channel region will be the solution that propels the transistor into the next
scale reduction and carries the weight of Moore’s Law through the next decade and perhaps long
after that.
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