Carbon-based nanoelectronics: from simulations to applications | the Royal Society
Transcript
- So, good evening, ladies, and gentlemen. My name is Peter Bruce. I'm the physical secretary
- and one of the vice presidents here at the Royal Society, and it's a delight for me to welcome you
- all here tonight to this award lecture to Carlton House Terrace here in London. It's a particular
- pleasure for me to introduce this speaker and to present him with the Rising Star Africa Prize. So,
- just a little bit of housekeeping. Try to remember to make sure your phones are off or on silent.
- There is no planned fire drill, so if the fire alarm does sound, it's likely to be real. You can
- give it a couple of seconds just to make sure it doesn't go off instantly, and if it does continue,
- then there are fire exits here and here, and if you can, then assemble on the Duke of York
- steps. I'm sure it won't happen, but I just want to be safe and sound. So let me just say something
- a little bit about the Rising Star prize first, and then I'll say a few words about the speaker.
- 91TV Rising Star Africa Prize is intended to recognise early-career research
- scientists based in Africa who are making innovative contributions to the physical,
- mathematical, and engineering sciences. The prize was established in memory of Paul O'Brien,
- a fellow of the Royal Society, and someone that I knew personally. His work always focused on
- encouraging science and education in Africa. It was one of his passions, and so it's very
- appropriate that we have this award, supported by him and in his name. The winner receives £14,000
- towards their research and a personal gift of £1,000. It's open to any African citizen. It's
- restricted to early career scientists, so no more than 15 years since achieving their PhD.
- So that's the remit of the award. Now, coming on to our winner and lecturer this evening,
- our winner of the 2022 prize, Khalil Tamersit, received his PhD degree in medical electronics
- from Batna University in Algeria in 2018, so he definitely falls into the 15 years since
- a PhD category. He is currently the associate professor in the Department of Electronics and
- Telecommunications at the Faculty of Science and Technology, University, 8th May 1945,
- at Guelma in Algeria. He's published numerous papers in highly reputable international journals.
- His research interests include carbon-based nanoelectronics and bioelectronics, the modelling
- and simulation of sensors and biosensors at the micro and nanoscale, and the application
- of computational intelligence and smart sensing and nanoelectronics simulations. He's received
- a number of awards, including from the Institute of Electrical and Electronics Engineers. I mean,
- he's a very worthy winner of our distinguished Rising Star Africa Prize. So, with that, I'm
- going to invite Kahlil to come to the podium and present his lecture, Carbon-based Nanoelectronics
- from Simulations to Applications.
- Thank you for this kind introduction. Hello, everyone. Thank you for coming. Today,
- I will talk about carbon-based nanoelectronics from simulations to applications. The presentation
- consists of four parts. I'll start with an introduction on the nanoscale electronics,
- transistors, and device downscaling. Then, I'll present an overview on the carbon nanomaterials,
- their applications in electronic devices,
- and the relevant simulation approaches. Next, I'll introduce the carbon-based nanoelectronics,
- including our main contributions, and finally, I'll present a conclusion and some perspectives.
- Let's start with the definition of nanometre to have an idea on nanoelectronics. Considering
- one millimetre, if we go down by a factor of one thousand, that will be one micrometre or micron.
- If we go down by another factor of one thousand, that will be one nanometre or nano.
- These figures are some natural and manmade things distributed according to their sizes.
- In fact, the nanoscale is the size range between approximately one nanometre and
- one hundred nanometres. For example, in the nano world, we can't find the deoxyribonucleic acid and
- tens of silicon atoms as natural elements, and we can find manmade elements such as
- carbon nanotube, and some nano devices and systems based on nanomaterials.
- Moving to the electronics to have the link between the electronics and the nanoscale,
- the electronics can be found inside the daily instruments and devices.
- Defining it, it can be understood by the connection of some electronic devices
- in charge of controlling the electrons flow, in the aim of doing some functions.
- In fact, the main device in microprocessor is called transistor, where one modern
- microprocessor or advanced microprocessor can have billions of these elementary devices.
- Seventy-five years ago, the transistor was born. Now, the commonly used type of transistors is
- called field effect transistor, based on metal oxide semiconductor, as shown in figure A.
- This device consists of a gate, a channel region connecting source and drain,
- and a barrier separating the gate from the channel.
- In fact, the working principle of a conventional field-effect transistor relies
- on the control of channel conductivity by means of an applied gate voltage.
- It can also be defined as a controlled resistor.
- Note that the field effect transistor has two states, the off-state and on-state. For example,
- considering a given during to source voltage applied between source and drain electrodes,
- and if the gate voltage is zero volts, we get a high channel resistance, as shown in figure
- B. Thus, the electrons from source don't reach the drain through the channel. This is the off-state,
- and if the gate voltage is one volt, we get low channel resistance, as shown in figure C.
- Thus, the electrons from source can reach the drain through the channel. This is the on-state
- that is happening with the NMOS device that has the circuit symbol,
- as shown in figure D. The PMOS device works in reverse to NMOS device.
- In other words, if the gate voltage is zero volts, we get on-state instead of off-state in
- the case of NMOS device, and if the gate voltage is one volt, we get off-state instead of on-state.
- Considering the basic switch circuit, the NMOS and PMOS transistors can be used to
- switch on and off the light of this basic switch circuit by applying the appropriate gate voltage,
- as shown previously. In logic circuits or generally CMOS technology, which
- means complementary metal oxide semiconductor, these two transistors can work in conjunction.
- What can we do with these devices? We can build some logic circuits, as shown in these figures,
- such as the well-known inverter that gives the output one if the input is zero,
- and the inverse is true. The NAND function is another logic circuit.
- Its output gives zero if the two inputs are one, and the output is one for the remaining
- combinations of inputs. For NOR function, the output is one if the two inputs are zero, and
- the output is zero for the remaining combinations of inputs. By combining the inverter with NAND and
- NOR functions, we get AND and OR functions, respectively, as shown in these figures.
- Note that the AND and OR functions are the inverse of NAND and NOR functions in terms
- of logical tables or truth tables. By combining these logic gates with others, we can build a
- huge number of archived, let me say, a huge number of electronic circuits that can perform advanced
- logic functions, such as this basic Arithmetic Logic Unit, which is an integrated circuit.
- Conceptually, making the electronic ship, or the integrated circuit,
- more powerful is based on increasing the number of transistors. To do so, the size of transistors
- should be reduced, and this is called, ladies and gentlemen, the miniaturisation approach.
- This is what happened during more than five decades. The transistors have shrank from several
- micrometres to several nanometres, as shown in figure B, and this reflects the great development
- of electronics that we are witnessing today. In this context, we talk about Moore's Law, which is
- an amazing empirical prediction given by Gordon Moore, who was a co-founder of Intel Corporation.
- In fact, Moore's law is the observation that the number of transistors in a dense
- integrated circuit doubles about every two years, as shown in figure D. We can see the
- increase of transistors number over time for different generations of microprocessors.
- Now, the scientific community works to satisfy,
- let me say, the downscaling requirements for the next years, while ensuring the high performance
- and energy efficiency of the future field effect transistors. Let's have a view on
- the performance and improvements that should be considered with the downscaling approach.
- In fact, the switching behaviour between off-state and on-state, as shown previously, is not ideal
- in real scenario. In the real scenario, there is the off-state current, or leakage current,
- which is a current when the potential barrier attributed to the electrostatic gating
- opposes the electrons flow from source to drain, as shown in figure B. This off-state
- current should be minimised, because it's proportional to the standby power consumption.
- There is the on-state current, or ion, as shown in figure A,
- which is attributed to the thermionic emission over the potential barrier,
- as shown in figure C, because we can modulate the height of the potential barrier by varying the
- gate voltage to have off-state and on-state, as shown previously. There is the subthreshold swing,
- or, in other words, the swing factor, which can be defined as the required gate voltage to change the
- drain current by about one order of magnitude. In fact, minimising this important electrical
- parameter, I want to say the subthreshold swing loads to reduce the power supply voltage,
- as shown in figure A by VDD, which is the voltage needed to switch on and off our device.
- Why we do that, because having a huge number of transistors in a dense,
- integrated circuit will certainly consume energy, and for low power applications, we have to reduce
- the power supply voltage. For this reason, we need small values of subthreshold swing.
- The main issues that oppose the downscaling with the required performance are the Short Channel
- Effects and the theoretical limit of subthreshold swing. In fact, the Short Channel Effects are
- attributed to the rapprochement of source and drain, due to the ultra-scaling that degrades the
- control of electrostatic gating over the carrier transport in the channel between source and drain.
- The second issue is that if we want to decrease the power supply voltage, as I said,
- from VDD to VDD prime, while ensuring sound switching behaviour between off-state and
- on-state, we need, in this scenario, a small value of subthreshold swing. Or,
- in other words, steep slope drain current, as shown by this green dashed line.
- However, the subthreshold swing is limited to 60 millivolts per decade,
- and this is exactly the main issue in today's nanoelectronics. As solutions, the use of
- emerging nanomaterials in field-effect transistors as channels is considered as an intriguing option.
- We cite, among others, the TMD-based nanomaterials, as shown in figure A,
- and the carbon-based materials, as shown in figure B, such as the carbon nanotube, graphene sheet,
- graphene, nanoribbons, bilayer, graphene, and so on. The use of advanced gate configurations
- is also considered as a solution. For example, we can use the gate all around.
- The tree gate, omega gate, the double gate configuration, and so on.
- Note that the use of advanced gate configurations aims to improve the control
- of electrostatic gating over the carrier transport in the channel between source and drain.
- The third approach is the use of innovative transistors with non-classical working principle
- and operating regimes, such as tunnelling transistors, fuel injection transistors, and
- electromechanical transistors. In this context, the theoretical assessments and simulations can
- play an important role in the development of the future field-effect transistors. Actually,
- our research works focus on exploiting the carbon-based materials, namely the carbon
- nanotubes and graphene nanoribbons, to boost the performance of nanoscale field effect transistors.
- Considering the carbon-based materials, graphene is two-dimensional material consisting of a
- single layer of carbon atoms arranged in a honeycomb structure, as shown in figure A.
- This two-dimensional material was isolated in 2004 by Professor Andre Geim and Professor Kostya
- Novoselov of the University of Manchester. Then, in 2010, they were awarded the Nobel Prize in
- Physics for their groundbreaking experiments regarding this two-dimensional material.
- As one can see in the second figure, the huge number of archived documents and papers linked
- to the graphene, while indicating the main relevant fields, including engineering, in
- which we find electronics and nanoelectronics. So, the main advantages of graphene for electronics
- and nanoelectronics are its high electron mobility, where graphene can conduct electrons
- 100 times more easily than silicon, which is very beneficial for high-speed applications.
- It's purely two-dimensional structure with atomic thickness, which can serve the miniaturisation
- approach. It has high sensitivity to its local environment, including sensing and biosensing
- environments, as well as the electrostatic environment in the case of, for example,
- nanoscale field-effect transistors. It is readily dopable, where it can be p-type doped material and
- n-type doped material, which is very important for nanoelectronics applications.
- It has high flexibility and mechanical strength, which empowers it to be used in
- variable structures and sensors and devices, and its cost is low. For example, using the
- exfoliation approach, which is the simplest method to manufacture this two-dimensional material.
- However, graphene is a gapless material,
- which limits its use in digital and switching applications.
- If we say digital and switching applications, we say switching the device between off-state and
- on-state in very large scale integration. Yet, a bandgap can be opened if we apply
- vertical potential on bilayer graphene, or if graphene sheet is cut into small
- ribbons to have lateral confinements, and that will be graphene nanoribbons.
- Note that there are two types of graphene nanoribbon, the armchair edge and zigzag
- graphene nanoribbon. For the first type, there are three families that can provide energy band gaps
- according to their dimer number, as shown in figure C, in the width direction.
- As one can see in figure D, the variation of energy bandgap as a function of
- nanoribbon width for the three families of armchair edge graphene nanoribbon. So,
- the graphene nanoribbon is a material with tunable bandgap,
- which is very important for advanced electronic devices, including the nanotransistors.
- Carbon nanotubes are also one-dimensional, are also unique one-dimensional let me say,
- materials which can be viewed as [unclear word 0:23:05] sheets of graphene. This ruling can
- be done at different fashion, resulting in different carbon nanotube properties.
- Note that there are three types of carbon nanotubes; the zigzag, chiral,
- and armchair carbon nanotubes, and there are semiconducting and metallic carbon nanotubes.
- For the semiconducting carbon nanotubes, which are the most relevant to nanoelectronics,
- their energy band gaps are inversely proportional to their diameters.
- Note that the advantages of carbon nanotubes for modern electronics and nanoelectronics
- are somewhat similar to those of graphene nanoribbons.
- We cite, among others, the nanotubes have atomic thicknesses and small diameters,
- which is beneficial for the miniaturisation approach or the downscaling approach. They have
- extremely high carrier mobility, which can serve the many, let me say, the high-speed applications.
- They are characterised by saturation, velocity, ballistic transport, excellent stability, and
- tunable bandgap, and all these features are in the benefit of advanced nano devices, including nano
- transistors. If we integrate the carbon nanotubes and graphene nanoribbons
- in field effect transistors as channels, as shown in these figures, we get these kind of nanoscale
- field effect transistors, which can be endowed with advanced gate configurations, to improve the
- control of electrostatic gating over the carrier transport in the channel between source and drain.
- In this context, the computational assessments and simulations
- can play an important role in the development of these carbon-based nanotransistors.
- The simulations and technology computer-aided design form an important step that paves the
- way to optimised fabrication and industry. Among the main benefits of simulations and technology,
- computer-aided design, we cite the reduction of development costs by
- reducing the time and manufacturing cycles spent to develop semiconductor technologies.
- In addition to the visualisation of internal physical processes, while allowing to see inside
- the device, which is very important for the well understanding of cutting-edge nanotransistors.
- The simulation and technology computer-aided design can also provide the possibility to
- perform design technology co-optimisation and virtual experimentations, which is very beneficial
- for technologies roadmaps. Moving to the commonly used computational approaches, figure A shows the
- scheme of the different levels of approximation for the modelling of materials' electronic
- properties, hierarchically ranked according to their accuracy and computational demand.
- Interfaces for the exchange of information between different methods are also shown in this figure.
- At the base of this hierarchy, we find the ab initio calculations, or the first principles,
- which are the most accurate, but with the computational cost. The next level
- of approximation, or the atomistic models, constitutes the next level of approximation.
- They are quantum mechanical in form, but require inputs from the ab initio calculations, as shown
- in figure A at the bottom. In our case, we have used the tight-binding approximation to describe
- the carbon-based channel material, in our case, the carbon nanotubes and graphene nanoribbons.
- Figure B shows the different approaches for the modelling of electron devices,
- hierarchically ranked according to their accuracy and computational demand.
- The information from the material's band structure feeds each method differently,
- as shown in figure B at the bottom. In our case, we have used the non-equilibrium Green's function,
- fed with a tight-binding Hamiltonian or,
- in other words, with a tight-binding approximation, as shown in figure A,
- while forming a powerful quantum simulation tool that can predict and match the experimental
- results, which is the most important thing in such computational approaches. In most
- of our quantum simulations, we have used the non-equilibrium Green's function to
- deal with the carrier transport in the channel between source and drain. While we have used the
- finite difference method and finite element method to deal with the electrostatics, or,
- in other words, with the Poisson equation, in its 2D or 3D form.
- Note that the non-equilibrium Green's function solver and the Poisson equation solver are
- coupled self-consistently until a convergence. After the convergence, we can extract the drain
- current and the device characteristics using the non-equilibrium Green's function quantities.
- Note that the main computational procedure in the non-equilibrium Green's function
- simulation is the calculation of the retarded Green's function, given by equation one,
- that comprises the Hamiltonian matrix given by H, which is an atomistic description of the
- carbon-based channel material. In our case, carbon nanotubes and graphene nanoribbons.
- In addition to the self-energies shown by the two last terms describing the physical
- coupling between source, drain, electrodes, and the channel, as shown in the two top figures.
- Is to note that this computational approach can consider the most
- electrostatic and quantum phenomena, making it a powerful quantum simulation method.
- After the development of the source code that usually contains few thousands of lines,
- it's mandatory to check the simulator accuracy. To do so, benchmarking should be performed
- against some experimental and theoretical results available in the relevant literature.
- We can see in these figures the good agreement, in terms of transfer and output characteristics
- issued from the non-equilibrium Green's function-based simulator, and some
- experimentations and advanced quantum simulators for the two cases. Namely, the carbon nanotube
- field effect transistor and graphene nanoribbons field effect transistor.
- In fact, the nanoscale carbon nanotube and graphene nanoribbon field effect transistors
- have already been proposed in literature, where some improvements have been recorded
- in comparison to the silicon-based transistors, which are the conventional transistors that can
- exist. For example, in the microprocessors of our smartphones. However, some issues are faced with
- the continuous downscaling, such as the increase in off-state current, which is not beneficial for
- low-power applications. This increase in off-state current, as shown in figures A and A prime,
- is due to the direct source to drain tunnelling, as shown in figure B,
- which is attributed to two main reasons. The first reason is the rapprochement of source and drain,
- due to the ultra-scaling or, in other words, devices with gate lengths less than
- ten nanometres. The second reason is the light effective mass of carriers in carbon nanotubes
- and graphene nanoribbons that supports this kind of tunnelling mechanism. In fact, there is another
- issue called band-to-band tunnelling, as shown in figure C, which is a card in the device off-state,
- leading to a non-bipolar behaviour, as shown in the transfer characteristic of figure D
- in the green area. In fact, it's a kind of having two switching behaviours. The first one,
- when we increase the gate voltage from off-state to on-state, and the second when we decrease it.
- Note that each switching is considered as a high off-state current to the other switching, which
- is a disadvantage in terms of power consumption. I emphasise here that the direct-source-to-drain
- tunnelling and the band-to-band tunnelling are the two main quantum phenomena responsible for
- the deterioration of the device switching performance in the nanoscale regime.
- Our improvement approaches are aiming to mitigate the aforementioned quantum effects that
- become severe with the gate down scaling. In this context, we have proposed some innovative designs,
- as shown here, based on specific channel doping and improved electrostatics, using
- electrical or electrostatic doping and chemical doping, in order to engineer the band structure
- and fix the carrier transport in the channel, while improving the device performance.
- It's worth noting that the use of electrostatic doping lows to realise an accurate,
- abrupt junction in nanoscale regime, which is difficult to be realised using chemical doping.
- Note that the realisation of accurate, abrupt junctions in nanoscale regime is
- very important for nanoelectronics, especially to guarantee the device performance, and,
- let me say, the reliability. As a contribution, we have proposed junctionless carbon nanotube and
- graphene nanoribbon field effect transistors, which are similar to the conventional devices,
- accepting the use of n-n-n doping profile or, in other words, a uniform n-type doping profile,
- instead of n-i-n doping profile, in the case of conventional devices.
- In fact, the main benefits of the junctionless paradigm has… We cite, let me say,
- the facilitation of the manufacturing processes, while avoiding the realisation of accurate,
- abrupt junctions in nanoscale regime. The second benefit is that it allows to engineer the band
- structure and fix the carrier transport in the channel, while enhancing the device performance.
- For example, considering the off-state regime in figure A for the inversion mode case,
- we can see that the direct-source-to-drain tunnelling spectrum is somewhat moderate,
- leading to a high leakage current. While in figure B for the junctionless mode, we can see that the
- direct-source-to-drain tunnelling intensity is decreased in comparison to that of inversion
- mode case. This decrease in leakage current, or in the direct-source-to-drain tunnelling,
- is attributed to a kind of dilation in potential barrier, induced by the junctionless paradigm
- that opposes these direct tunnelling components. It's worth noting that the recorded improvements
- include the decrease in leakage current, as shown here. The increase in current threshold, the
- improvement of subthreshold swing or swing factor, and some mitigations in terms of short-channel
- effects. The second improvement approach is based on the gate work function engineering.
- In fact, the work function can be defined
- as the minimum energy needed to remove an electron from a solid surface to a point in the vacuum.
- In this approach, we have used the binary emitter Holloway gate electrodes
- that has two different values of work function at the extremities, forming a graded work function
- from high work function to low work function. Aiming to engineer the band structure and fix
- the carrier transport in the channel, while enhancing the device performance.
- We can see in the electron density distribution that the band-to-band tunnelling is decreased
- using the electrostatic gating effect of the binary emitter Holloway gate electrode,
- as shown in figure B by the graded potential profile. This behaviour leads to, let me say, a
- decrease in the whole induced barrier lowering and leakage current, which are detrimental effects.
- As improvements, we have recorded decrease in leakage current and increase in current ratio,
- as shown in the transfer characteristic of figure C. We have also recorded a
- decrease in subthreshold swing, which is very beneficial by finding the optimal, let me say,
- work function difference of the binary emitter Holloway gate electrode that can lead to the best
- subthreshold swing, and we have obtained some mitigations of short channel effects. The third
- improvement approach is based on the use of split gate configuration, as shown in figure two,
- where the band-to-band tunnelling was significantly mitigated, as shown in the
- current spectrum of figure B. This mitigation of band-to-band tunnelling is attributed to a barrier
- induced by the N-gated region between the two gates, as shown in figure two,
- at the medium of the N-type doped carbon nanotube,
- or graphene nanoribbons, because it's about junctionless transistor.
- This N-gated region or engineered region lowers the bands and opposes the band-to-band tunnelling.
- As a result, we have recorded a decrease in leakage current, as shown in the bottom spectra,
- by comparing the two cases, namely the conventional and the proposed design.
- We have also recorded an increase in current ratio, as shown in figure C,
- by varying the N-gated region and modulating this barrier, while reducing the off-state
- current and increasing the current ratio, which is very beneficial for digital applications.
- We have obtained some improvement in switching speed and switching energy,
- which are the time and energy needed to switch on and off our field-effect transistors.
- The fourth improvement approach is based on
- the use of hybrid gate consisting of metal-ferroelectric-metal structure.
- In this approach, if we apply the gate voltage on the external metal, we get an amplified
- potential at the internal metal by means of the negative, let me say, the ferroelectric induced
- negative capacitance phenomenon. This step-up voltage transformer, as shown in figure A,
- allows to speed up the switching behaviour between off-state and on-state, using,
- let me say, reduced power supply voltage, as shown by VDD prime in comparison to VDD, which is the
- power supply voltage for the MOSFET device or the conventional device. It's worth mentioning that
- the recorded improvements include the most characteristics of graphene nanoribbons and carbon
- nanotube field effect transistors, including the on and off-state current, the current ratio,
- the switching speed, and the switching energy. The subthreshold swing, the scaling capability, and
- the radio frequency and analogue characteristics which is very important. As shown previously,
- the carbon nanotubes and graphene nanoribbons field effect transistors
- exhibit a non-bipolar behaviour. In other words, two switching behaviours in the same
- transfer characteristic, as shown in figure one. The on-state of the left switching is occurred in
- the band-to-band tunnelling regime, as shown in figure D, and the right on-state current,
- let me say, is occurred in the thermal regime, as shown in figure A. Note that the two switching
- behaviours share the same off-state current, which is dominated by the direct-source-to-drain
- tunnelling, as shown in the two figures in the middle. In the band-to-band tunnelling regime,
- the thermionic tunnelling current is ignored. Thus, the subthreshold swing is not limited
- by the Boltzmann limit of 60 millivolts per decade, as shown in figure C and F,
- which is an important and promising feature to exploit. In this context, we have proposed some
- approaches based on electrostatic and chemical doping engineering, as shown previously,
- in order to further boost the performance of the carbon nanotubes and graphene nanoribbons
- field effect transistors, working in the band-to-band tunnelling regime.
- It's worth mentioning that many improvements have been recorded
- in terms of downscaling, high performance, and energy efficiency. The carbon-based
- tunnel field effect transistor is another promising device for future nanoelectronics,
- which is similar to the devices shown previously. It consists of a gate, channel region connecting
- source and drain electrodes, and barrier or oxide or dielectric, separating the gate
- from the channel. The unique difference is the P-type doping of source instead of N-type doping
- in the case of conventional device. The working principle of these devices relies predominantly
- on the band-to-band tunnelling regime, while providing the possibility to reach sub-thermionic
- subthreshold swing, which is very important and beneficial for low-power applications.
- However, the downscaling of these devices degrades their benefits. In this context,
- we have proposed some designs based on electrical and chemical doping,
- in order to improve their immunity against the downscaling approach, or the miniaturisation,
- by engineering the band structures and fix the carrier transport in the channel, while improving
- the device performance. We can see in the three bottom figures that our proposed design can boost
- the performance of the carbon-based tunnel field effect transistors, in terms of downscaling.
- Where we have recorded an increase in on-state current, decrease in off-state
- current, a mitigation in ambipolar behaviour, and sub-thermionic subthreshold swing,
- which are prerequisites for future digital applications.
- In fact, the field-effect transistors can also be used in sensing and biosensing applications.
- In order to do so, one or more of their compounds should be sensitive to specific
- analytes or targets. For example, figure A shows the field effect transistor based biosensor with
- a bare channel as sensing element. As one can see, the direct contact between the
- biomolecule and the bare channel. Figure B shows a field effect transistor-based biosensor with
- lateral sensing cavities, or nano gaps for introducing biomolecules or bio targets.
- Figure C shows a field effect transistor-based biochemical sensor with metal gate as sensing
- elements, and figure D shows the well-known ion-sensitive field
- effect transistors with buffer solution-based gating system. It is to note that the presence
- and/or the quantity variation of these analytes, or other measurements, produce electrical
- modulations of the field effect transistors, such as the drain current, as shown in figure E.
- By tracking these electrical modulations using, let me say, the appropriate readout circuits,
- we can get information on the measurements and their quantities. Note that the measurement
- means the quantity intended to be measured using, with
- site, among others, the DNA gas, pressure, ionic strength, temperature, etc. These two devices
- are two proposed biosensors based on graphene nanoribbon and carbon nanotube field effect
- transistors. Their working principle is based on the concept of dielectric modulation,
- which refers to the variation of, let me say, dielectric constant induced by the biomolecules.
- From simulation point of view, the bio information, or the information of the
- biomolecules, is embedded in the discretised Poisson equation in the concerned area, using the
- finite difference method or finite element method. It's worth mentioning that the dielectric constant
- can be defined as a measure of material's ability
- to store the electrical energy. Let's have a clearer view on this concept.
- Experimentally, in order to create, let me say, sensing cavity between the metal gate
- and the channel, an oxide layer is used as sacrificial layer, as shown in this figure,
- that can be removed using wet etching techniques. In this context, the dielectric constant is unity,
- since it is filled with air. If we introduce the single-stranded DNA molecules or probes
- to search for, let me say, a complementary sequence, or a target,
- the dielectric constant of the nanogap increases.
- Note that these DNA probes are attached to the metal gate using some techniques,
- or self-assembled monolayer techniques, which is validated experimentally in the literature.
- If the DNA hybridisation occurs in the cavity area, the dielectric constant of the nanogap
- increases again, and if this concentration of the double-stranded DNA molecules increases,
- the dielectric constant increases again and again. These biomolecules induced dielectric constant
- modulation affect the potential profile or the potential barrier shown previously, leading to a
- modulation in terms of drain current, or transfer characteristics, including the threshold voltage.
- By tracking these electrical modulations using appropriate, let me say, readout circuits,
- we can get bio information on these biomolecules without labelling processes. This is called,
- ladies and gentlemen, the label-free DNA detection. Another proposal. It involves
- gas nanosensors based on carbon nanotube and graphene nanoribbon field effect transistors.
- The working principle of these devices is based on the work function modulation,
- which, let me say, the work function modulation, which refers to
- the variation of the metal gate work function induced by the gas molecules.
- We can see in figure C, the variation of work function as a function of gas pressure,
- considering two different gases and two different metal gates. According to our simulations, we have
- found that the use of electrostatic engineering and the unique properties of carbon nanomaterials,
- including the band-to-band tunnelling, can boost the performance of these devices,
- including the sensitivity. Another proposal, it involves the dosimeters, or radiation nanosensors,
- based on graphene nanoribbon and carbon nanotube field effect transistors. The
- working principle of these devices is based on the use of interfacial traps,
- implanted intentionally to store the electron-hole pairs generated by radiation.
- These radiation induced rapid charge densities affect the potential profile and electron
- density distribution in the channel, as shown in figure C, leading to a change in drain current,
- including the threshold voltage, as shown in the inset of figure D. By tracking these electrical
- modulations using appropriate readout circuit, we can get information on the radiation dose,
- radiation nature, and the radiation rate. Note that these kinds of dosimeters can
- be used in medical applications, such as radiotherapy, due to their, let me say,
- low sizes, high performance, high sensitivity, and energy efficiency.
- We have also proposed partial microsensors based on graphene field effect transistors.
- As shown in figure one, this device is endowed with a narrow nanogap between the metal gate and
- the top gate oxide, making the metal gate movable. In this context, if we apply a pressure or nano
- force on the metal gate, we get a modulation in terms of the thickness of the air nano gap.
- If we say a modulation in terms of nano gap thickness, we say modulation in terms of top
- coupling capacitance. This leads to modulation in drain current, including the direct point, which
- is defined as the minimum drain current, as shown in figure three in the transfer characteristic. By
- tracking these electrical modulations, we can get information on the applied pressure and forces.
- On the other hand, we have used a genetic algorithm-based approach in conjunction with
- our numerical model, in order to find the electrical, dimensional, and the physical
- parameters of this sensor that can lead to the best performance, including the best sensitivity.
- Another contribution in, let me say, in the computational domain.
- In fact, the non-equilibrium Green's function is known by its high computational cost that prevents
- its broader use. In this context, we have proposed a computationally efficient hybrid approach based
- on the artificial neural network and wavelet transform as shown in the flowchart of this of our
- proposed approaches. The artificial neural network has been used to imitate the non-equilibrium
- Green's function, using, let me say, by training a neural model, using well-created database
- using the conventional simulator, and the wavelet computation has been used
- for two main reasons. The first reason is to compress the issue with matrices
- from the finite difference method or finite element method, making them sparse,
- as shown in figure three, while keeping the most important information only. The second reason
- is the incorporation of the wavelet-based adaptive mission to in the Poisson solver and, as a result,
- we have found that our computationally efficient hybrid approach is fast and accurate. By this,
- I reach the end of my presentation, ladies and gentlemen. As a conclusion, with the
- great advances in nanotechnology, the simulations and technology computer aided design will play a
- significant role for nanoelectronics development, while paving the way to high-performance, advanced
- systems, which will have great reflections on the progress of all fields, including medicine.
- According to our results and those available in the relevant literature, we can say that
- the carbon-based electronic devices are very promising for the post-silicon era.
- As insight regarding the Moore's Law,
- I think that the future of electronics may require the invention of innovative devices
- and new information processing schemes, but that doesn't mean that Moore's Law has to end.
- As perspectives, our future works will focus on performing nanoengineering
- inside the inner environment of the carbon nanotube, while exploiting its
- inner and outer environment in order to boost the performance of carbon nanotube
- field effect transistors in digital sensing and biosensing applications.
- It's worth mentioning that the Royal Society grant will significantly help us at all levels.
- Here, we can see the worldwide reach of our scientific collaborations.
- As accomplishments, I have published with my colleagues and collaborators more than
- 60 papers in international journals and conferences, most of them as first and
- corresponding author. As I have obtained some awards, I would like to thank my teachers who
- taught me. I thank my mentors and supervisors. I thank my collaborators from all over the world.
- I warmly thank my family, my friends, my colleagues, and all the people who have
- contributed, directly or indirectly, to my works. Many thanks to the Ministry of Higher
- Education and Scientific Research of Algeria, and I am deeply indebted to the Royal Society
- for giving me the chance to take part of this awarding event. Thank you for your attention.
- Thank you very much, Khalil. An excellent lecture. Now, we have time for questions,
- and, of course, we also - I should have mentioned - we're live streaming this,
- so we might have questions also from our slightly wider audience. I think [?Shafia
- 00:56:46] will bring those to us if we have any. So, who would like to start off with a question?
- Richard, please, and there's a microphone, so if you just hold your hand up.
- Well, first, thank you for a very interesting, stimulating lecture. I
- was interested in all of it, but particularly fascinated by the applications in sensing.
- I can see that these could be very, very sensitive in sensing. How selective are they?
- I mean, if you're using it for gas sensing, can you really pick out individual molecules?
- Yes, I think you talk about the selectivity. The selectivity is linked to - because we have worked
- on the work function, working principle - and if we choose, let me say, a metal gate for to sense
- gas A, we can use another metal to sense gas B. Let me say, if the most used materials are
- the conducting polymers, we can evaluate the selectivity according to the measurements.
- Okay. Thanks very much.
- Yes, please. I have a question here.
- The first CNT transistors were made some seven years or so ago,
- but I'm unable to locate any commercial production of them as yet. Is there some reason for that?
- In fact, the simulation and technology computer-aided
- design form an important step that paves the way to optimise fabrication and industry.
- Among the main benefits of our work is the visualisation of internal physical processes,
- while allowing to see inside the device on a macro, micro, or nano scale, in order to
- boost the performance of these devices. These processes allow us to project, let me say,
- the promising devices by following the International Technology Roadmap
- for semiconductors, that becomes, in 2016, the International Roadmap for Devices and Systems.
- It's about working today for tomorrow. For example, in the last decade,
- the scientific community worked to satisfy the International Technology Roadmap for
- Semiconductors for the next few years in terms of high performance and downscaling, which are within
- reach in today's technology. The same approach is applied for the next years, and I believe that the
- synergy of emerging nanomaterials, the advanced computational approaches, and innovative designs
- can boost the performance of these devices and can give new impulses to the field.
- Thank you.
- why haven't we seen any commercial production?
- I know why we haven't seen anything coming out of graphene because, of course, graphene has proved
- impossible to make, actually, in a form where you get the high mobility in a material which is
- capable of making devices, but carbon nanotubes shouldn't necessarily face the same objection.
- So it's not clear why there hasn't been a company which has been, essentially,
- working on your kind of simulations and producing devices, which have actual application. Is
- there some reason why that is not happening?
- graphene nanoribbon, which is very suitable for transistor applications. We did some simulations
- by following the International Technology Roadmap for semiconductors, in order to propose
- some devices that can be very beneficial for digital applications in the future.
- I think the same point is still valid, that you can make graphene ribbons, but the mobility is
- nothing like the mobility that you get in single crystals, because, as in ordinary semiconductors,
- you're just inhibited by the grain boundaries.
- In fact, the graphene nanoribbon, the most, let me say, challenge to produce graphene nanoribbon
- is its dimer number, or, in the case of carbon nanotube, the chirality, but I think with the
- great advances in nanotechnology processes, these proposed designs are within reach.
- Other questions?
- Hi. Thanks very much. I very much enjoyed that. I guess the first nanotube transistor
- was Cees Dekker in 1998, so we've had a bit of a longer wait than that. I think there has
- been a lot of work from IBM, for example, on devices, and maybe one of the issues is around
- contacts. You didn't say very much about how we should make the source and drain contact,
- and I wondered if you're assuming that ohmic contact, and if so, what kind of materials you
- think might be suitable for the contacts?
- in our simulations, we have assumed that the ideal contacts
- we have used the graphene nanoribbon as channel material, and for sake of simplicity,
- we have considered ideal contacts.
- Ohmic contacts, yes.
- right. If not, let's thank the speaker.
- Then, I have the pleasurable task of presenting Khalil with his medal and his scroll, so he's
- going to come out here so the photographer can get a chance to take a picture! I shall
- do a juggling act, holding all these things at the same time. Well, let's do the handshaking first
- of all. Do it that way.
- so much.
- Thank you very much. Thank you.
- [Inaudible 01:04:45] Sorry. Thank you. One more handshake, if that's
- okay.
- Well done.
- Thank you very much.
- Okay, so that's the official business over. I think some people are having drinks at the back,
- but for those of you who aren't, thank you very much for coming. I
- hope we'll see you again at a future lecture presentation.
The Rising Star Africa Prize 2022 lecture given by Dr Khalil Tamersit.
Boosting the performance of electronic instruments and systems through the continual miniaturization of electronic devices was an approach that has been followed by Semiconductor Industry for more than five decades, and has led to the significant electronics development that we are witnessing today. The basic material that has accompanied this spectacular evolution is Silicon. Now, there are billions of silicon transistors in the current microprocessors, whereas there were only thousands in the 70s. However, silicon-based nanotransistors are reaching their limits in terms of miniaturization and energy efficiency. Thus, different nanodevices based on emerging nanomaterials are urgently needed to continue in this evolutionary approach while extending the lifetime of Moore's Law.
Dr. Khalil Tamersit will describe how the simulation approaches and computational models are used to propose and optimize new carbon-based electronic devices (eg nanotransistors, nanosensors, nanobiosensors, and micromachines) while paving the way for a post-silicon future.
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