Imperial College London

DrKristelFobelets

Faculty of EngineeringDepartment of Electrical and Electronic Engineering

Reader in Microelectronics Engineering
 
 
 
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Contact

 

+44 (0)20 7594 6236k.fobelets Website CV

 
 
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Assistant

 

Ms Susan Brace +44 (0)20 7594 6215

 
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Location

 

714Electrical EngineeringSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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167 results found

Kang MJ, Kim MS, Jang SH, Fobelets Ket al., 2021, Internal Thermoelectric Cooling in Nanosheet Gate-All-Around FETs Using Schottky Drain Contacts, IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol: 68, Pages: 4156-4160, ISSN: 0018-9383

Journal article

Minin IV, Minin OV, Salvador-Sanchez J, Delgado-Notario JA, Calvo-Gallego J, Ferrando-Bataller M, Fobelets K, Velazquez-Perez JE, Meziani YMet al., 2021, Responsivity enhancement of a strained silicon field-effect transistor detector at 0.3 THz using the terajet effect, OPTICS LETTERS, Vol: 46, Pages: 3061-3064, ISSN: 0146-9592

Journal article

Panteli C, Georgiou P, Fobelets K, 2021, Reduced Drift of CMOS ISFET pH Sensors Using Graphene Sheets, IEEE SENSORS JOURNAL, Vol: 21, Pages: 14609-14618, ISSN: 1530-437X

Journal article

Calvo-Gallego J, Delgado-Notario JA, Velazquez-Perez JE, Ferrando-Bataller M, Fobelets K, El Moussaouy A, Meziani YMet al., 2021, Numerical Study of the Coupling of Sub-Terahertz Radiation to n-Channel Strained-Silicon MODFETs, SENSORS, Vol: 21

Journal article

Delgado-Notario JA, Calvo-Gallego J, Velazquez-Perez JE, Ferrando-Bataller M, Fobelets K, Meziani YMet al., 2020, Effect of the Front and Back Illumination on Sub-Terahertz Detection Using n-Channel Strained-Silicon MODFETs, APPLIED SCIENCES-BASEL, Vol: 10

Journal article

Fobelets K, 2020, Knitted coils as breathing sensors, Sensors and Actuators A: Physical, Vol: 306, Pages: 1-5, ISSN: 0924-4247

A new implementation of a wearable respiratory inductive plethysmography garment is obtained by knitting a 250 μm thin insulted Cu wire simultaneously with yarn in the round. This was used to integrate a knitted coil in the body of a baby romper suit. During simulated breathing the diameter of knitted coil changes by stretching the knit circularly, causing a variation of the self-inductance of the coil. Coils with 5 rows of integrated metal wire with different stitch types and patterns were investigated to determine their influence on inductance, series resistance and sensitivity. We observed that knit styles that reduce the resistance of the coil, such as lace and jacquard also reduce the inductance and flexibility of the garment. Jacquard with three colours and one metal wire for each colour, gave the highest coil quality factor but also the poorest flexibility. We found that 1/1 rib stitch has the highest self-inductance for all yarn types. Its sensitivity of 0.5 – 0.6 μH/cm is similar to stockinette stitch except when elastic viscose yarn is used. Coils in stockinette stitch and elastic viscose yarn have the highest sensitivity of 0.84 μH/cm. No hysteresis in self-inductance was observed for circumference variations between 44 and 53 cm of the body of the baby romper in 1/1 rib stitch due to the elasticity of knitted garments.

Journal article

Kang MJ, Myeong I, Fobelets K, 2020, Geometrical influence on Self Heating in Nanowire and Nanosheet FETs using TCAD Simulations, IEEE Electron Devices Technology and Manufacturing Conference (EDTM), Publisher: IEEE

Conference paper

Li CB, Fobelets K, Tymieniecki MS, Hamayun M, Durrani ZAK, Green Met al., 2019, Bunch-free Electroless-etched Si Nanowire Array, 218th ECS Meeting

Conference paper

Fobelets K, Thielemans K, Mathivanan A, Papavassiliou Cet al., 2019, Characterization of nnitted coils for e-textiles, IEEE Sensors Journal, Vol: 19, Pages: 7835-7840, ISSN: 1530-437X

Inductor coils are integrated in many wearable garments for EM wave screening, heating and health monitoring. This paper presents a critical evaluation of the inductor characteristics of circular weft knitted coils for applications in e-textiles. Inductors are knitted using circular needles with thin insulated metal wire and yarn knitted together. The resulting helical coils are characterized as a function of number of turns, coil diameter, needle size, and insulated metal wire material. The results are compared to wound coils. Simulations of the knitted and wound coils show close agreement with the experimental results and confirm a higher inductance for the knits compared to the wound coils with the same pitch between turns. The parasitic coil capacitance is higher in the knit due to the vertical legs of the stitches, absent in wound coils. Knits with thin Cu and Litz wires result in flexible and wearable textile coils.

Journal article

Moser N, Panteli C, Fobelets K, Georgiou Pet al., 2019, Mechanisms for enhancement of sensing performance in CMOS ISFET arrays using reactive ion etching, Sensors and Actuators B: Chemical, Vol: 292, Pages: 297-307, ISSN: 0925-4005

In this work, we investigate the impact of successively removing the passivation layers of ISFET sensors implemented in a standard CMOS process to improve sensing performance. Reactive ion etching is used as a post-processing technique of the CMOS chips for uniform and accurate etching. The removal of the passivation layers addresses common issues with commercial implementation of ISFET sensors, including pH sensitivity, capacitive attenuation, trapped charge, drift and noise. The process for removing the three standard layers (polyimide, Si3N4 and SiO2) is tailored to minimise the surface roughness of the sensing layer throughout an array of more than 4000 ISFET sensors. By careful calibration of the plasma recipe we perform material-wise etch steps at the top and middle of the nitride layer and top of the oxide layer. The characterisation of the ISFET array proves that the location of the trapped charge in the passivation layers is mainly at the interface of the layers. Etching to the top of the oxide layer is shown to induce an improvement of 80% in the offset range throughout the array and an increase in SNR of almost 40 dB compared to the non-processed configuration. The performance enhancement demonstrates the benefit of a controlled industry-standard etch process on CMOS ISFET array system-on-chips.

Journal article

Marchese M, Angeli L, Ilieva M, Huang Y, van de Ven A, Strangers AP, Palm B, Fobelets Ket al., 2019, C-EXTENDED: EXTENDING THE ERASMUS EXPERIENCE BEYOND MOBILITY, 11th International Conference on Education and New Learning Technologies (EDULEARN), Publisher: IATED-INT ASSOC TECHNOLOGY EDUCATION & DEVELOPMENT, Pages: 4053-4061, ISSN: 2340-1117

Conference paper

Fobelets K, Panteli C, Sydoruk O, Li Cet al., 2018, Ammonia sensing using arrays of silicon nanowires and graphene, Journal of Semiconductors, Vol: 39, ISSN: 1674-4926

Ammonia (NH3) is a toxic gas released in different industrial, agricultural and natural processes. It is also a biomarker for some diseases. These require NH3 sensors for health and safety reasons. To boost the sensitivity of solid-state sensors, the effective sensing area should be increased. Two methods are explored and compared using an evaporating pool of 0.5 mL NH4OH (28% NH3). In the first method an array of Si nanowires (Si NWA) is obtained via metal-assisted-electrochemical etching to increase the effective surface area. In the second method CVD graphene is suspended on top of the Si nanowires to act as a sensing layer. Both the effective surface area as well as the density of surface traps influences the amplitude of the response. The effective surface area of Si NWAs is 100 × larger than that of suspended graphene for the same top surface area, leading to a larger response in amplitude by a factor of ~7 notwithstanding a higher trap density in suspended graphene. The use of Si NWAs increases the response rate for both Si NWAs as well as the suspended graphene due to more effective NH3 diffusion processes.

Journal article

Panteli C, Georgiou P, Fobelets K, 2018, Performance improvement of commercial ISFET sensors using reactive ion etching, Microelectronic Engineering, Vol: 192, Pages: 61-65, ISSN: 0167-9317

Reactive Ion Etching (RIE) is used to improve the performance of commercial Complementary Metal Oxide Semiconductor (CMOS) Ion-Sensitive Field-Effect Transistors (ISFETs) by thinning the top passivation layers inherent of the CMOS fabrication process. Using a combination of O2 and SF6 in 50% ratio, both polyimide and Si3N4 layers are etched in one etching step. Etching for different times we find the right remaining layer thickness for best ISFET performance to be ∼1 μm of SiO2. The results show an increase in pH sensitivity of 125%, a 5700% increase in passivation capacitance and a 96% reduction in capacitive attenuation. The RIE etch recipe can be used on multi-project wafers (MPW) to boost CMOS sensor performance.

Journal article

Delgado-Notario JA, Javadi E, Calvo-Gallego J, Diez E, Meziani YM, Velazquez-Perez JE, Fobelets Ket al., 2018, Sub-THz Response of Strained-Silicon MODFETs, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, Vol: 215, ISSN: 1862-6300

Journal article

Delgado-Notario JA, Velazquez-Perez JE, Meziani YM, Fobelets Ket al., 2018, Sub-THz imaging using non-resonant HEMT detectors, Sensors, Vol: 18, ISSN: 1424-2818

Plasma waves in gated 2-D systems can be used to efficiently detect THz electromagnetic radiation. Solid-state plasma wave-based sensors can be used as detectors in THz imaging systems. An experimental study of the sub-THz response of II-gate strained-Si Schottky-gated MODFETs (Modulation-doped Field-Effect Transistor) was performed. The response of the strained-Si MODFET has been characterized at two frequencies: 150 and 300 GHz: The DC drain-to-source voltage transducing the THz radiation (photovoltaic mode) of 250-nm gate length transistors exhibited a non-resonant response that agrees with theoretical models and physics-based simulations of the electrical response of the transistor. When imposing a weak source-to-drain current of 5 μA, a substantial increase of the photoresponse was found. This increase is translated into an enhancement of the responsivity by one order of magnitude as compared to the photovoltaic mode, while the NEP (Noise Equivalent Power) is reduced in the subthreshold region. Strained-Si MODFETs demonstrated an excellent performance as detectors in THz imaging.

Journal article

shougee A, Konstantinou F, Albrecht T, Fobelets Ket al., 2018, Cyclic voltammetry peaks due to deep level traps in Si nanowire array electroes, IEEE Transactions on Nanotechnology, Vol: 17, Pages: 154-160, ISSN: 1536-125X

When metal-assisted chemical etching (MACE) is used to increase the effective surface area of Si electrodes for electrochemical capacitors, it is often found that the cyclic voltammetry characteristics contain anodic and cathodic peaks. We link these peaks to the charging-discharging dynamics of deep level traps within the nanowire system. The trap levels are associated with the use of Ag in the MACE process that can leave minute amounts of Ag residue within the nanowire system to interact with the H2O layer surrounding the nanowires in a room temperature ionic liquid. The influence of the traps can be removed by shifting the Fermi level away from the trap levels via spin-on doping. These results in lower capacitance values but improved charge-discharge cycling behavior. Low-frequency noise measurements proof the presence or absence of these deep level traps.

Journal article

Delgado Notario JA, Javadi E, Velazquez JE, Diez E, Meziani YM, Fobelets Ket al., 2017, Detection of terahertz radiation using submicron field effect transistors and their use for inspection applications, Conference on Millimetre Wave and Terahertz Sensors and Technology X, Publisher: SPIE-INT SOC OPTICAL ENGINEERING, ISSN: 0277-786X

Conference paper

Hamid A, Fobelets K, Enrique Velazquez-Perez J, 2017, Thermo-Electric Power Generators using Gated Silicon Nanowires, 11th UKSim-AMSS European Modelling Symposium on Computer Modelling and Simulation (EMS), Publisher: IEEE, Pages: 168-173, ISSN: 2473-3539

Conference paper

Konstantinou F, Shougee A, Albrecht T, Fobelets Ket al., 2017, TiO2 coated Si nanowire electrodes for electrochemical double layer capacitors in room temperature ionic liquid, Journal of Physics D: Applied Physics, Vol: 50, ISSN: 0022-3727

Three TiO2 deposition processes are used to coat the surface of Si nanowire array electrodes for electrochemical double layer capacitors in the room temperature ionic liquid [Bmim][NTF2]. The fabrication processes are based on wet chemistry only and temperature treatments are kept below 450°C. Successful TiO2 coatings are found to be those that are carried out at low pressure and with low TiO2 coverage to avoid nanowires breakage. The best TiO2 coated Si nanowire array electrode in [Bmim][NTF2] showed energy densities of 0.9 Whcenterdotkg-1 and power densities of 2.2 kWcenterdotkg-1 with a nanowire length of ~10 μm.

Journal article

Moser N, Panteli C, Ma D, Toumazou C, Fobelets K, Georgiou Pet al., 2017, Improving the pH Sensitivity of ISFET Arrays withReactive Ion Etching, BioCAS 2017, Publisher: IEEE

In this paper, we report a method to improvesensitivity for CMOS ISFET arrays using Reactive Ion Etching(RIE) as a post-processing technique. The process etches awaythe passivation layers of the commercial CMOS process, using anoxygen (O2) and sulfur hexafluoride (SF6) plasma. The resultingattenuation and pH sensitivity are characterised for five diesetched for 0 to 15 minutes, and we demonstrate that capacitiveattenuation is reduced by 196% and pH sensitivity increasedby 260% compared to the non-etched equivalent. The spread oftrapped charge is also reduced which relaxes requirements on theanalogue front-end. The technique significantly improves the performanceof the fully-integrated sensing system for applicationssuch as DNA detection.

Conference paper

Delgado Notario JA, Javadi E, Clerico V, Fobelets K, Otsuji T, Diez E, Velazquez-Perez JE, Meziani YMet al., 2017, Experimental and theoretical studies of Sub-THz detection using strained-Si FETs, 20th International Conference on Electron Dynamics in Semiconductors, Optoelectronics and Nanostructures (EDISON), Publisher: IOP PUBLISHING LTD, ISSN: 1742-6588

Conference paper

Panteli C, Georgiou P, Fobelets K, 2017, Optimising the performance of commercial ISFET sensors using Reactive Ion Etching, MNE 2017

Conference paper

Panteli C, Fobelets K, Sydoruk O, 2017, Graphene Suspended on Silicon Nanowire Arrays for Enhanced Gas Sensing, 231st ECS Meeting, ISSN: 2151-2043

Conference paper

Delgado Notario JA, Javadi E, Calvo-Gallego J, Diez E, Velazquez JE, Meziani YM, Fobelets Ket al., 2017, Sub-Micron Gate Length Field Effect Transistors as Broad Band Detectors of Terahertz Radiation, 5th Russia-Japan-USA-Europe Symposium on Fundamental and Applied Problems of Terahertz Devices and Technologies (RJUSE-TeraTech), Publisher: WORLD SCIENTIFIC PUBL CO PTE LTD, Pages: 87-95, ISSN: 1793-1274

Conference paper

Panteli C, Liu D, Sydoruk O, Fobelets Ket al., 2016, Through graphene etching of porous Si by electroless metal assisted chemical etching, MNE

Conference paper

Qiao L, Shougee A, Albrecht T, Fobelets Ket al., 2016, Oxide-coated silicon nanowire array capacitor electrodes in room temperature ionic liquid, Electrochimica Acta, Vol: 210, Pages: 32-37, ISSN: 0013-4686

Improved performance of Si nanowire arrays for capacitor electrodes in ionic liquid [Bmim][NTf2], is obtained by spin-on-doping the nanowires followed by hot, concentrated nitric acid oxidation. n- and p-type Si nanowire arrays are fabricated via a 2-step metal-assisted chemical etch process to increase the effective surface area. Spin-on-doping increases the doping density of the nanowires, enhancing the current by a factor of more than 3. The well-controlled HNO3 oxidation defines a thin, dense oxide layer on the Si nanowires increasing chemical stability, both expanding the electrochemical window and increasing the current further by a factor >2. Specific capacitances of 238 μF cm−2 (∼0.4 F g−1, 159 mF cm−3) and 404 μF cm−2 (∼0.7 F g−1, 269 mF cm−3) are obtained for n- and p-type Si nanowire arrays, respectively.

Journal article

shougee A, Konstantinou F, Albrecht T, Fobelets Ket al., 2016, Silicon nanowire arrays for high capacitance electrodes in room temperature ionic liquid electrolyte, 229th ECS Meeting

Conference paper

Shougee A, Konstantinou F, Albrecht T, Fobelets Ket al., 2016, Silicon nanowire arrays for high capacitance electrodes in room temperature ionic liquid electrolyte, 229th ECS Meeting

Conference paper

Fobelets K, 2015, Conductivity and 1/f noise in Si nanowire arrays, E-MRS spring meeting

Conference paper

Rahman T, Fobelets K, 2015, Efficient tool flow for 3D photovoltaic modelling, Computer Physics Communications, Vol: 193, Pages: 124-130, ISSN: 1879-2944

Performance predictions and optimisation strategies in current nanotechnology-based photovoltaic (PV) require simulation tools that can efficiently and accurately compute optical and electrical performance parameters of intricate 3D geometrical structures. Due to the complexity of each type of simulation it is often the case that a single package excels in either optical or electrical modelling, and the other remains a bottleneck. In this work, an efficient tool flow is described in order to combine the highly effective optical simulator Lumerical with the excellent fabrication and electrical simulation capability of Sentaurus. Interfacing between the two packages is achieved through tool command language and Matlab, offering a fast and accurate electro-optical characteristics of nano-structured PV devices.

Journal article

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