Papers by Nonofo Ditshego
Journal of Nano Research, Nov 1, 2018
A highly sensitive low-doped ZnO nanowire field effect transistor (NWFET) biosensor has been fabr... more A highly sensitive low-doped ZnO nanowire field effect transistor (NWFET) biosensor has been fabricated and measured. The low doped biosensor with NWFET transducer was used to sense charge of the following substances: lysozyme (LYSO), phosphate buffered saline (PBS), bovine serum albumin (BSA). It achieved maximum sensitivity of -543.2 % for the PBS-LYSO protein and 13,069 % for the PBS-BSA protein. These results were achieved because the electrical measurement and characterisation was focused on the charge effect of the LYSO and BSA acting on the ZnO nanowire subthreshold region. The nano-fabrication process is stable and reproducible. The high sensitivity of the ZnO NWFET biosensor can be exploited for selective analyte detection by functionalizing the nanowire surface with antibodies and/or other biomolecular probe molecules.
Microelectronic Engineering, Jun 1, 2016
This paper describes the optimisation of top-down fabrication process of the ZnO-based dual nanow... more This paper describes the optimisation of top-down fabrication process of the ZnO-based dual nanowire field effect transistors (NWFETs) based on the spacer method. The approach uses the top-down nanowire process with reduced sidewall roughness during pattern transfer to improve the electrical characteristics. The main feature of the process involves a reflow of the photoresist performed at a temperature of 130°C and dry oxidation of the etched silicon sidewalls. The process optimisation leads to a significant reduction of the root-mean-square (rms) roughness of the photoresist from 23.2nm to 3.6nm and the ZnO nanowire rms surface roughness from 11.2nm to 5.5nm. The ZnO-based NWFET fabricated with the resist reflow process operates in depletion mode with a threshold voltage of -6V, a subthreshold slope of 0.80V/decade, an on-off current ratio of 106, a transconductance of 5.9nS and field effect mobility of 7.7cm2/Vs. Schematic of the fabrication process for (a) non-reflow resist spacer method and (b) reflow resist spacer method.Display Omitted The optimisation of top-down fabrication of ZnO nanowire field effect transistor by sidewall smoothing is proposed.Reflow of photoresist after developed smoothed off the pattern's sidewall.Top-down reflow resist spacer method produced Zno nanowire with reduce surface roughness.Smooth nanowire has good output electrical characteristics.
Microelectronic Engineering, Mar 1, 2016
In this work, we demonstrate a wafer-level zinc oxide (ZnO) nanowire fabrication process using io... more In this work, we demonstrate a wafer-level zinc oxide (ZnO) nanowire fabrication process using ion beam etching and a spacer etch technique. The proposed process can accurately define nanowires without an advanced photolithography and provide a high yield over a 6-inch wafer. The fabricated nanowires are 36 nm wide and 86 nm thick and present excellent transistor characteristics. The pH sensitivity using a liquid gate was found to be 46.5 mV/pH, while the pH sensitivity using a bottom gate showed a sensitivity of 366 mV/pH, which is attributed to the capacitance coupling between the top-and bottom-gates. The maximum process temperature used in the fabrication of the nanowire sensors is optimized to be 200°C (after wet oxidation) which makes it applicable to low-cost substrates such as glass and plastic. The Ion Beam Etching (IBE) process in this work is shown to be highly transferable and can therefore be directly used to form nanowires of different materials, such as polysilicon and molybdenum disulfide, by only an adjustment of the etch time.
Top-down method was used oxide (ZnO) nanowire field effect tran biosensor. The nanosensor was use... more Top-down method was used oxide (ZnO) nanowire field effect tran biosensor. The nanosensor was used to meas characteristics of lysozyme (LYSO) and bovin (BSA) protein solutions in phosphate buffe The LYSO and BSA proteins are oppos measurement pH of 7.4. Subthreshold voltag and 700 mV due to surface charge effect on t is obtained for the LYSO and BSA solution NWFET sensitivity of 72 % is achieved for th while the BSA proteins resulted in a sensitivity
Nanotechnology, Mar 8, 2016
We demonstrate the advantages of dual-gate polysilicon nanoribbon biosensors with a comprehensive... more We demonstrate the advantages of dual-gate polysilicon nanoribbon biosensors with a comprehensive evaluation of different measurement schemes for pH and protein sensing. In particular, we compare the detection of voltage and current changes when top-and bottom-gate bias is applied. Measurements of pH show that a large voltage shift of 491 mV/pH is obtained in the subthreshold region when the top-gate is kept at a fixed potential and the bottomgate is varied (voltage sweep). This is an improvement of 16 times over the 30 mV/pH measured using a top-gate sweep with the bottom-gate at a fixed potential. A similar large voltage shift of 175 mV is obtained when the protein avidin is sensed using a bottom-gate sweep. This is an improvement of 20 times compared with the 8.8 mV achieved from a topgate sweep. Current measurements using bottom-gate sweeps do not deliver the same signal amplification as when using bottom-gate sweeps to measure voltage shifts. Thus, for detecting a small signal change on protein binding, it is advantageous to employ a double-gate transistor and to measure a voltage shift using a bottom-gate sweep. For top-gate sweeps, the use of a dual-gate transistor enables the current sensitivity to be enhanced by applying a negative bias to the bottom-gate to reduce the carrier concentration in the nanoribbon. For pH measurements, the current sensitivity increases from 65% to 149% and for avidin sensing it increases from 1.4% to 2.5%.
Journal of Nano Research, Jun 29, 2021
Indium gallium zinc oxide fin-field effect transistor (IGZO FinFET) characteristics are investiga... more Indium gallium zinc oxide fin-field effect transistor (IGZO FinFET) characteristics are investigated and then compared with Zinc oxide fin-field effect transistor (ZnO FinFET) and the Silicon fin-field effect transistor (Si FinFET). This was done using 3D simulation. The threshold voltage for Si, ZnO, and IGZO is 0.75 V, 0.30 V and 0.05 V respectively. The silicon device has the highest transconductance (5.0 x 10-7 S) and performs better than the other devices because it has less fixed charge defects. IGZO has the second-best value of Gm (3.6 x 10-7 S), ZnO has the least value of Gm (3.4 x 10-7 S). Si device has the least drain current (IDS) value of 2.0 x 10-7 A, ZnO device has a better IDS value of 6.2 x 10-6 A while IGZO device has the best IDS value of 1.6 x 10-5 A. IGZO is better than Si by two (2) order magnitude. The field effect mobility is 50.0 cm2/Vs for all three devices.
International journal of electrical and electronic engineering and telecommunications, 2022
An ion sensitive field effect transistor can outperform conventional ion-selective electrodes. Th... more An ion sensitive field effect transistor can outperform conventional ion-selective electrodes. Thus, a zinc oxide (ZnO) nanowire field effect transistor (NWFET) pH sensor was fabricated and measured. The sensor contained a channel with 1.710 18 cm -3 donor concentration and 100 ZnO nanowires in parallel, each with the following dimensions: 10 µm120 nm20 nm. The active channel is passivated with an 18 nm Al2O3 layer. The device was measured under a controlled environment with and without pH solutions. The pH range was 3-9 with a sensitivity of 2.48 mV to 10.3 mV. The voltage sensitivity translates to a percentage value of 15%. The measurements obtained before and after the pH solution treatment demonstrate the possibility of re-use of the device by rinsing and brushing the sensing layer.
Advanced engineering forum, Nov 16, 2021
This present work is about simulating and analysing a Vertical Cavity Surface Emitting Laser (VCS... more This present work is about simulating and analysing a Vertical Cavity Surface Emitting Laser (VCSEL) structure used in optical fibre communication systems. In this paper a VCSEL structure made of seven Quantum Wells of Indium Gallium Arsenide Phosphide (InGaAsP) emitting at 1550 nm is simulated. The device is analysed looking at the following characteristics: Direct current current and voltage (IV) characteristics, light power against electrical bias, optical gain against electrical bias, light distribution over the structure, output power and threshold current. Specification of material characteristics, ordinary physical models settings, initial VCSEL biasing, mesh declarations, declaration of laser physical models, their optical and electrical parameters were defined using Atlas syntax. Mirror ratings and quantum wells are the two main parameters that were studied and analysed to come up with structure trends. By determining important device parameters such as proper selection of the emission wavelength and choice of material; a VCSEL with an output power of 9.5 mW was simulated and compared with other structures.
Microelectronic Engineering, Sep 1, 2015
Nanotechnology, Jun 3, 2016
In this work, we investigate how the sensitivity of a nanowire or nanoribbon sensor is influenced... more In this work, we investigate how the sensitivity of a nanowire or nanoribbon sensor is influenced by the subthreshold slope of the sensing transistor. Polysilicon nanoribbon sensors are fabricated with a wide range of subthreshold slopes and the sensitivity is characterized using pH measurements. It is shown that there is a strong relationship between the sensitivity and the device subthreshold slope. The sensitivity is characterized using the current sensitivity per pH, which is shown to increase from 1.2%/pH to 33.6%/pH as the subthreshold slope improves from 6.2 V/dec to 0.23 V/dec respectively. We propose a model that relates current sensitivity per pH to the subthreshold slope of the sensing transistor. The model shows that sensitivity is determined only on the subthreshold slope of the sensing transistor and the choice of gate insulator. The model fully explains the values of current sensitivity per pH for the broad range of subthreshold slopes obtained in our fabricated nanoribbon devices. It is also able to explain values of sensitivity reported in the literature, which range from 2.5%/pH to 650%/pH for a variety of nanoribbon and nanowire sensors. Furthermore, it shows that aggressive device scaling is not the key to high sensitivity. For the first time, a figure-of-merit is proposed to compare the performance of nanoscale field effect transistor sensors fabricated using different materials and technologies.
Journal of Nano Research, Apr 1, 2019
ZnO NWFETs were fabricated with and without Al2O3 passivation. This was done by developing a new ... more ZnO NWFETs were fabricated with and without Al2O3 passivation. This was done by developing a new recipe for depositing the thin film of ZnO. By using a high donor concentration of 1.7 x 10 18 cm -3 for the thin film, contact resistance values were lowered (passivated device had Rcon = 2.5 x 10 4 Ω; unpassivated device had Rcon = 3.0 x 10 5 Ω). By depositing Zn first instead of O2, steep subthreshold slopes were obtained. The passivated device had a subthreshold slope of 225 mV/decade and the unpassivated device had a slope of 125 mV/decade. Well-behaved electrical characteristics have been obtained and the passivated device shows field effect mobility of 10.9 cm 2 /Vs and the un-passivated device shows a value of 31.4 cm 2 /Vs. To verify the results, 3D simulation was also carried out which shows that the obtained values of sub-threshold slope translate into interface state number densities of -1.86 x 10 13 cm -2 for the unpassivated device and 3.35 x 10 14 cm -2 for the passivated device. The passivated device is suitable for biosensing applications.
IntechOpen eBooks, Jul 14, 2021
The last 19 years have seen intense research made on zinc oxide (ZnO) material, mainly due to the... more The last 19 years have seen intense research made on zinc oxide (ZnO) material, mainly due to the ability of converting the natural n-type material into p-type. For a long time, the p-type state was impossible to attain and maintain. This chapter focuses on ways of improving the doped ZnO material which acts as a channel for nanowire field-effect transistor (NWFET) and biosensor. The biosensor has specific binding which is called functionalization that is achieved by attaching a variety of compounds on the designated sensing area. Reference electrodes and buffers are used as controllers. Top-down fabrication processes are preferred over bottom-up because they pave way for mass production. Different growth techniques are reviewed and discussed. Strengths and weaknesses of the FET and sensor are also reviewed.
Research & Innovation Symposium 2019, Jun 1, 2019
Indium gallium zinc oxide thin film transistor (IGZO TFT) characteristics are investigated, impro... more Indium gallium zinc oxide thin film transistor (IGZO TFT) characteristics are investigated, improved and then compared with the standard metal-oxide semiconductor field-effect transistor (MOSFET). The device tends to operate with a negative threshold voltage which is undesirable as it means the device is 'ON' at 0.0 V. For the device to be an effective CPU switch, it needs to operate with voltage values between 0.0 V and 5.0 V where the lower value means it is completely OFF with no leakage currents. Negative fixed charge was introduced to help turn the device OFF whereas the MOSFET had negligible values. The TFT was driven at a drain voltage of 1.0 V whereas the MOSFET was at 0.1 V. This made the two devices comparable with similar threshold voltage of 0.6 V and an ideal subthreshold swing of 60 mV/decade. The experiment shows that the subthreshold voltage for the IGZO TFT is excellent at lower drain currents but degrades at higher currents. The quick degradation observed on the subthreshold region can also be attributed to short channel effects (SCEs). Keywords: Indium gallium zinc oxide thin film transistor (IGZO TFT), metal oxide semiconductor field effect transistor (MOSFET), scaling, short channel effects (SCE)
<jats:p>Germanium (Ge) is envisioned as a suitable channel candidate for field-effect trans... more <jats:p>Germanium (Ge) is envisioned as a suitable channel candidate for field-effect transistors (FET). Properties of Ge such as high carrier mobility, compatibility with Si and adaptability with high-k materials makes it comparable to silicon. This paper presents a detailed design of a 30 nm Ge based FinFET by parameter optimization using Silvaco software. Poisson and Schrodinger equation is used to come up with an analytical quantum model. The quantum model is developed based on theory of a double gate (DG) FET but the final design is a trigate (TG) device since they are more scalable. The quantum attributes of DG MOSFET are acquired by adopting the coupled Poisson–Schrodinger equation with the aid of the variational approach. The ratio of channel length (<jats:italic>L</jats:italic><jats:italic><jats:sub>C</jats:sub></jats:italic>) to fin height (<jats:italic>H</jats:italic><jats:italic><jats:sub>fin</jats:sub></jats:italic>) to fin thickness (<jats:italic>t</jats:italic><jats:italic><jats:sub>fin</jats:sub></jats:italic>) is 4:2:1. The channel length is taken as the gate length (<jats:italic>L</jats:italic><jats:italic><jats:sub>G</jats:sub></jats:italic>) although they are slightly differ mathematically due to side diffusion of the implanted ions. Simulation results show that physical parameters such as dimensions influence electrical characteristics of the device such as threshold voltage (<jats:italic>V</jats:italic><jats:italic><jats:sub>TH</jats:sub></jats:italic>). Much focus is on optimization of the on/off current ratio (<jats:italic>I</jats:italic><jats:italic><jats:sub>ON</jats:sub></jats:italic>/<jats:italic><jats:sub>OFF</jats:sub>)</jats:italic> and <jats:italic>V</jats:italic><jats:italic><jats:sub>TH</jats:sub></jats:italic> performances. <jats:italic>I</jats:italic><jats:italic><jats:sub>ON</jats:sub></jats:italic>/<jats:italic><jats:sub>OFF</jats:sub></jats:italic><jats:italic>≈ </jats:italic>10<jats:sup>6</jats:sup> is achieved at carrier concentration in the range 1 <jats:italic>× </jats:italic>10<jats:sup>18</jats:sup><jats:italic>≤ </jats:italic><jats:italic>n</jats:italic><jats:italic><jats:sub>d</jats:sub></jats:italic><jats:italic>≤ </jats:italic>1<jats:italic>.</jats:italic>22 <jats:italic>× </jats:italic>10<jats:sup>18</jats:sup> and in this scenario, <jats:italic>V</jats:italic><jats:italic><jats:sub>TH</jats:sub></jats:italic> = 0<jats:italic>.</jats:italic>4V . Systematical investigation is presented using IV characteristics to demonstrate the sensitivity or how critical design parameters of Ge FinFET are to the device's figure of merits. Device performs well at low voltages but breaks down at higher drain voltages (<jats:italic>V</jats:italic><jats:italic><jats:sub>DS</jats:sub></jats:italic><jats:italic>≥ </jats:italic>4V). Gate source voltages (<jats:italic>V</jats:italic><jats:italic><jats:sub>GS</jats:sub></jats:italic>) range between 0<jats:italic>.</jats:italic>05V<jats:italic>≤ </jats:italic><jats:italic>V</jats:italic><jats:italic><jats:sub>GS </jats:sub></jats:italic><jats:italic>≤ </jats:italic>1V and conductance is dependent on it. Effects of DIBL, which is around 0.031, and velocity saturation are studied to determine how they can be suppressed during the design process.</jats:p>
In this work, we investigate how the sensitivity of a nanowire or nanoribbon sensor is influenced... more In this work, we investigate how the sensitivity of a nanowire or nanoribbon sensor is influenced by the subthreshold slope of the sensing transistor. Polysilicon nanoribbon sensors are fabricated with a wide range of subthreshold slopes and the sensitivity is characterized using pH measurements. It is shown that there is a strong relationship between the sensitivity and the device subthreshold slope. The sensitivity is characterized using the current sensitivity per pH, which is shown to increase from 1.2% to 33.6% as the subthreshold slope improves from 6.2 V/dec to 0.23 V/dec respectively. We propose a model that relates current sensitivity per pH to the subthreshold slope of the sensing transistor. The model shows that sensitivity is determined only on the sub-threshold slope of the sensing transistor and the choice of gate insulator. The model fully explains the values of current sensitivity per pH for the broad range of subthreshold slopes obtained in our fabricated nanoribbon...
This paper presents a smart energy meter for automatic metering and billing system. The integrati... more This paper presents a smart energy meter for automatic metering and billing system. The integration of a mircocontroller and GSM short message service (SMS) provides the meter reading system with automatic functions that are predefined. The GSM module requires a SIM (Subscriber Identity Module) card just like mobile phones to activate communication with the network. Direct current (DC) components are used to control alternating current (AC) loads. To isolate these components from each other, relays with a network of resistors and diodes are used. Users can recharge and control loads remotely. Utility companies also have remote access to the system such as fault diagnosis and communicating with clients. The proposed energy meter system (EMS) transmits data like consumed energy in kWh and generates a bill over a GSM mobile network. Other advantages include that the system provides domestic power consumption accurately, safely and with a relatively fast update rate. Keywords—SMART ener...
Indium gallium zinc oxide fin-field effect transistor (IGZO FinFET) characteristics are investiga... more Indium gallium zinc oxide fin-field effect transistor (IGZO FinFET) characteristics are investigated and then compared with Zinc oxide fin-field effect transistor (ZnO FinFET) and the Silicon fin-field effect transistor (Si FinFET). This was done using 3D simulation. The threshold voltage for Si, ZnO, and IGZO is 0.75 V, 0.30 V and 0.05 V respectively. The silicon device has the highest transconductance (5.0 x 10-7 S) and performs better than the other devices because it has less fixed charge defects. IGZO has the second-best value of Gm (3.6 x 10-7 S), ZnO has the least value of Gm (3.4 x 10-7 S). Si device has the least drain current (IDS) value of 2.0 x 10-7 A, ZnO device has a better IDS value of 6.2 x 10-6 A while IGZO device has the best IDS value of 1.6 x 10-5 A. IGZO is better than Si by two (2) order magnitude. The field effect mobility is 50.0 cm2/Vs for all three devices.
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Papers by Nonofo Ditshego