Md. Sherajul Islam

A Study on Theoretical Performance of Graphene FET using Analytical Approach with Reference to High Cutoff Frequency

This paper presents a detailed study of theoretical performance of graphene field effect transistor (GFET) using analytical approach. GFET shows promising performance in terms of faster saturation as well as extremely high cutoff frequency (3.9THz). A significant shift of the Dirac point as well as an asymmetrical ambipolar behavior is observed on the transfer characteristics. Similarly, an approximate symmetrical capacitance–voltage (C–V) characteristics is obtained where it has guaranteed the consistency because it shows a significant saturation both in the accumulation and inversion region. In addition, a high transconductance of 6800uS at small channel length (20nm) along with high cutoff frequency (3.9THz) has been observed which demands for high speed field effect devices.

Polarized micro Raman scattering spectroscopy for curved edges of epitaxial graphene

This letter performed polarized microscopic laser Raman scattering spectroscopy on the curved edges of transferred epitaxial graphene on SiO2/Si. The intensity ratio between the parallel and perpendicular polarized D band is evolved, providing a spectroscopy-based technique to probe the atomic-scale edge structures in graphene. A detailed analysis procedure for non-ideal disordered curved edges of graphene is developed combining the atomic-scale zigzag and armchair edge structures along with some point defects. These results could provide valuable information of the realistic edges of graphene at the atomic-scale that can strongly influence the performance of graphene-based nanodevices.

High efficiency In x Ga 1−x N-based quantum well solar cell

This paper reports on the In<inf>x</inf>Ga<inf>1−x</inf>N-based quantum well (QW) cell as a promising candidate for future high efficiency solar cells. The performances of the proposed quantum well cell are assessed using an analytical model and different device parameters are optimized. A maximum efficiency of 36.49% is obtained with at an optimized band gap of 1.38 eV and at a well depth of 0.2eV. The effects of well number on short circuit current density, open circuit voltage and on the conversion efficiency of the In<inf>x</inf>Ga<inf>1−x</inf>N-based QW solar cell are also studied. The highest efficiency found for In<inf>x</inf>Ga<inf>1−x</inf>N-based QW cell for 35 well was 40.2% while for Al<inf>x</inf>Ga<inf>1−x</inf>As -based QW solar cell this was 35.55%. Comparison of In<inf>x</inf>Ga<inf>1−x</inf>N-based QW solar cell and Al<inf>x</inf>Ga<inf>1−x</inf>As -based QW solar cell implies that In<inf>x</inf>Ga<inf>1−x</inf>N-based quantum well cell offers the better efficiency.

Numerical Analysis on Phonon Localization of Vacancy Type Disordered Graphene

The phonon properties of hydrogen passivated graphene with vacancy defects are studied using the forced vibrational method. The phonon density of states (PDOSs), typical mode patterns and localization length are calculated over a broad range of vacancies. We find that phonon properties of graphene strongly depend on the system size. We observe a broadening and softening of the PDOS peaks with the increase of vacancy concentrations. We find an increasing C–H stretching mode with the increase of defect density. Our numerical experiments reveal that the typical mode pattern for the K point in-plane TO (iTO) modes phonon show the spatial localized vibrations persuaded by vacancies, which are in conceptually good agreement with the large D band peak of the Raman spectra comes from the imperfections of crystal. For the K point iTO mode phonons, a typical localization length is on the order ≈ 5 nm for vacancy-type defects at high concentrations of 30% is observed. The localization effects manifest themselves in the projected temperature behavior of the constant-volume specific heat capacity of pristine and disordered graphene samples.

Vacancy and curvature effects on the phonon properties of single wall carbon nanotube

Single wall carbon nanotube (SWCNT) is considered as an ideal candidate for next-generation nanoelectronics owing to its unusual properties. Here we have performed an in-depth theoretical analysis of the effect of vacancy defects and curvature on the phonon properties of and SWCNTs using the forced vibrational method. We report that Raman active E2g mode softens towards the low-frequency region with increasing vacancies and curvature in both types of CNTs. Vacancy induces some new peaks at low-frequency region of the phonon density of states. Phonon localization properties are also manifested. Our calculated mode pattern and localization length show that optical phonon at Raman D-band frequency is strongly localized in vacancy defected and large curved CNTs. Our findings will be helpful in explaining the thermal conductivity, specific heat capacity, and Raman spectra in vacancy type disordered CNTs, as well as electron transport properties of CNT-based nanoelectronic devices.

Probing the neuronal status for cerebrovascular disease using EEG

Cerebrovascular disease (CVD) such as stroke is the leading cause of long-term disability and the third most common reason of death in the world. In this work, an indication for the assessment of neuronal activity and the degree of severity of stroke patient has been studied using electroencephalogram (EEG). A parallel study has also been carried out on healthy volunteers of under fifteen years to find the comparison of neural activity between different age groups. In order to assess the brain pathophysiology in supratentorial brain lesion patients the quantitative EEG parameters such as delta/alpha ratio (DAR) and delta-plus-theta to alpha-plus-beta ratio (DTABR) have been used. It is observed that the DAR and DTABR values of the left cerebral hemisphere of the patient are much higher than the right cerebral hemisphere. It is also higher in both cerebral hemispheres than the control. A threshold value of ∼3.7 for DAR and ∼3.5 for DTABR has been obtained. It is found that delta and DAR indices of the old age are twice than the child, indicating the diminishing of neuronal activity of old age is half of the child. These results could be important for stroke diagnosis, prognosis, re-habitation strategies, and proper neurological treatment.

Theoretical analysis of substrate effects on the DC performance of AlGaN/GaN high electron mobility transistor

This paper reports the theoretical analysis of substrate (Sapphire, Si, SiC, Diamond) effects on the saturation drain current and transconductance of Al.27Ga.73N/GaN high-electron mobility transistor using analytical approach. This model includes polar optical phonon scattering, source-drain resistance and self-heating effects with a wide temperature ranges. It is found that substrates have significant effects on the dc performance of AlGaN/GaN HEMT. The model reports very good performance of HEMT fabricated on Diamond substrate due to its excellent thermal conductivity properties and a comparative analysis of the performance on different substrates at high voltage level is presented.

Effect of phosphor distribution on junction temperature reduction in white LEDs

High efficiency white LEDs are expected to take the place of conventional lamps for general lighting applications. High junction temperature is one of the key parameters that affects both electrical and optical properties. In this work, effects of phosphor distribution on junction temperature reduction is studied. The results showed that, if proper phosphor distribution is ensured so that they form thermal bridge between the chip and convective boundary, the junction temperature decreases. As the phosphor concentration increases from 0.3 to 0.4, the junction temperature significantly reduces from 187 to 143°C at an ambient temperature of 25°C. It can also be demonstrated from the study that the lower the ambient temperature, the better the performance.

Numerical analysis on vibrational properties of vacancy-type disordered graphane

We theoretically explore the vacancy induced vibrational properties of graphane using the forced vibrational method. We find strong changes in the phonon density of states for vacancy-type disordered graphane, revealing the significant impacts on the electron transport properties. The phonon eigenvectors estimated for the K point in-plane transverse optical mode for the defective graphane show the strongly localized vibrations. The localization effects manifest themselves in the projected temperature behavior of the constant-volume specific heat capacity of pristine and disordered graphane samples.

DC and RF characteristics of graphene FET using analytical approach

This paper presents a detailed study of DC and RF characteristics of GFET using analytical approach. GFET shows promising performance in terms of faster saturation as well as extremely high cut-off frequency. A significant shift of the Dirac point as well as an asymmetrical ambipolar behavior is observed on the transfer curve.