Mehdi Hasan

Photonic nanojet-enhanced nanometer-scale germanium photodiode.

A design challenge for photodiodes yielding both high speed and responsivity is the necessity to concentrate incident light into a subwavelength active volume region. Photonic nanojets have been reported in the literature as a means to focus an incident plane wave to a subwavelength-waist propagating beam with applications ranging from next-generation DVDs to characterizing subwavelength features within dielectric targets. In the present work, a new application of photonic nanojets is proposed, focusing electromagnetic energy into a photodiode. Three-dimensional finite-difference time-domain solutions are conducted to determine the advantages of photonic nanojet-enhanced photodiodes at near-infrared wavelengths (1310 nm). We find that photonic nanojets provide a factor of 26 increase in the volume-integrated electric field within the subwavelength active volume of the photodiode of size 0.0045 μm³. Furthermore, this increase is achieved independent of the incident polarization and over a broad bandwidth. Photonic nanojets may thus serve as an attractive alternative to plasmonics for some applications.

A feasibility study of microjets applied to breast cancer detection

The feasibility of detecting small tumors of diameter 0.5mm inside a human breast is demonstrated using microjets. Microjets are photonic nanojets scaled to microwave frequencies wherein a plane-wave-illuminated dielectric sphere produces a high-intensity, narrow beam of light extending from the shadow-side surface. Contrasting electromagnetic properties (permittivity and conductivity) of normal and malignant breast tissue are the basis for the detection via a simple image processing approach to extract the backscattering from the tumor. Simple proof-of-concept simulation results are presented to illustrate the effectiveness of the new approach.

A Continuous Compact DC Model for Dual-Independent-Gate FinFETs

Multiple-independent-gate (MIG) silicon FinFETs were recently shown capable of enabling: 1) device-level polarity control; 2) dynamic threshold modulation; and 3) subthreshold slope tuning down to ultra-steep-slope operation. These operation mechanisms can unlock a myriad of opportunities in digital as well as analog design. Here we discuss a continuous compact direct-current (dc) model, capable of describing the current-voltage characteristics of a class of MIG FinFETS, namely dual-independent-gate (DIG) FinFETs, over all its biasing design space. This model captures some of the unique features of DIG FinFETs including the dependence of its super-steep subthreshold swing on drain bias and polarity gate bias. An excellent agreement is shown between the model and measured experimental current-voltage characteristics in these devices. Moreover, the predictive nature of the model is evaluated by foreseeing the perspectives of DIG FinFETs as efficient RF detectors at very high frequencies.

Digital, analog and RF design opportunities of three-independent-gate transistors

Field-Effect Transistors (FETs) with Three Independent Gates (TIG) can achieve different modes of operation according to the bias of the gate terminals. In particular, TIG FETs were recently shown capable of (i) device-level polarity control, (ii) dynamic threshold modulation and (iii) subthreshold slope tuning down to ultra-steep-slope operation. Experimentally demonstrated using both contemporary FinFETs and emerging silicon nanowires channel technologies, TIGFETs unlock several design opportunities. In this paper, we comment on the digital, analog and RF design capabilities offered by this new class of transistors.

Effect of the intra-layer potential distributions and spatial currents on the performance of graphene SymFETs

In this paper, a two-dimensional (2-D) model for a graphene symmetric field effect transistor (SymFET), which considers (a) the intra-graphene layer potential distributions and (b) the internal current flows through the device, is presented and discussed. The local voltages along the graphene electrodes as well as the current-voltage characteristics of the device are numerically calculated based on a single-particle tunneling model. Our numerical results show that: (i) when the tunneling current is small, due to either a large tunneling thickness (≥ 2 atomic layers of BN) or a small coherence length, the voltage distributions along the graphene electrodes have almost zero variations upon including these distributed effects, (ii) when the tunnel current is large, due to either a small tunneling thickness (∼ 1 atomic layer of BN) or due to a large coherence length, the local voltage distributions along the graphene electrodes become appreciable and the device behavior deviates from that predicted by a 1-D approximation. These effects, which are not captured in one-dimensional SymFET models, can provide a better understanding about the electron dynamics in the device and might indicate potential novel applications for this proposed device.

A compact DC model for dual-independent-gate FinFETs

To: (i) reduce the power consumption in digital integrated circuits, (ii) increase the transistor trans-conductance generation efficiency in analog circuits, and (iii) attain a very sensitive nonlinear response to RF, transistors exhibiting very steep room-temperature subthreshold slope (SS) are required. The subthreshold slope of conventional MOSFETs is limited to >60mV/dec due to their current turn-on mechanism being thermionic emission. During the last decade, several emerging transistor concepts, based on alternative current transport mechanisms, have been proposed so to overcome this fundamental limitation. For instance, Tunnel FETs (TFETs) have emerged as one of the most attractive alternatives to traditional MOSFETs, with experimental demonstrations of SS below 30 mV/dec, due to the current turn-on mechanism in such devices being band-to-band tunneling. In this context, dual-independent-gate (DIG) FinFETs have been also demonstrated capable of achieving a very steep subthreshold slope [1, 2]. The reason behind this super steep slope is a positive feedback induced by weak impact ionization in the device. Experimental demonstrations of DIG FinFETs have shown SS of 3.4 mV/dec at room-temperature over 5 decades of current swing [1, 2]. In this paper, we discuss a simple, closed-form analytic model for the current-voltage characteristics of DIG FinFETs, which can be of interest for many applications including circuit-design and application oriented device performance evaluation.

Perspectives of DIG FinFETs for efficient terahertz detection applications

Dual-independent-gate (DIG) silicon FinFETs were recently shown capable of operating with ultra-steep subthreshold slope of 3.4 mV/dec at room-temperature due to a weak impact ionization induced positive feedback. In this work we discuss the perspectives of these devices for room-temperature terahertz detector applications. Our analysis shows that DIG-FinFETs can enable room-temperature current-responsivities up to two orders of magnitude larger than those of regular FET and Schottky diode detectors at room-temperature. The device operation, detector configurations, and the sources of noise in the device are discussed and rigorously analyzed; moreover the device is also benchmarked against other present-day direct detector technologies.

Three-Independent-Gate Transistors: Opportunities in digital, analog and RF applications

This paper provides a comprehensive review of Three-Independent-Gate Field-Effect Transistors (TIGFETs). In parallel to the focus on transistor scaling, an alternative approach to push further the performance of computing systems consists in increasing the functionalities of the basic transistors by means of additional gate controls. TIGFETs belong to this category of devices and can achieve different modes of operation according to the bias of the gate terminals. In particular, these devices are capable of (i) device-level polarity control, (ii) dynamic threshold modulation and (iii) subthreshold slope tuning down to ultra-steep-slope operation. The functionality increase at the device level leads to several design opportunities for digital, analog and RF applications.

Two-dimensional distributed effects in graphene SymFETs

The perfect symmetry in the band structure of graphene has motivated the investigation of single-particle inter-layer tunneling in finite area two-terminal graphene-insulator-graphene hetero-structures. Based on this transport mechanism, Zhao et al. [1] recently proposed a symmetric graphene tunneling field-effect transistor (SymFET), where current flows by resonant tunneling between an n-type graphene electrode and a p-type graphene electrode. This device was analyzed in previous works employing a single particle tunneling model and assuming a 1-D approximation of the device. However, two-dimensional effects can be important in tunneling devices based on 2-D materials and alter the predicted characteristics of such devices. In this work, we introduce a rigorous 2-D electrostatic model that takes into consideration: (a) the intra-graphene-layer potential distributions, and (b) the internal current flows through the device, as suggested in the work of Zhao et al. [1] as a proposed future element to be considered in further work.

Super-enhanced optical energy concentration through a subwavelength aperture using a photonic nanojet

Summary fom only given. Optical transmission through resonant subwavelength apertures in optically thick metal films have received an explosion of interest for their ability to overcome the diffraction limit of light and concentrate light efficiently into a subwavelength volume. This achievement has attracted the use of subwavelength apertures in numerous applications, i.e. in near-field optical microscopy, fluorescence correlation spectroscopy, nanoscale optical recording, optical lithography, ultra small photodetectors, novel nanoscale light source, and nonlinear optical processes, etc. In a separate line of research, photonic nanojets have been discovered and proposed for a number of applications ranging from optical data disk storage to high-speed photodetectors. A photonic nanojet is a sub-wavelength (as small as λ/3) narrow electromagnetic beam that can propagate multiple wavelengths from the shadow-side surface of a dielectric sphere. We present here a means to significantly compress the transverse width of a photonic nanojet by placing a plasmonic nano-aperture in its path. Three-dimensional (3-D) finite-difference time-domain (FDTD) modeling is used to demonstrate the superenhanced optical energy concentration capability of the photonic nanojet nano-aperture light-collection system. Specifically, 3-D FDTD results demonstrate that a gold nano-aperture illuminated by a nanojet compresses the nanojet from λ/3 to λ/6, which corresponds for an incident wavelength, λ = 633 nm to a reduction of the intensity full-width at half-maximum (FWHM) from ~ 220 nm to ~ 140 nm. Further, we achieve an absorption enhancement factor of nearly 350 in a subwavelength volume of 0.004 μm3 on the shadow-side of the gold nano-aperture for an incident wavelength, λ of 633 nm. The superenhanced, subwavelength concentration of light is achieved for both resonant and non-resonant plasmonic nano-apertures. This phenomenon may find utility in a wide range of applications, such as high-speed photodetectors, optical data storage, optical lithography, near-field optical microscopy, novel nanoscale light source, localized detection of embedded ultrasubwavelength inhomogeneity, fluorescence correlation spectroscopy, biosensors, etc.