Bandstructure Effects in Ultra-Thin-Body DGFET: A Fullband Analysis

This paper discusses a few unique effects of ultra-thin-body double-gate NMOSFET that are arising from the bandstructure of the thin film Si channel. The bandstructure has been calculated using 10-orbital $sp^3d^5s^*$ tight-binding method. A number of intrinsic properties including band gap, density of states, intrinsic carrier concentration and parabolic effective mass have been derived from the calculated bandstructure. The spatial distributions of intrinsic carrier concentration and $$ effective mass, arising from the wavefunction of different contributing subbands are analyzed. A self-consistent solution of Poisson-Schrodinger coupled equation is obtained taking the full bandstructure into account, which is then applied to an insightful analysis of volume inversion. The spatial distribution of carriers over the channel of a DGFET has been calculated and its effects on effective mass and channel capacitance are discussed.Comment: 13 pages, 21 figure

Estimation of background carrier concentration in fully depleted GaN films

Buffer leakage is an important parasitic loss mechanism in AlGaN/GaN HEMTs and hence various methods are employed to grow semi-insulating buffer layers. Quantification of carrier concentration in such buffers using conventional capacitance based profiling techniques is challenging due to their fully depleted nature even at zero bias voltages. We provide a simple and effective model to extract carrier concentrations in fully depleted GaN films using capacitance-voltage (C-V) measurements. Extensive mercury probe C-V profiling has been performed on GaN films of differing thicknesses and doping levels in order to validate this model. Carrier concentrations as extracted from both the conventional C-V technique for partially depleted films having the same doping concentration, and Hall measurements show excellent agreement with those predicted by the proposed model thus establishing the utility of this technique. This model can be readily extended to estimate background carrier concentrations from the depletion region capacitances of HEMT structures and fully depleted films of any class of semiconductor materials.Comment: 16 pages, 6 figure

Effects of Parasitics and Interface Traps On Ballistic Nanowire FET In The Ultimate Quantum Capacitance Limit

In this paper, we focus on the performance of a nanowire Field Effect Transistor (FET) in the Ultimate Quantum Capacitance Limit (UQCL) (where only one subband is occupied) in the presence of interface traps ($D_{it}$), parasitic capacitance ($C_L$) and source/drain series resistance ($R_{s,d}$) using a ballistic transport model and compare the performance with its Classical Capacitance Limit (CCL) counterpart. We discuss four different aspects relevant to the present scenario, namely, (i) gate voltage dependent capacitance, (ii) saturation of the drain current, (iii) the subthreshold slope and (iv) the scaling performance. To gain physical insights into these effects, we also develop a set of semi-analytical equations. The key observations are: (1) A strongly energy-quantized nanowire shows non-monotonic multiple peak C-V characteristics due to discrete contributions from individual subbands; (2) The ballistic drain current saturates better in the UQCL compared to CCL, both in presence and absence of $D_{it}$ and $R_{s,d}$; (3) The subthreshold slope does not suffer any relative degradation in the UQCL compared to CCL, even with $D_{it}$ and $R_{s,d}$; (4) UQCL scaling outperforms CCL in the ideal condition; (5) UQCL scaling is more immune to $R_{s,d}$, but presence of $D_{it}$ and $C_L$ significantly degrades scaling advantages in the UQCL.Comment: Accepted at IEEE Transactions on Electron Device

HFinFET: A Scalable, High Performance, Low Leakage Hybrid N-Channel FET

In this letter we propose the design and simulation study of a novel transistor, called HFinFET, which is a hybrid of a HEMT and a FinFET, to obtain excellent performance and good off state control. Followed by the description of the design, 3D device simulation has been performed to predict the characteristics of the device. The device has been benchmarked against published state of the art HEMT as well as planar and non-planar Si NMOSFET data of comparable gate length using standard benchmarking techniques.Comment: 3 pages, 4 figure

Intrinsic Limits of Subthreshold Slope in Biased Bilayer Graphene Transistor

In this work, we investigate the intrinsic limits of subthreshold slope in a dual gated bilayer graphene transistor using a coupled self-consistent Poisson-bandstructure solver. We benchmark the solver by matching the bias dependent bandgap results obtained from the solver against published experimental data. We show that the intrinsic bias dependence of the electronic structure and the self-consistent electrostatics limit the subthreshold slope obtained in such a transistor well above the Boltzmann limit of 60mV/decade at room temperature, but much below the results experimentally shown till date, indicating room for technological improvement of bilayer graphene.Comment: 10 pages, 2 figure

Optical Phonon Limited High Field Transport in Layered Materials

An optical phonon limited velocity model has been employed to investigate high-field transport in a selection of layered 2D materials for both, low-power logic switches with scaled supply voltages, and high-power, high-frequency transistors. Drain currents, effective electron velocities and intrinsic cut-off frequencies as a function of carrier density have been predicted thus providing a benchmark for the optical phonon limited high-field performance limits of these materials. The optical phonon limited carrier velocities of a selection of transition metal dichalcogenides and black phosphorus are found to be modest as compared to their n-channel silicon counterparts, questioning the utility of these devices in the source-injection dominated regime. h-BN, at the other end of the spectrum, is shown to be a very promising material for high-frequency high-power devices, subject to experimental realization of high carrier densities, primarily due to its large optical phonon energy. Experimentally extracted saturation velocities from few-layer MoS2 devices show reasonable qualitative and quantitative agreement with predicted values. Temperature dependence of measured vsat is discussed and found to fit a velocity saturation model with a single material dependent fit parameter.Comment: 8 pages, 6 figure

External Bias Dependent Direct To Indirect Bandgap Transition in Graphene Nanoribbon

In this work, using self-consistent tight-binding calculations, for the first time, we show that a direct to indirect bandgap transition is possible in an armchair graphene nanoribbon by the application of an external bias along the width of the ribbon, opening up the possibility of new device applications. With the help of Dirac equation, we qualitatively explain this bandgap transition using the asymmetry in the spatial distribution of the perturbation potential produced inside the nanoribbon by the external bias. This is followed by the verification of the bandgap trends with a numerical technique using Magnus expansion of matrix exponentials. Finally, we show that the carrier effective masses possess tunable sharp characters in the vicinity of the bandgap transition points.Comment: Accepted for publication in Nano Letter

Photoresponse of atomically thin MoS2 layers and their planar heterojunctions

MoS2 monolayers exhibit excellent light absorption and large thermoelectric power, which are, however, accompanied with very strong exciton binding energy - resulting in complex photoresponse characteristics. We study the electrical response to scanning photo-excitation on MoS2 monolayer (1L) and bilayer (2L) devices, and also on monolayer/bilayer (1L/2L) planar heterojunction and monolayer/few-layer/multi-layer (1L/FL/ML) planar double heterojunction devices to unveil the intrinsic mechanisms responsible for photocurrent generation in these materials and junctions. Strong photoresponse modulation is obtained by scanning the position of the laser spot, as a consequence of controlling the relative dominance of a number of layer dependent properties, including (i) photoelectric effect (PE), (ii) photothermoelectric effect (PTE), (iii) excitonic effect, (iv) hot photo-electron injection from metal, and (v) carrier recombination. The monolayer and bilayer devices show peak photoresponse when the laser is focused at the source junction, while the peak position shifts to the monolayer/multi-layer junction in the heterostructure devices. The photoresponse is found to be dependent on the incoming light polarization when the source junction is illuminated, although the polarization sensitivity drastically reduces at the monolayer/multi-layer heterojunction. Finally, we investigate laser position dependent transient response of photocurrent to reveal trapping of carriers in SiO2 at the source junction is the critical factor to determine the transient response in 2D photodetectors, and also show that, by systematic device design, such trapping can be avoided in the heterojunction devices, resulting in fast transient response. The insights obtained will play an important role in designing fast 2D TMDs based photodetector and related optoelectronic and thermoelectric devices.Comment: Nanoscale, 201