Swaroop Ganguly

Bulk Planar Junctionless Transistor (BPJLT): An Attractive Device Alternative for Scaling

We propose a novel highly scalable source-drain-junction-free field-effect transistor that we call the bulk planar junctionless transistor (BPJLT). This builds upon the idea of an isolated ultrathin highly doped device layer of which volume is fully depleted in the off-state and is around flatband in the on-state. Here, the leakage current depends on the effective device layer thickness, and we show that with well doping and/or well bias, this can be controllably made less than the physical device layer thickness in a bulk planar junction-isolated structure. We demonstrate by extensive device simulations that these additional knobs for controlling short-channel effects reduce the off-state leakage current by orders of magnitude for similar on-state currents, making the BPJLT highly scalable.

Effect of Band-to-Band Tunneling on Junctionless Transistors

We evaluate the impact of band-to-band tunneling (BTBT) on the characteristics of n-channel junctionless transistors (JLTs). A JLT that has a heavily doped channel, which is fully depleted in the off state, results in a significant band overlap between the channel and drain regions. This overlap leads to a large BTBT of electrons from the channel to the drain in n-channel JLTs. This BTBT leads to a nonnegligible increase in the off-state leakage current, which needs to be understood and alleviated. In the case of n-channel JLTs, tunneling of electrons from the valence band of the channel to the conduction band of the drain leaves behind holes in the channel, which would raise the channel potential. This triggers a parasitic bipolar junction transistor formed by the source, channel, and drain regions induced in a JLT in the off state. Tunneling current is observed to be a strong function of the silicon body thickness and doping of a JLT. We present guidelines to optimize the device for high on-to-off current ratio. Finally, we compare the off-state leakage of bulk JLTs with that of silicon-on-insulator JLTs.

Enhanced Electrostatic Integrity of Short-Channel Junctionless Transistor With High-

We propose the use of a high-κ spacer to improve the electrostatic integrity and, thereby, the scalability of silicon junctionless transistors (JLTs) for the first time. Using extensive simulations of n-channel JLTs, we demonstrate that the high-κ spacers improve the electrostatic integrity of JLTs at sub-22-nm gate lengths. Electric field that fringes through the high-κ spacer to the device layer on either sides of the gate results in an effective increase in electrical gate length in the off-state. However, the effective gate length is unaffected in the on-state. Hence, the off-state leakage current is reduced by several orders of magnitude with the use of a high-κ spacer with concomitent improvements in the subthreshold swing and drain-induced barrier lowering. A marginal improvement in the on-state current is observed with the use of the high-κ spacer, and this is related to the reduction in parasitic resistance in the silicon under the spacer due to fringe fields.

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We have developed a double-channel high electron mobility transistor with back barriers for carrier confinement. We have observed that the double-channel devices may suffer from the lack of gate control particularly for the lower channel. However, the problem can be contained by using a suitable back barrier for the lower channel. The double-channel back-barrier devices show good current gain and power gain cutoff frequencies. These devices can be operated with excellent gain linearity up to a larger value for input power and frequency.

Spacers

We present nonlocal empirical pseudopotential calculations for SiGe alloys employing a novel nonlinear interpolation scheme. Our interpolation scheme is able to correctly model for the first time the band gap bowing observed in relaxed SiGe alloys. The valence-band-edge and conduction-band-edge energies in relaxed Si1−xGex for arbitrary x, which are difficult to obtain by experimental techniques, have been evaluated using pseudopotential calculations. We have also calculated the band energies of pseudomorphic [100]-strained Si1−xGex alloys grown over unstrained Si1−yGey substrates. The energy gaps, valence and conduction band offsets, effective masses, and strain induced splittings in pseudomorphic SiGe layers are calculated for the whole range of alloy compositions x and y.

Double-Channel AlGaN/GaN High Electron Mobility Transistor With Back Barriers

Controlling the morphology of inorganic nanocrystals is important because many of their electronic attributes are highly sensitive to shape and aspect ratio. FePt nanocrystals have potential as advanced magnetic materials for ultrahigh-density memory. This is due to their high shape and/or magnetocrystalline anisotropy, which allows bits as small as 3 nm to be thermally stable over typical data storage periods of 10 years. Herein, nanocrystals were simply fabricated by simultaneous reduction of platinum acetylacetonate and thermal decomposition of iron pentacarbonyl in properly chosen conditions of solvent/surfactant proportions and temperature for rational design of their shape and magnetic properties. This work has combined magnetometry measurements and micromagnetic simulations to illustrate the role of the external shape on the rotation of the magnetization vector for colloidal assemblies.

Band gap bowing and band offsets in relaxed and strained Si1−xGex alloys by employing a new nonlinear interpolation scheme

Modeling and experimental investigation of B equilibrium diffusivity and its activation in Si in the presence of other species, including ab initio calculations, are presented here. The results suggest that incorporating other species along with B into the Si substrate can achieve shallower junctions and higher B activation in semiconductor device applications.

Origin of shape anisotropy effects in solution-phase synthesized FePt nanomagnets

We predict that the Curie temperature of a ferromagnetic, resonant tunneling diode will decrease abruptly, by approximately a factor of 2, when the downstream chemical potential falls below the quantum-well resonance energy. This property follows from elementary quantum-transport-theory notions combined with a mean-field description of diluted, magnetic, semiconductor ferromagnetism. We illustrate this effect by solving coupled nonequilibrium Greens function, magnetic mean-field, and electrostatic Poisson equations self-consistently to predict the bias voltage and temperature dependence of the magnetization of a model system.

Boron diffusion and activation in the presence of other species

We model B diffusion in the category of equilibrium diffusivity in the presence of other species using ab initio calculations. The result shows that in the presence of other atoms X (X=F, N, C, Al, Ga, In, and Ge), the migration energy for B along the migration pathway is larger than the case if X=Si (0.33 eV). This suggests the reduction of B diffusion. If another B occupies that position (X=B), then a smaller migration energy is observed and enhanced diffusion is expected. The simulation results are consistent with experiments. The reason for the migration energy difference can be understood in terms of electronic and strain effects. Estimates of the enthalpy of bond formation agree with the simulation except for the low-enthalpy cases where the strain effect caused by atom-size misfit dominates.

Bias-voltage-controlled magnetization switch in ferromagnetic semiconductor resonant tunneling diodes

We demonstrate a method for nanowire formation by natural selection during wet anisotropic chemical etching in boiling phosphoric acid. Nanowires of sub-10 nm lateral dimensions and lengths of 700 nm or more are naturally formed during the wet etching due to the convergence of the nearby crystallographic hexagonal etch pits. These nanowires are site controlled when formed in augmentation with dry etching. Temperature and power dependent photoluminescence characterizations confirm excitonic transitions up to room temperature. The exciton confinement is enhanced by using two-dimensional confinement whereby enforcing greater overlap of the electron-hole wave-functions. The surviving nanowires have less defects and a small temperature variation of the output electroluminescent light. We have observed superluminescent behaviour of the light emitting diodes formed on these nanowires. There is no observable efficiency roll off for current densities up to 400 A/cm2.