Nima Dehdashti Akhavan

Junctionless multigate field-effect transistor

This paper describes a metal-oxide-semiconductor MOS transistor concept in which there are no junctions. The channel doping is equal in concentration and type to the source and drain extension doping. The proposed device is a thin and narrow multigate field-effect transistor, which can be fully depleted and turned off by the gate. Since this device has no junctions, it has simpler fabrication process, less variability, and better electrical properties than classical MOS devices with source and drain PN junctions.

Reduced electric field in junctionless transistors

The electric field perpendicular to the current flow is found to be significantly lower in junctionless transistors than in regular inversion-mode or accumulation-mode field-effect transistors. Since inversion channel mobility in metal-oxide-semionductor transistors is reduced by this electric field, the low field in junctionless transistor may give them an advantage in terms of current drive for nanometer-scale complementary metal-oxide semiconductor applications. This observation still applies when quantum confinement is present.

Junctionless Multiple-Gate Transistors for Analog Applications

This paper presents the evaluation of the analog properties of nMOS junctionless (JL) multigate transistors, comparing their performance with those exhibited by inversion-mode (IM) trigate devices of similar dimensions. The study has been performed for devices operating in saturation as single-transistor amplifiers, and we have considered the dependence of the analog properties on fin width Wfin and temperature T. Furthermore, this paper aims at providing a physical insight into the analog parameters of JL transistors. For that, in addition to device characterization, 3-D device simulations were performed. It is shown that, depending on gate voltage, JL devices can present both larger Early voltage VEA and larger intrinsic voltage gain AV than IM devices of similar dimensions. In addition, VEA and AV are always improved in JL devices when the temperature is increased, whereas they present a maximum value around room temperature for IM transistors.

Low subthreshold slope in junctionless multigate transistors

The improvement of subthreshold slope due to impact ionization is compared between “standard” inversion-mode multigate silicon nanowire transistors and junctionless transistors. The length of the region over which impact ionization takes place, as well as the amplitude of the impact ionization rate are found to be larger in the junctionless devices, which reduces the drain voltage necessary to obtain a sharp subthreshold slope.

Mobility improvement in nanowire junctionless transistors by uniaxial strain

Improvement of current drive in n- and p-type silicon junctionless metal-oxide-semiconductor-field-effect-transistors (MOSFETs) using strain is demonstrated. Junctionless transistors have heavily doped channels with doping concentrations in excess of 10(19) cm(-3) and feature bulk conduction, as opposed to surface channel conduction. The extracted piezoresistance coefficients are in good agreement with the piezoresistive theory and the published coefficients for bulk silicon even for 10 nm thick silicon nanowires as narrow as 20 nm. These experimental results demonstrate the possibility of enhancing mobility in heavily doped silicon junctionless MOSFETs using strain technology

Improvement of carrier ballisticity in junctionless nanowire transistors

In this work we show that junctionless nanowire transistor (JNT) exhibits lower degree of ballisticity in subthreshold and higher ballisticity above threshold compare to conventional inversion-mode transistors, according to quantum mechanical simulations. The lower degradation of the ballisticity above threshold region gives the JNT near-ballistic transport performance and hence a high current drive. On the other hand, lower ballisticity in subthreshold region helps reducing the off-current and improves the subthreshold slope. A three-dimensional quantum mechanical device simulator based on the nonequilibrium Green’s function formalism in the uncoupled mode-space approach has been developed to extract the physical parameters of the devices.

Effect of intravalley acoustic phonon scattering on quantum transport in multigate silicon nanowire metal-oxide-semiconductor field-effect transistors

In this paper we investigate the effects of intravalley acoustic phonon scattering on the quantum transport and on the electrical characteristics of multigate silicon nanowire metal-oxide-semiconductor field-effect transistors. We show that acoustic phonons cause a shift and broadening of the local DOS in the nanowire, which modifies the electrical characteristics of the device. The influence of scattering on off-state and on-state currents is investigated for different values of channel length. In the ballistic transport regime, source-to-drain tunneling current is predominant, whereas in the presence of acoustic phonons, diffusion becomes the dominant current transport mechanism. A three-dimensional quantum mechanical device simulator based on the nonequilibrium Greens function formalism in uncoupled-mode space has been developed to extract device parameters in the presence of electron-phonon interactions. Electron-phonon scattering is accounted for by adopting the self-consistent Born approximation and using the deformation potential theory

Properties of Accumulation-Mode Multi-Gate Field-Effect Transistors

In this work, we analyze the conduction mechanisms and the electrical performance of intrinsic and doped accumulation-mode (AM) p-type, triple-gate silicon-on-insulator (SOI) metal oxide semiconductor field-effect transistors (MOSFETs). Both long- and short-channel devices with different fin widths are investigated on the basis of experimental and simulation data. The analysis shows that for a small fin width, the threshold voltages associated with both body current and side channels in heavily doped devices coincide, thereby preventing the increase in leakage current caused by body conduction that is conventionally observed in planar AM fully-depleted (FD) SOI MOSFETs. Shortchannel effects (SCEs) are minimized in these devices owing to the good electrostatic control by the surrounding gate. The experimental data indicate that SCEs are comparable to those observed in inversion-mode (IM) devices with a gate length of 50 nm. This makes AM triple-gate (or more generally, multigate) MOSFETs interesting devices for digital applications.

A method of removing the valence band discontinuity in HgCdTe-based nBn detectors

A method is described where the valence band discontinuity in HgCdTe-based nBn detectors will be eliminated. The method relies on doping modulation technique, where grading the material composition and doping concentration of the barrier layer at the same time will lead to elimination of the valence band discontinuity in HgCdTe-based nBn detectors. The method is not limited to the nBn structure and can be applied to any barrier detector structure with xBx (with x = n, p) to eliminate the energy band discontinuity in the valence band or conduction band.

Engineering the Bandgap of Unipolar HgCdTe-Based nBn Infrared Photodetectors

Design of practically realizable unipolar HgCdTe nBn photodetectors has been studied in detail by numerical analysis. The simulations reported herein reveal that, by optimization of barrier doping, dark current levels can be reduced and collection efficiency substantially improved. It is shown that p-type doping of the barrier layer can significantly reduce the effective potential barrier arising from the valence band offset between the absorber and barrier regions, thus enabling HgCdTe nBn detector operation under near zero-bias conditions. However, relatively high electric fields in the space charge regions near the barrier/absorber interface result in enhanced trap-assisted Shockley–Read–Hall thermal generation. Our calculations indicate that nBn HgCdTe detectors with barriers engineered by use of HgTe/Hg0.05Cd0.95Te superlattices have, potentially, substantially better valence band alignment without the need for p-type doping.