S. Qureshi

A Novel Partial-Ground-Plane-Based MOSFET on Selective Buried Oxide: 2-D Simulation Study

A novel partial-ground-plane (PGP)-based MOSFET on a selective buried oxide (SELBOX), named PGP-SELBOX, is proposed. An extensive simulation study and the comparative analysis of the key characteristics of the PGP-SELBOX, the SELBOX, and the conventional silicon-on-insulator (SOI) devices has been performed using the 2-D device simulator Medici. The simulations have revealed that the PGP-SELBOX and the SELBOX structures are more thermally efficient than the conventional SOI device. Further, the magnitude of the short-channel effects (SCEs) is lower in the PGP-SELBOX in comparison to the SELBOX device. Though the SCE suppression is best in the thin-film SOI device, the PGP-SELBOX shows better improvement in SCE suppression in comparison to the SELBOX device. The suppression of self-heating effects and SCEs in the PGP-SELBOX results in a significant reduction in leakage current. An improved performance in terms of I ON/I OFF ratio is obtained in the PGP-SELBOX device. Further, the fT values of the PGP-SELBOX are comparable to those of the SELBOX and the SOI devices. A process flow in which a low-dose separation by implantation of oxygen technique can be employed for the fabrication of the PGP-SELBOX is also proposed.

Amorphous silicon based radiation detectors

We describe the characteristics of thin(1μm) and thick (>30μm) hydrogenated amorphous silicon p-i-n diodes which are optimized for detecting and recording the spatial distribution of charged particles, x-rays and γ rays. For x-ray, γ ray, and charged particle detection we can use thin p-i-n photosensitive diode arrays coupled to evaporated layers of suitable scintillators. For direct detection of charged particles with high resistance to radiation damage, we use the thick p-i-n diode arrays.

Signal, recombination effects and noise in amorphous silicon detectors

Abstract Some properties of hydrogenated amorphous silicon diodes are described. Back biased diodes of the Schottky, p-i-n type, in thicknesses ranging from 5–15 μm, have been tested with 6 MeV alpha particles and with 1 and 2 MeV protons. Large signal saturation, due to electron-hole recombination, occurs for high LET particles. Diodes have been exposed to fast neutron fluences up to 10 13 cm −2 and shown to have better radiation resistance than similarly exposed crystalline silicon detectors. From our measurements we extrapolate that minimum ionizing particles can be detected with stacked layers 100–120 μm thick, with adequate signal/noise levels.

A novel δ-doped partially insulated dopant-segregated Schottky barrier SOI MOSFET for analog/RF applications

In this paper, a comparative analysis of single-gate dopant-segregated Schottky barrier (DSSB) SOI MOSFET and raised source/drain ultrathin-body SOI MOSFET (RSD UTB) has been carried out to explore the thermal efficiency, scalability and analog/RF performance of these devices. A novel p-type δ-doped partially insulated DSSB SOI MOSFET (DSSB Pi-OX-δ) has been proposed to reduce the self-heating effect and to improve the high-frequency performance of DSSB SOI MOSFET over RSD UTB. The improved analog/RF figures of merit such as transconductance, transconductance generation factor, unity-gain frequency, maximum oscillation frequency, short-circuit current gain and unilateral power gain in DSSB Pi-OX-δ MOSFET show the suitability of this device for analog/RF applications. The reduced drain-induced barrier lowering, subthreshold swing and parasitic capacitances also make this device highly scalable. By using mixed-mode simulation capability of MEDICI simulator a cascode amplifier has been implemented using all the structures (RSD UTB, DSSB SOI and DSSB Pi-OX-δ MOSFETs). The results of this implementation show that the gain-bandwidth product in the case of DSSB Pi-OX-δ MOSFET has improved by 50% as compared to RSD UTB and by 20% as compared to DSSB SOI MOSFET. The detailed fabrication flow of DSSB Pi-OX-δ MOSFET has been proposed which shows that with the bare minimum of steps the performance of DSSB SOI MOSFET can be improved significantly in comparison to RSD UTB.

Engineering spacers in dopant-segregated Schottky barrier SOI MOSFET for nanoscale CMOS logic circuits

In this paper, it has been shown that employing an underlap channel created by using the dual spacers in dopant-segregated Schottky barrier (DSSB) SOI MOSFET not only reduces the off-state leakage, short-channel effects and the parasitic overlap capacitances but also suppresses the variability induced by process fluctuations in the Schottky barrier height, dopant-segregation length and SOI film thickness of the device. However, the reduced effective gate voltage due to voltage drop across the underlap lengths also reduces the on-state drive current of the device. To alleviate this trade-off, a novel dual-k spacer underlap channel DSSB SOI structure has also been proposed in which the increased fringing electric field effect due to high-k inner spacer layer not only improves the on-state drive current but also reduces the off-state leakage current in both n-channel and p-channel devices. Despite the presence of high-k inner spacer layer increasing the fringing gate capacitance, the scalability in an optimized dual-k spacer underlap structure has improved by ∼60% and ∼35%, respectively over the conventional spacer overlap and underlap channel structures. In addition, the variability in an optimized dual-k spacer underlap structure has also been reduced by ∼50% and ∼30% respectively over the conventional spacer overlap and underlap channel structures. This clearly indicates that the proposed dual-k spacer underlap structure is a better choice for low-variability nanoscale CMOS logic circuits. The detailed fabrication flow of this novel device has also been proposed which demonstrates the use of conventional CMOS processes.

A novel high breakdown voltage lateral bipolar transistor on SOI with multizone doping and multistep oxide

A novel high breakdown voltage lateral bipolar junction transistor (LBJT) on silicon-on- insulator (SOI) is proposed. The novelty of the device is the use of the combination of multistep-doped drift region and multistep buried oxide. The steps in doping and in oxide thickness have been used as a replacement for much complex linearly varying drift doping and linearly varying oxide thickness. The LBJT structure incorporating the combination of multistep doping and multistep oxide is analyzed for electrical characteristics using a two-dimensional numerical simulator MEDICI. Numerical simulation has demonstrated that the breakdown voltage of the proposed device with a two-zone step doped (TZSD) drift region is >150% higher than the conventional device. It has been observed that increasing the number of doping zones to 3 from 2 results in a >40% rise in breakdown voltage. The proposed device gives high breakdown voltage even at high doping concentration in the collector drift region. This reduces the on-resistance of the device and thus improves its speed. The dependence of breakdown voltage on various device parameters has been extensively studied to achieve optimum device performance. A process flow for the device fabrication is also being proposed.

THEORETICAL STUDY ON THE EFFECT OF VACANCY DEFECT RECONSTRUCTION ON ELECTRON TRANSPORT IN Si-C NANOTUBES

We investigate the effect of vacancy defect reconstruction on electron transport properties in a (4, 0) zigzag and (5, 5) armchair silicon-carbide nanotubes (SiCNTs) by applying self consistent non-equilibrium Greens function formalism in combination with the density-functional theory to a two probe molecular junction constructed from SiCNTs. The geometry optimization results show that single vacancies and di-vacancies in SiCNTs have different reconstructions. A single vacancy when optimized, reconstructs into a 5-1DB configuration in both zigzag and armchair SiCNTs, and a di-vacancy reconstructs into a 5-8-5 configuration in zigzag and into a 5-2DB configuration in armchair SiCNTs. Analysis of frontier molecular orbitals (FMO) and transmission spectrum show that the vacancy defect increases the band gap of (4, 0) metallic SiCNT and decreases the band gap of (5, 5) semiconducting SiCNT. Bias voltage dependent current characteristic show reduction in overall current in metallic SiCNT and an increase in overall current in semiconducting SiCNT.

Applications of a-Si:H radiation detectors

Abstract Detection of various kinds of radiation (charged particles, X-ray and γ ray) with hydrogenated amorphous silicon (a-Si:H) layers is discussed. Some applications of a-Si:H radiation detectors in high energy physics, medical imaging, materials and life sciences are described in this paper.

Impact of Segregation Layer on Scalability and Analog/RF Performance of Nanoscale Schottky Barrier SOI MOSFET

In this paper, the impact of segregation layer density (NDSL) and length (LDSL) on scalability and analog/RF performance of dopant-segregated Schottky barrier (DSSB) SOI MOSFET has been investigated in sub-30 nm regime. It has been found that, although by increasing the NDSL the increased off-state leakage, short-channel effects and the parasitic capacitances limits the scalability, the reduced Schottky barrier width at source-to-channel interface improves the analog/RF figures of merit of this device. Moreover, although by reducing the LDSL the increased voltage drop across the underlap length reduces the drive current, the increased effective channel length improves the scalability of this device. Further, the gain-bandwidth product in a commonsource amplifier based on optimized DSSB SOI MOSFET has improved by ~40% over an amplifier based on raised source/drain ultrathin-body SOI MOSFET. Thus, optimizing NDSL and LDSL of DSSB SOI MOSFET makes it a suitable candidate for future nanoscale analog/RF circuits

Detection of minimum-ionizing particles in hydrogenated amorphous silicon

Abstract Based on previously-reported results of the successful detection of alpha particles and 1 and 2 MeV protons with hydrogenated amorphous silicon (a-Si:H) diodes, detection of a single minimum-ionizing particle will require a total sensitive thickness of approximately 100–150 μm, either in the form of a single thick diode, or as a stack of several thinner diodes. Signal saturation at high d E d x makes it necessary to simulate minimum ionization in order to evaluate present detectors. Two techniques, using pulsed infrared light, and pulsed X-rays, give single-pulse signals large enough for direct measurements. A third, using beta rays, requires multiple-transit signal averaging to produce signals measurable above noise. Signal amplitudes from the a-Si: H limit at 60% of the signal size from Si crystals extrapolated to the same thickness. This is consistent with an a-Si: H radiation ionization energy, W = 6 eV/electron-hole pair. Beta-ray signals are observed at the expected amplitude.