Band Gap Regulation Research Articles

Theoretical study of the bandgap regulation of a two-dimensional GeSn alloy under biaxial strain and uniaxial strain along the armchair direction.

Recently, two-dimensional germanium-tin (2D-GeSn) alloys have attracted considerable attention because they have been predicted to possess a direct bandgap, and this bandgap can be tuned by changing the Sn concentration. However, the tuning efficiency of alloying Sn is still relatively low, and alloying more Sn in 2D-GeSn is difficult to accomplish. To address this issue, the band structures for 2D-GeSn under different strain types (including biaxial and uniaxial strain along the armchair direction, as well as compressive and tensile strain) are investigated using a first-principles method based on density functional theory combined with a GGA+U method and special quasirandom structures. For tensile strain, the results indicate that both biaxially and uniaxially strained 2D-GeSn alloys exhibit direct bandgaps, and their bandgaps decrease as the strain strength increases. The bandgap tuning efficiency for biaxial strain is higher than that for uniaxial strain. For compressive strain, both biaxially and uniaxially strained 2D-GeSn alloys exhibit a large indirect bandgap area, and their bandgaps increase as the strain strength increases; however, their distribution shapes are slightly different. To uncover the physical origin of the difference between them, the projected band, the projected density of the states, the bond length and the bond angle for 2D-GeSn are analyzed. Overall, these results indicate that the combination of alloying Sn and applying an external strain is a good way to reduce the necessary Sn concentration, and this may provide comprehensive theoretical guidance for the strain energy band engineering of 2D-GeSn.

Layer Stacked Iodine and Phosphorus Co‐doped C3N4 for Enhanced Visible‐Light Photocatalytic Hydrogen Evolution

AbstractHetero‐element doping is one of the most effective methods for band gap regulation of semiconductor photocatalysts. Here, a novel layer stacking of iodine and phosphorus co‐doped carbon nitride (I/P‐CN) was prepared from a novel supramolecular precursor. The obtained I/P‐CN exhibits significantly enhanced visible‐light photocatalytic hydrogen production (93.9 μmol h−1) compared with bulk carbon nitride. Moreover, it reveals a consistent cycle stability for photocatalytic application. The marked improvement in photocatalytic activity is the result of iodine and phosphorus co‐doping bringing about a decrease in the band gap and an increase of absorption intensity and width. The unique layer in the stacking structure facilitates an increase in specific surface area and active reaction sites. Our research opens up a new strategy to synthesize multiple elements co‐doped carbon nitride as a promising metal‐free photocatalyst for hydrogen evolution under visible light.

Modulation of Metal and Insulator States in 2D Ferromagnetic VS2 by van der Waals Interaction Engineering.

2D transition-metal dichalcogenides (TMDCs) are currently the key to the development of nanoelectronics. However, TMDCs are predominantly nonmagnetic, greatly hindering the advancement of their spintronic applications. Here, an experimental realization of intrinsic magnetic ordering in a pristine TMDC lattice is reported, bringing a new class of ferromagnetic semiconductors among TMDCs. Through van der Waals (vdW) interaction engineering of 2D vanadium disulfide (VS2 ), dual regulation of spin properties and bandgap brings about intrinsic ferromagnetism along with a small bandgap, unravelling the decisive role of vdW gaps in determining the electronic states in 2D VS2 . An overall control of the electronic states of VS2 is also demonstrated: bond-enlarging triggering a metal-to-semiconductor electronic transition and bond-compression inducing metallization in 2D VS2 . The pristine VS2 lattice thus provides a new platform for precise manipulation of both charge and spin degrees of freedom in 2D TMDCs availing spintronic applications.

Photovoltaic Behaviors Regulated by Band-Gap and Bipolar Electrical Cycling in Holmium-Doped Bi5Ti3FeO15 Ferroelectric Films

The pure and Ho-doped Bi5Ti3FeO15 ferroelectric films with layered perovskite structure were prepared via a chemical solution deposition. One investigated the regulation of band gap and bipolar electrical cycling on photovoltaic behaviors including photovoltaic spectral response, open-circuit voltage, and short-circuit current for the films. It is shown that the photovoltaic response peak of Ho-doped Bi5Ti3FeO15 films shifts toward the visible region and the open-circuit voltage as well as short-circuit current are improved compared with the pure Bi5Ti3FeO15 films, which is attributed to the narrowed band gap after doping with Ho. And the spectral response, open-circuit voltage as well as short-circuit current of Ho-doped Bi5Ti3FeO15 films do not suffer from the fatigue like the pure Bi5Ti3FeO15 films with the action of bipolar electrical cycling. It just appears extended response peak without blue shift from the visible region. Likewise, Ho-doped Bi5Ti3FeO15 films still enjoy strong open-circuit voltage ...

A Low Quiescent Biased Regulator With High PSR Dedicated to Micropower Sensor Circuits

A new micropower high power-supply-rejection (PSR) regulator, on the basis of op-amp-less architectural design, is presented in this paper. The proposed regulator is based on the embodiment of the Brokaw bandgap regulator circuit in an additional feedback control loop so as to achieve high efficiency in terms of referenced PSR bandwidth per current. The measured PSR performance without any filtering capacitor for a 50-kΩ nominal resistive load at 1 kHz and 1 MHz are obtained as -76 and -43 dB, respectively. The regulator, making use of native driving devices to improve the headroom, supports the supply range from 2 to 3.3 V. The output voltage is close to 1.8 V, having an untrimmed temperature coefficient of 117 ppm/°C from - 40°C to 125°C while drawing the quiescent current of only 5.65 μA. The regulator occupies an area of 0.047 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> using current source model 1.8 V/3.3 V CMOS 0.18 μm triple-well process technology. The circuit is verified using the realistic BSIM3 models for simulation. Finally, the performance comparison with respect to the measured results of reported works, using technology normalized figure of merit, has validated that the proposed circuit technique offers good PSR results while having the advantages of low quiescent power, small size, and full integration without any off-chip capacitor. The proposed regulator is particularly suitable for micropower sensor circuits.

STYLE: a statistical design approach based on nonparametric performance macromodeling

Despite their potential benefits, statistical design/optimization techniques are often not routinely used by industry because of prohibitive costs and/or lack of generality. To overcome these obstacles, we present a general approach to statistical design which is computationally inexpensive and applicable to an extensive spectrum of analog-digital circuits, unlike previous inexpensive methods which limit the user to specific technologies, standard device models, and/or purely digital designs. This general applicability and cost efficiency is maintained through four factors. First, an efficient screening algorithm minimizes the dimensionality of the input parameter space by identifying the important input parameters. Second, a general, accurate, parsimonious nonparametric regression model is formulated so that a wide range of behavior between input parameters and output performances can be mapped, not just those with a linear regression relationship. Third, this approach tightly couples with a compiler-methodology based simulator which sustains the achieved flexibility through its model independence. Finally, a waveform analysis capability completely automates the procedure. An industry ECL bandgap regulator circuit illustrates the benefits of this method. >

Error amplifier limitations in low voltage regulators using only NPN transistors

The performance of low-voltage bandgap regulators built with only npn transistors is examined. The power supply rejection ratio (PSRR) is significantly constrained, more than any other performance parameter, by the exclusion of a complimentary active load. The PSRR for an inherently stable (single operating point) bandgap regulator built using only npn transistors is bounded below by 2.5 (nV/sub T//V/sub cc/)/sup n/=0.7 mV/V or 0.04 mV/V for circuits with n=2 or 3 stages, respectively. This represents a "gain" constraint imposed by the coincidence of signal and bias paths in bipolar circuits which employ current differencing or bootstrapping techniques. Base current errors can significantly degrade PSRR. Two all-npn circuits are presented with PSRR /spl les/0.1 mV/V that exhibit a single operating point under most circumstances.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>