Field-effect Transistor Configuration Research Articles

ENHANCEMENT OF SUPERCONDUCTIVE CRITICAL TEMPERATURES IN ALMOST EMPTY OR FULL BANDS IN TWO DIMENSIONS: POSSIBLE RELEVANCE TO β-HfNCl, C60 AND MgB2

We examine the possibility of enhancement of superconductive critical temperature in two-dimensions using the BCS theory. When the Fermi level is near an edge of a band the density of states (DOS) is not particularly high but k-dependent contribution to DOS (electron density on the Fermi surface) has a divergent peak at the bottom or top of the band (van Hove singularity). The attractive phonon-mediated interaction depends on q (momentum transfer) due to finite screening (non-local interaction). Enhancement of the critical temperatures is shown for the first time in this case. The results are qualitatively consistent with the superconductive behaviors of HfNCl (T c ≤ 25 K ) and ZrNCl (T c ≤ 15 K ), C 60 with a field-effect transistor configuration (T c = 52 K ), and MgB 2 (T c ≈ 40 K ) which have unexpectedly high critical temperatures.

Theoretical investigation on the magnetic interaction of the tetrathiafulvalene–nitronyl nitroxide stacking model: possibility of organic magnetic metals and magnetic superconductors

The possibility of realizing ferromagnetic conducting crystals was investigated theoretically. Detailed theoretical calculations were carried out for tetrathiafulvalene–nitronyl nitroxide (TTF–NN) and one- and two-hole doped states of its dimer. The result indicated that the sign of effective exchange interactions between TTF–NN is largely dependent on the stacking modes and hole doping states. Well-controlled stacking and hole doping modes of spin-polarized TTF–NN molecule led to positive effective exchange integrals ( J ab), indicating its possibility as an organic ferromagnetic metal. Thus, charge-transfer (CT) complexes with radical groups will exhibit a ferromagnetic interaction and electronic conductivity may be expected along the donor column, if appropriate columnar stacking and partially hole-doped state were realized. Implications of the calculated results are discussed in relation to the field-induced superconductivity in the field-effect-transistor (FET) and spin FET configurations.

Possibilities of molecular magnetic metals and high Tc superconductors in field effect transistor configurations

AbstractAb initio molecular orbital (MO) and density functional theory (DFT) calculations of neutral and anion radical states of anthracene are performed to elucidate their vibrational frequencies. They are used to calculate the transition temperatures of superconductivity of these species in the field effect transistor (FET) configuration on the basis of the electron–phonon coupling mechanism. Anderson and Kondo lattice models are also examined in relation to possible electronic phases of segregated columns of donors or acceptors with radical groups, and conducting polymers with radical groups in the FET configuration. The effective exchange interactions within conduction electrons and localized spins are calculated by theoretical methods. The electron or hole doping into molecular crystals and/or thin films of these systems in the FET configuration are studied in relation to possible magnetic states, Kondo and dense Kondo states, and superconducting states via spin fluctuations. The cooperative mechanism between the electron–phonon and spin fluctuation is also discussed to search the high Tc superconductor. Finally, the magnetic modifications of helical conducting polymers and DNA chains with chiral spin sources are discussed in relation to possibility of molecular solenoid, which may be generated in the FET configuration. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

Superconductivity at 52 K in hole-doped C60.

Superconductivity in electron-doped C60 was first observed almost ten years ago. The metallic state and superconductivity result from the transfer of electrons from alkaline or alkaline-earth ions to the C60 molecule, which is known to be a strong electron acceptor. For this reason, it is very difficult to remove electrons from C60--yet one might expect to see superconductivity at higher temperatures in hole-doped than in electron-doped C60, because of the higher density of electronic states in the valence band than in the conduction band. We have used the technique of gate-induced doping in a field-effect transistor configuration to introduce significant densities of holes into C60. We observe superconductivity over an extended range of hole density, with a smoothly varying transition temperature Tc that peaks at 52 K. By comparison with the well established dependence of Tc on the lattice parameter in electron-doped C60, we anticipate that Tc values significantly in excess of 100 K should be achievable in a suitably expanded, hole-doped C60 lattice.

Charge transport in the normal state of electron- or hole-dopedYBa2Cu3O7−x

By using a field-effect transistor configuration, we have demonstrated that it is possible to dope electrons or holes into an initially underdoped ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ film at room temperature. The results of systematic measurements of the dual-type transconductance indicate comparable field-effect mobilities for electrons and holes. We propose a model based on band bending and localized electronic states within the band gap of the Mott insulator to explain the dual-type charge transport.

Acoustic spectroscopy of deep centres in GaAs/AlGaAs heterostructures

Two versions of the acoustic deep-level transient spectroscopy (A-DLTS) technique based on the acoustoelectric effect resulting from the interaction between an acoustic wave and heterostructure interfaces have been used to study deep centres in GaAs/AlGaAs heterostructures. The former uses the high frequency transverse acoustoelectric signal (TAS) arrising from the interaction of surface acoustic wave electric field and free carriers at the heterostructure interfaces. The latter uses an acoustoelectric response signal (ARS) produced by the heterostructure interface when a longitudinal acoustic wave propagates through the heterostructure. Planar GaAs/AlGaAs heterostructure capacitors with electrodes in a field effect transistor configuration were investigated by these versions of the A-DLTS technique. Several deep centres were found and their activation energies and corresponding cross-sections determinated. Both the appearance of some A-DLTS peaks and the shift of practically all peaks of the A-DLTS spectrum with increasing bias voltage can be considered to be the characteristic features of interface states.

Transformer coupled stacked FET power amplifiers

The development of high-power linear ultrahigh-frequency amplifiers is made difficult by the low impedance of the devices used in the output stage, which causes matching difficulties and high radio-frequency current levels. A stacked field-effect transistor (FET) configuration is shown to reduce these problems with its increased output impedance and lower current required for a given output power. A linear analysis of the stacked FET configuration is given. Two class A monolithic microwave integrated circuit amplifiers are developed and subjected to one- and two-tone tests to demonstrate the performance of the stacked FET as a power amplifier at 900 MHz.

Two-dimensional clusters in SiGe/Si heterostructures and their effect on field effect transistor transport characteristics

Heterostructures of Si0.80Ge0.20/Si (100) were grown by pulsed laser deposition consisting of alternating 10 nm SiGe and silicon layers for a total of 10 periods. The presence of alloy clustering at SiGe/Si interfaces was investigated by parallel transport in transistor structures. Photoluminescence line broadening indicated the presence of Ge-rich clusters (20–30 nm) in layers grown at a substrate temperature of 600○C using a excimer laser at 248 nm wavelength.The 2D clusters were modeled, assuming a spherical shape and a potential of the form δΔU where δ is the composition variation and ΔU is the disorder potential. A scattering matrix element was derived which included intraband scattering. Transport effects were calculated using Monte Carlo techniques. Of interest was lateral transport in a field effect transistor configuration. Therefore, velocity versus field curves were calculated where electron motion is confined to the 2D-like Si/SiGe interface layers. Using concepts of charge control, current versus voltage relationships were derived. The derived models showed that for the case of a 250 μm conducting channel lateral length with 2D clusters evenly placed at every 10 nm, the transconductance increase is 25% and is a function of both cluster size and separation.In order to measure the effect of clustering, high electron mobility transistors were fabricated with two conducting 2D channels consisting of Si/SiGe/Si grown on high resistivity (100) silicon. Using 0.5 μm gates, a transconductance of 125 mS/mm was obtained in transistors without alloy clustering which decreased to less than 80 mS/mm when alloy clustering was present. The present investigation has related the presence of 2D nano-clusters to device performance and has also shown agreement between the experimental results and theoretical calculations.

Analysis of self-aligned MOSFETs with modulation-doped SiGe channels

An analysis of enhanced performance field-effect transistor (FET) structures, incorporating the self-aligned gate feature of conventional Si MOSFETs and the higher carrier-mobility-channel configuration of modulation-doped FETs, is presented. A modified charge-control model is developed to account for the incorporation of the MOS gate in a modulation-doped FET structure. Electrical characteristics are computed for these devices having strained SiGe layers on Si and/or Ge substrates. Theoretical results are computed using the ultra-high hole mobility reported in the literature for the modulation-doped SiGe-Ge-SiGe p-MOSFETs. It is shown that these devices will extend the use of SiGe based FETs to over the 80 GHz range for a 0.25 μm gate at 77 K, while maintaining the high packing density inherent in Si MOSFETs.

Surface-superlattice effects in a grating-gate GaAs/GaAlAs modulation doped field-effect transistor

We report transport phenomena exhibited by a two-dimensional electron gas at the interface of a modulation doped GaAs/GaAlAs heterostructure in the presence of a field-effect-controlled periodic potential modulation. By means of x-ray lithography and lift-off, a 0.2-μm-period Schottky barrier grating gate was fabricated in lieu of the common continuous gate in a field-effect transistor configuration. Conductance measurements at 4.2 K provide evidence of a superlattice effect.

Resonant tunneling transistors with controllable negative differential resistances

Three-terminal devices based on resonant tunneling through two quantum barriers separated by a quantum well are presented and analyzed theoretically. Each proposed device consists of a resonant tunneling double barrier heterostructure integrated with a Schottky barrier field-effect transistor configuration. The essential feature of these devices is the presence, in their output current-voltage ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_{D} - V_{D}</tex> ) curves, of negative differential resistances controlled by a gate voltage. Because of the high-speed characteristics associated with tunnel structures, these devices could find applications in tunable millimeter-wave oscillators, negative resistance amplifiers, and high-speed digital circuits.