Schottky-barrier Field-effect Transistors Research Articles

Analysis of Strain Effects in Ballistic Carbon Nanotube FETs

The effect of uniaxial and torsional strain on the performance of ballistic carbon nanotube (CNT) Schottky-barrier (SB) field-effect transistors (FETs) is examined by self-consistently solving the Poisson equation and the Schroumldinger equation using the nonequilibrium Green's function formalism. A mode space approach can be used to reduce the computational cost of atomistic simulations for the strained CNTs by orders of magnitude. It is shown that even a small amount of uniaxial (< 2%) or torsional (<5deg) strain can result in a large effect on the performance of the CNTFETs due to the variation of the band gap and band-structure-limited velocity. Semiconducting CNT channels with different chiralities are influenced in drastically different ways by a certain applied strain, which is determined by a (n-m) mod 3 rule. In general, a type of strain which produces a larger band gap results in increased SB height and decreased band-structure-limited velocity, and hence a smaller minimum leakage current, smaller on current, larger maximum achievable I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> /I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OFF</sub> , and larger intrinsic delay. The other type of strain that reduces the band gap results in the opposite effect on the device performance metrics of the CNTFETs

Scaling Behaviors of Graphene Nanoribbon FETs: A Three-Dimensional Quantum Simulation Study

The scaling behaviors of graphene nanoribbon (GNR) Schottky barrier field-effect transistors (SBFETs) are studied by self-consistently solving the nonequilibrium Green's function transport equation in an atomistic basis set with a 3-D Poisson equation. The armchair edge GNR channel shares similarities with a zigzag carbon nanotube; however, it has a different geometry and quantum confinement boundary condition in the transverse direction. The results indicate that the I-V characteristics are ambipolar and strongly depend on the GNR width because the bandgap of the GNR is approximately inversely proportional to its width, which agrees with recent experiments. A multiple gate geometry improves immunity to short channel effects; however, it offers smaller improvement than it does for Si MOSFETs in terms of the on-current and transconductance. Reducing the oxide thickness is more useful for improving transistor performance than using a high-k gate insulator. Significant increase of the minimal leakage current is observed when the channel length is scaled below 10 nm because the small effective mass facilitates strong source-drain tunneling. The GNRFET, therefore, does not promise to extend the ultimate scaling limit of Si MOSFETs. The intrinsic switching speed of a GNR SBFET, however, is several times faster than that of Si MOSFETs, which could lead to promising high-speed electronics applications, where the large leakage of GNR SBFETs is of less concern.

A drain current model for Schottky-barrier CNT-FETs

We present here a physics-based drain current model for Schottky-barrier carbon nanotube field-effect transistors. The model captures a number of features exhibited by these transistors such as thermionic and tunnel emission, ambipolar conduction, ballistic transport, multimode propagation and electrostatics dominated by the nanotube capacitance

Silicon-Nanowire Transistors with Intruded Nickel-Silicide Contacts

Schottky barrier field effect transistors based on individual catalytically-grown and undoped Si-nanowires (NW) have been fabricated and characterized with respect to their gate lengths. The gate length was shortened by the axial, self-aligned formation of nickel-silicide source and drain segments along the NW. The transistors with 10-30 nm NW diameters displayed p-type behaviour, sustained current densities of up to 0.5 MA/cm2, and exhibited on/off current ratios of up to 10(7). The on-currents were limited and kept constant by the Schottky contacts for gate lengths below 1 microm, and decreased exponentially for gate lengths exceeding 1 microm.

Heat dissipation in carbon nanotube transistors

Heat dissipation and its effect on current in carbon nanotube (CNT) Schottky barrier field-effect transistors are studied by solving nonequilibrium Green’s function transport equation self-consistently with a heat transport equation. Temperature rise in the semiconducting CNT channel is significantly smaller than its metallic counterpart because (i) the percentage of total power dissipated in the semiconducting CNT channel is smaller, and (ii) the heat dissipation reaches peak values at two ends of the channel. The simulation indicates that in the bias regime of interest to nanoelectronic applications, the effect of heating on the transistor I-V characteristics is small.

Intrinsic characteristics of semiconducting oxide nanobelt field-effect transistors

Field-effect transistors (FETs) based on individual semiconducting oxide (SnO2 and ZnO) nanobelts with multiterminal electrical contacts have been fabricated and characterized. Simultaneous two-terminal and four-terminal measurements enable direct correlation of the FET characteristics with the nature of the contacts. Devices with high-resistance non-Ohmic contacts exhibit a Schottky barrier FET behavior. In contrast, low-resistance Ohmic contacts on the nanobelt lead to high-performance n-channel depletion mode FETs with well-defined linear and saturation regimes, large “on” current, and an on/off ratio as high as 107. The FET characteristics of such devices show a significant modification by a 0.2% H2 gas flow at room temperature. The excellent intrinsic characteristics of these nanobelt FETs make them ideal candidates as nanoscale biological and chemical sensors based on field-effect modulation of the channel conductance.

Simulation of carbon nanotube based p–n junction diodes

Taking advantage of the unique characteristics of an ambipolar carbon nanotube field effect transistor (CNTFET), a ‘p–n junction’ is simulated along the single-walled carbon nanotube channel using two separate gates close to the source and drain of the CNTFET, respectively. The current–voltage characteristics of the double-gated CNTFET are calculated using a semiclassical method based on the Schottky barrier field effect transistor mechanism. The calculation results show a good rectification performance of the p–n junction.

A new model of noise characteristics of SiC Schottky barrier MESFET with deep impurity levels and traps

Noise characteristics of silicon carbide Schottky barrier field effect transistors (MESFET) are examined for the case of the operation in small-signal regime and the presence of deep impurity levels and electron traps in the band gap of the channel. A new model of calculations of noise is suggested. It is shown that the noise measure of the short channel MESFET can be decreased within certain high frequency range.

Accurate numerical modelling the GaAs MESFET current-voltage characteristics

In this paper, we present a computing model of the current-voltage (I-V) characteristics of a gallium arsenide Schottky barrier field effect transistor called GaAs MESFET. This physical model is based on the two-dimensional analysis of the Poisson equation in the active region under the gate. In this frame, we elaborated a simulation software based on analysis of expressions that we have previously set up (1-3), the obtained theoretical results are discussed and compared to the experimental ones.

The effect of hydrogenation on the sink breakdown voltage of transistors based on ion-doped gallium arsenide structures

It was found that the hydrogenation of ion-doped gallium arsenide structures leads to an increase in the sink breakdown voltage of high-power microwave Schottky barrier field effect transistors based on such structures (from 7 up to 17 V) and in the power of related microwave integration circuits (by a factor of up to 2.4). Data characterizing the dependence of the increase in the transistor breakdown voltage and the device power on the hydrogenation regime are presented. Possible mechanisms explaining the effect of hydrogenation on the electrical properties of gallium arsenide and the parameters of semiconductor devices are considered.

Microwave-stimulated relaxation of internal strains in GaAs-based device heterostructures

We have studied the effect of microwave radiation (frequency, 2.45 GHz; specific power density, 1.5 W/cm2) on the relaxation of internal mechanical strains in the (i)n-n+-GaAs structures, (ii) Au-Ti-n-n+-GaAs diode structures with Schottky barriers, and (iii) GaAs-based Schottky-barrier field-effect transistors (SFETs). It is shown that exposure of the samples to the microwave radiation for a few seconds leads to relaxation of the internal mechanical strains and improves the quality of the semiconductor surface layer structure. This results in improved parameters of the GaAs-based device structures of both (diode and SFET) types, as manifested by increased Schottky barrier height, reduced ideality factor and back current in the diode structures, and increased gain slope and initial drain current in the SFETs.

A 30-nm-gate field-effect transistor

The fabrication technology is developed for and characteristics are investigated of a GaAs Schottky-barrier field-effect transistor (SBFET) with an effective gate length of 30 nm. The SBFET power gain cutoff frequency is 150 GHz. The noise factor at 12–37 GHz is comparable with that of two-dimensional electron gas transistors. The theoretical electron transit time under the gate is below 0.1 ps.

Observation of a long-range action effect in ion-bombarded GaAs transistor structures

Changes in the carrier concentration and mobility in the active layers of Schottky-barrier field-effect transistors are observed when the structures are bombarded with argon ions on the nonworking side of the GaAs substrate.

Fluctuating deep-level trap occupancy model for Hooge’s 1/f noise parameter for semiconductor resistors

A theoretical expression for Hooge’s 1/f noise parameter α, for a Schottky barrier field-effect transistor, which has been biased at a small drain-source voltage (a gate-controlled semiconductor resistor), has been derived. The theory is based on the fluctuating occupancy of deep level traps in the depletion region. The theory explains the large variations in the observed values of Hooge’s parameter since the derived expression shows that α varies approximately as n−7/2, where n is the electron density, and that α is sensitive to the trap concentration, the gate (or semiconductor surface) potential, the thickness of the semiconductor conducting layer, and the low-field electron mobility-depletion depth profile. Detailed experimental characterization of a semiconductor resistor has been carried out resulting in the accurate determination of α over a range of the applied gate voltage, Vg. We obtain excellent agreement between the theoretical and experimental dependence of α on Vg.

Operation of Schottky-barrier field-effect transistors of 3C-SiC up to 400 °C

Schottky-barrier field-effect transistors have been fabricated from 3C-SiC and transistor operation has been studied at temperatures up to 400 °C. B-doped high-resistivity and undoped n-type 3C-SiC epilayers were successively grown on p-type Si substrates by chemical vapor deposition. Au and Al electrodes were used for Schottky-barrier gate contacts and source and drain contacts for n-type 3C-SiC, respectively. Transconductances of 1.7 and 0.15 mS/mm were obtained at room temperature and 400 °C, respectively.

Schottky-barrier field-effect transistors of 3C-SiC

Schottky-barrier field-effect transistors have been fabricated first from 3C-type SiC. Al-doped p-type and nondoped n-type 3C-SiC epilayers were successively grown on p-type Si substrates by chemical vapor deposition. Au and Al electrodes were used for Schottky-barrier gate contacts and ohmic (source and drain) contacts for n-type SiC. Transistor operation was observed for the first time for the field-effect transistors of 3C-SiC.

Measurement of the low-field electron mobility and compensation ratio profiles in GaAs field-effect transistors

A novel technique for measuring the low-field electron mobility as a function of depth into the active layer of a nonuniformly doped GaAs Schottky barrier field-effect transistor (MESFET) has been developed. This technique is based on measurements of the current-voltage characteristics and the transconductance under low-field conditions as well as capacitance-voltage measurements. By comparison of the measured electron mobility with previous theoretical results the compensation ratio profile can be determined. Measurements on ion-implanted and epitaxial GaAs MESFET’s show that, for depths &amp;gt;0.1 μm, the nonuniform mobility profile is correlated with the donor density profile and sensitive to the compensation ratio, indicating that ionized impurity scattering is important over the range of carrier densities typically used in these devices.

Characteristics and mechanism of 1/f noise in GaAs Schottky barrier field-effect transistors

Measurements of the absolute spectral density of 1/f noise in GaAs Schottky barrier field-effect transistors (MESFET’s) show that 1/f noise consists of a surface component and a separable uncorrelated bulk component. The surface component is caused by trapping of electrons by surface states. The bulk component is correlated with the low field electron mobility and deep level trap concentration versus depletion depth profiles and can only be explained by the random fluctuations in the occupancy of deep level traps in the depletion region. A simple model quantitatively explains the observed characteristics of the bulk component of the noise at a low drain voltage. At normal operating drain voltages the observed 1/f noise characteristics are consistent with the model.

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.

A High-Speed Amorphous-Silicon Dytamic Circuit

AbstractA novel dynamic circuit composed of amorphous-silicon Schottkybarrier diodes and field-effect transistors has been proposed. The circuit response time is as short as the discharging time of a load capacitor through the driver transistor. The circuit having 1µm-long, self-aligned transistors has been predicted theoretically to be able to be operated at multi-MHz rates. Preliminary experimental results are also presented.