Thermionic Emission Diffusion Model Research Articles

Schottky barrier height engineering in vertical p-type GaN Schottky barrier diodes for high-temperature operation up to 800 K

We have fabricated vertical p-type gallium nitride (GaN) Schottky barrier diodes (SBDs) using various Schottky metals such as Pt, Pd, Ni, Mo, Ti, and Al. Current–voltage characteristics revealed that Schottky barrier heights determined using a thermionic emission (TE) model (ϕBTE) were ranged between 1.90 eV for Pt and 2.56 eV for Mo depending on the work function (ϕm) of the Schottky metals. Despite their low ϕm, Ti and Al gave unusually small ϕBTE probably due to the interfacial reaction between metal and p-type GaN. We also found that Mo/p-GaN SBDs exhibited a clear rectifying property even at 800 K, and the thermionic emission diffusion (TED) model explained well their high-temperature I–V characteristics. Furthermore, the temperature variation of Schottky barrier heights determined using a TED model (ϕBTED) almost agrees with half of the temperature variation of the bandgap energy. These findings will be helpful for the application of p-type GaN Schottky interfaces to high-power and high-temperature electronics.

Electrical characterization of GaN Schottky barrier diode at cryogenic temperatures

In this report, electrical characteristics of the Ni/GaN Schottky barrier diode grown on sapphire have been investigated in the range of 20 K–300 K, using current–voltage, capacitance–voltage, and deep level transient spectroscopy (DLTS). A unified forward current model, namely a modified thermionic emission diffusion model, has been developed to explain the forward characteristics, especially in the regime with a large ideality factor. Three leakage current mechanisms and their applicability boundaries have been identified for various bias conditions and temperature ranges: Frenkel–Poole emission for temperatures above 110 K; variable range hopping (VRH) for 20 K–110 K, but with a reverse bias less than 20 V; high-field VRH, in a similar form of Fowler–Nordheim tunneling, for cryogenic temperatures below 110 K, and relatively large bias (>25 V). Four trap levels with their energy separations from the conduction band edge of 0.100 ± 0.030 eV, 0.300 eV, 0.311 eV, and 0.362 eV have been tagged together with their capture cross sections and trap concentrations. The significantly reduced DLTS signal at 100 K suggested that traps practically became inactive at cryogenic temperatures, thus greatly suppressing the trap-assisted carrier hopping effects.

Effects of H2O2 treatment on the temperature-dependent behavior of carrier transport and the optoelectronic properties for sol–gel grown MoS2/Si nanowire/Si devices

The effects of H2O2 treatment on the temperature-dependent behavior of carrier transport and the optoelectronic properties of a MoS2/Si nanowire (SiNW)/n-Si device are studied. The MoS2 thin films are prepared using the sol–gel method. The thermionic emission–diffusion model is the dominant process in the MoS2/SiNW/n-Si device when there is no H2O2 treatment. However, carrier transport in MoS2/SiNW/n-Si devices that are subject to H2O2 treatment is dominated by thermionic emission, so it demonstrates reliable rectification. Passivation of the SiNW surface increases the responsivity to solar irradiation. There is a low trap density at the MoS2/SiNW interfaces so the increase in photocurrent density for the MoS2/SiNW/n-Si device that is subject to H2O2 treatment is due to greater internal power conversion efficiency. The photo-response results for MoS2/SiNW/n-Si devices that are subject to (are not subject to) H2O2 treatment confirm that the decay in the photocurrent is due to the dominance of long-lifetime (short-lifetime) charge trapping. MoS2/SiNW/n-Si devices that are subject to H2O2 treatment exhibit reliable responsivity to solar irradiation.

Responsivity to solar irradiation and the behavior of carrier transports for MoS2/Si and MoS2/Si nanowires/Si devices

The behavior of carrier transports and the responsivity to solar irradiation are studied for MoS2/n-type Si (n-Si) and MoS2/Si nanowires (SiNWs)/n-Si device. The MoS2 thin films were prepared by the sol–gel method. The MoS2/n-Si and MoS2/SiNWs/n-Si devices exhibit reliable rectification behavior. The thermionic emission-diffusion model is the dominant process in these fabricated MoS2/n-Si and MoS2/SiNWs/n-Si devices. By applying the insertion of the SiNWs, the responsivity to solar irradiation can be effectively increased. Because of the low reflectance values for the MoS2/SiNWs/n-Si sample, the increased photocurrent density for the MoS2/SiNWs/n-Si device is due to high external light injection efficiency. MoS2/SiNWs/n-Si devices benefit from the high surface to volume ratio for SiNWs leading to a high responsivity of the device. The photo-response results confirm that the decay in the photocurrent is due to the dominance of short-lifetime electron trapping in the MoS2 thin film.

Behavior of carrier transports and their sensitivity to solar irradiation for devices that use MoS2 that is directly deposited on Si using the chemical vapor method

To fabricate a MoS2/Si device, layers of MoS2 are directly deposited on an n-type Si substrate by chemical vapor deposition (CVD). No transfer processes are needed. The MoS2 thin film that is deposited on the n-type Si substrate exhibits p-type behavior and the MoS2/Si device exhibits stable rectification behavior. It is found that the thermionic emission–diffusion model is the dominant process in this fabricated MoS2/Si device. The MoS2/Si device also exhibits high sensitivity to solar irradiation. Because of the low reflectance values for the MoS2/Si samples, the enhanced sensitivity is due to high external light injection efficiency. This study provides valuable scientific information for multiple layered MoS2 films for other electronic and optoelectronic applications.

Gaussian distribution of Schottky barrier heights on SnO2 nanowires

ABSTRACTIn this work, we studied metal/SnO2 junctions using transport properties. Parameters such as barrier height, ideality factor and series resistance were estimated at different temperatures. Schottky barrier height showed a small deviation of the theoretical value mainly because the barrier was considered fixed as described by ideal thermionic emission-diffusion model. These deviations have been explained by assuming the presence of barrier height inhomogeneities. Such assumption can also explain the high ideality factor as well as the Schottky barrier height and ideality factor dependence on temperature.

Flexible Piezotronic Strain Sensor

Strain sensors based on individual ZnO piezoelectric fine-wires (PFWs; nanowires, microwires) have been fabricated by a simple, reliable, and cost-effective technique. The electromechanical sensor device consists of a single electrically connected PFW that is placed on the outer surface of a flexible polystyrene (PS) substrate and bonded at its two ends. The entire device is fully packaged by a polydimethylsiloxane (PDMS) thin layer. The PFW has Schottky contacts at its two ends but with distinctly different barrier heights. The I- V characteristic is highly sensitive to strain mainly due to the change in Schottky barrier height (SBH), which scales linear with strain. The change in SBH is suggested owing to the strain induced band structure change and piezoelectric effect. The experimental data can be well-described by the thermionic emission-diffusion model. A gauge factor of as high as 1250 has been demonstrated, which is 25% higher than the best gauge factor demonstrated for carbon nanotubes. The strain sensor developed here has applications in strain and stress measurements in cell biology, biomedical sciences, MEMS devices, structure monitoring, and more.

Polythiophene mesowires: synthesis by template wetting and local electrical characterisation of single wires

We report on the synthesis of semiconducting mesowires from regioregular poly(3-hexylthiophene) via melt injection into a porous alumina template. Following liberation from the template, mesowires with diameters ∼450 nm and average lengths of 10 µm were obtained. For two-terminal electrical contacting of individual mesowires on insulating substrates, the drain electrode was formed by shadow masking and metal evaporation, and a conducting probe atomic force microscope tip was employed as the source electrode. This approach enabled combined topographic imaging and spatially resolved electrical characterisation of individual mesowires, allowing the intrinsic mesowire resistivity (700 ± 300 Ω m) as well as the contact resistance (10 ± 6 GΩ) to be estimated. Fitting the measured data to a thermionic emission–diffusion model yielded a hole mobility ∼2 × 10−5 cm2 V−1 s−1 and a metal–polymer interface barrier height ∼0.1 eV.

Current Transport Modeling in an Amorphous Silicon Antifuse Structure

We have studied the transport mechanisms in thin film (≈ 2000 Å ) hydrogenated α-Si structures used as programmable antifuses in Field-Programmable-Gate-Arrays (FPGA) The antifuse was simulated using a back-to-back Schottky model for a metal/Si/metal thin film incorporating the thermionic-emission diffusion model for the metal-semiconductor contacts. The model predicts the current transport in the low voltage and high voltage regions. In the intermediate voltage range a linear field dependence of the barrier lowering is observed in the experimental results which leads to higher currents than predicted with the image force barrier lowering in the simulations. An important observation is that carrier multiplication is essential for breakdown which is evidence of impact ionization in α-Si. The permanent breakdown condition is modelled thermally, based on the steady- state solution to the heat equation.

Thermionic-emission diffusion model of current conduction in polycrystalline silicon

A comprehensive model of current conduction in polycrystalline silicon based on thermionic-emission diffusion theory is developed. On the basis of this model, a more general expression for the effective majority-carrier mobility is derived which reduces to thermionic-emission (TE) theory-based expressions under certain simplifying assumptions and also helps in understanding the physical significance of the scaling factor used by earlier workers to explain their experimental results.

Thermionic emission diffusion model of current conduction in polycrystalline silicon and temperature dependence of mobility

In this paper a comprehensive model of current conduction in polycrystalline silicon (polysilicon) based on the thermionic-emission-diffusion (TED) theory is developed, and on the basis of this model an expression for the effective majority carrier mobility μeff is derived. This expression is quite general in nature and some thermionic emission (TE) theory based expressions for μeff can be obtained from it straightaway under certain simplifying assumptions. In addition, it helps in understanding the physical significance of the scaling factor used by earlier workers to explain their experimental results. Also, the experimental data on Hall mobility, which we obtained under an ohmic conduction regime in the 300–440 K temperature range for dark and illuminated conditions in lightly doped n-type polysilicon samples of different grain sizes, are presented and are interpreted on the basis of the TED model. Under strong illumination, the Hall mobility μHL was observed to vary with temperature T according to the relation μHL=aT−b, where a depended on grain size and was found to be smaller for the smaller grain size. The dark mobility data fitted well into the TED-based expression for μeff considering the interface states associated with grain boundaries to be localized at Ev+0.63 eV in the band gap. The analysis reveals that, generally, the scaling factor is needed if the effect of diffusion is neglected in comparison with the thermionic emission while in essence it is appreciable to be considered. However, in the TED model, as diffusion contribution in controlling the current transport across the grain-boundary potential barrier is well taken care of, the scaling factor is not required.