Simulation Study of the Use of AlGaN/GaN Ultra-Thin-Barrier HEMTs with Hybrid Gates for Achieving a Wide Threshold Voltage Modulation Range

In this work, novel hybrid gate Ultra-Thin-Barrier HEMTs (HG-UTB HEMTs) featuring a wide modulation range of threshold voltages (VTH) are proposed. The hybrid gate structure consists of a p-GaN gate part and a MIS-gate part. Due to the depletion effect assisted by the p-GaN gate part, the VTH of HG-UTB HEMTs can be significantly increased. By tailoring the hole concentration of the p-GaN gate, the VTH can be flexibly modulated from 1.63 V to 3.84 V. Moreover, the MIS-gate part enables the effective reduction in the electric field (E-field) peak at the drain-side edge of the p-GaN gate, which reduces the potential gate degradation originating from the high E-field in the p-GaN gate. Meanwhile, the HG-UTB HEMTs exhibit a maximum drain current as high as 701 mA/mm and correspond to an on-resistance of 10.1 Ω mm and a breakdown voltage of 610 V. The proposed HG-UTB HEMTs are a potential means to achieve normally off GaN HEMTs with a promising device performance and featuring a flexible VTH modulation range, which is of great interest for versatile power applications.

High Thermoelectric Performance Achieved in Sb-Doped GeTe by Manipulating Carrier Concentration and Nanoscale Twin Grains

Lead-free and eco-friendly GeTe shows promising mid-temperature thermoelectric applications. However, a low Seebeck coefficient due to its intrinsically high hole concentration induced by Ge vacancies, and a relatively high thermal conductivity result in inferior thermoelectric performance in pristine GeTe. Extrinsic dopants such as Sb, Bi, and Y could play a crucial role in regulating the hole concentration of GeTe because of their different valence states as cations and high solubility in GeTe. Here we investigate the thermoelectric performance of GeTe upon Sb doping, and demonstrate a high maximum zT value up to 1.88 in Ge0.90Sb0.10Te as a result of the significant suppression in thermal conductivity while maintaining a high power factor. The maintained high power factor is due to the markable enhancement in the Seebeck coefficient, which could be attributed to the significant suppression of hole concentration and the valence band convergence upon Sb doping, while the low thermal conductivity stems from the suppression of electronic thermal conductivity due to the increase in electrical resistivity and the lowering of lattice thermal conductivity through strengthening the phonon scattering by lattice distortion, dislocations, and twin boundaries. The excellent thermoelectric performance of Ge0.90Sb0.10Te shows good reproducibility and thermal stability. This work confirms that Ge0.90Sb0.10Te is a superior thermoelectric material for practical application.

Temperature Dependent Carrier Transport in Hydrogenated Amorphous Semiconductors for Thin Film Memristive Applications

This paper demonstrates the transport of electron and hole carriers in two distinct hydrogenated amorphous semiconductor materials at different temperatures. Compared to crystalline materials, the amorphous semiconductors differ structurally, optically and electrically, hence the nature of carrier transport through such amorphous materials differ. Materials like hydrogenated amorphous silicon and amorphous IGZO have been used for the study of temperature dependent carrier transport in this paper. Simulation results have been presented to show the variation of free electron and hole concentration, trapped electron and hole concentration with energy at 300K for both the materials. The change in mobility with a change in the Fermi level has been plotted for different temperatures. The effect of temperature on Brownian motion mobility of electrons and holes in hydrogenated amorphous silicon and amorphous IGZO has been demonstrated towards the end of this paper.

Defect engineering of BCZT-based piezoelectric ceramics with high piezoelectric properties

The intrinsic conduction mechanism and optimal sintering atmosphere of (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (BCZT) ceramics were regulated by Mn-doping element in this work. By Hall and impedance analysis, the undoped BCZT ceramics exhibit a typical n-type conduction mechanism, and the electron concentration decreases with the increasing oxygen partial pressure. Therefore, the undoped ceramics exhibit best electrical properties (piezoelectrical constant d33 = 585 pC·N−1, electro-mechanical coupling factor kp = 56%) in O2. A handful of Mn-doping element would transfer the conduction mechanism from n-type into p-type. And the hole concentration reduces with the decreasing oxygen partial pressure for Mn-doped BCZT ceramics. Therefore, the Mn-doped ceramics sintered in N2 have the highest insulation resistance and best piezoelectric properties (d33 = 505 pC·N−1, kp = 50%). The experimental results demonstrate that the Mn-doping element can effectively adjust the intrinsic conduction mechanism and then predict the optimal atmosphere.

Calculating the Effect of AlGaN Dielectric Layers in a Polarization Tunnel Junction on the Performance of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes

In this work, AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) with AlGaN as the dielectric layers in p+-Al0.55Ga0.45N/AlGaN/n+-Al0.55Ga0.45N polarization tunnel junctions (PTJs) were modeled to promote carrier tunneling, suppress current crowding, avoid optical absorption, and further enhance the performance of LEDs. AlGaN with different Al contents in PTJs were optimized by APSYS software to investigate the effect of a polarization-induced electric field (Ep) on hole tunneling in the PTJ. The results indicated that Al0.7Ga0.3N as a dielectric layer can realize a higher hole concentration and a higher radiative recombination rate in Multiple Quantum Wells (MQWs) than Al0.4Ga0.6N as the dielectric layer. In addition, Al0.7Ga0.3N as the dielectric layer has relatively high resistance, which can increase lateral current spreading and enhance the uniformity of the top emitting light of LEDs. However, the relatively high resistance of Al0.7Ga0.3N as the dielectric layer resulted in an increase in the forward voltage, so much higher biased voltage was required to enhance the hole tunneling efficiency of PTJ. Through the adoption of PTJs with Al0.7Ga0.3N as the dielectric layers, enhanced internal quantum efficiency (IQE) and optical output power will be possible.

Realizing n-type gete through suppressing the formation of cation vacancies and bi-doping*

It is known that p-type GeTe-based materials show excellent thermoelectric performance due to the favorable electronic band structure. However, n-type doping in GeTe is of challenge owing to the native Ge vacancies and high hole concentration of about 1021 cm−3. In the present work, the formation energy of cation vacancies of GeTe is increased through alloying PbSe, and further Bi-doping enables the change of carrier conduction from p-type to n-type. As a result, the n-type thermoelectric performance is obtained in GeTe-based materials. A peak zT of 0.34 at 525 K is obtained for (Ge0.6Pb0.4)0.88Bi0.12Te0.6Se0.4. These results highlight the realization of n-type doping in GeTe and pave the way for further optimization of the thermoelectric performance of n-type GeTe.

Dominant acceptors in Li doped, magnetron deposited Cu2O films

Cu2O films deposited by reactive magnetron sputtering with varying Li concentrations have been investigated by a combination of temperature-dependent Hall effect measurement and thermal admittance spectroscopy. As measured by secondary ion mass spectrometry, Li concentrations up to 5x1020 Li/cm3 have been achieved. Li doping significantly alters the electrical properties of Cu2O and increases hole concentration at room temperature for higher Li concentrations. Moreover, the apparent activation energy for the dominant acceptors decreases from around 0.2 eV for undoped or lightly doped Cu2O down to as low as 0.05 eV for higher Li concentrations.

Investigation on electrical characteristics of TFTs fabricated with germanium films crystallized by atmospheric pressure micro-thermal-plasma-jet irradiation

Crystalline-germanium (c-Ge) is an attractive material for a thin-film transistor (TFT) channel because of its high carrier mobility and applicability to a low-temperature process. We present the electrical characteristics of c-Ge crystallized by atmospheric pressure micro-thermal-plasma-jet (µ-TPJ). The µ-TPJ crystalized c-Ge showed the maximum Hall mobility of 1070 cm2·V−1·s−1 with its hole concentration of ~ 1016 cm−3, enabling us to fabricate the TFT with field-effect mobility (μ FE) of 196 cm2·V−1·s−1 and ON/OFF ratio (R ON/OFF) of 1.4 × 104. On the other hand, RON/OFFs and μFEs were dependent on the scanning speed of the TPJ, inferring different types of defects were induced in the channel regions. These findings show not only a possibility of the TPJ irradiation as a promising method to make a c-Ge TFT on insulating substrates.

High Hole Mobility Polycrystalline GaSb Thin Films

In this paper, we report on the structural and electronic properties of polycrystalline gallium antimonide (poly-GaSb) films (50–250 nm) deposited on p+ Si/SiO2 by metalorganic vapour phase epitaxy at 475 °C. GaSb films grown on semi-insulating GaAs substrates are included as comparative samples. In all cases, the unintentionally doped GaSb is p-type, with a hole concentration in the range of 2 × 1016 to 2 × 1017 cm−3. Exceptional hole mobilities are measured for polycrystalline GaSb on SiO2 in the range of 9–66 cm2/Vs, exceeding the reported values for many other semiconductors grown at low temperatures. A mobility of 9.1 cm2/Vs is recorded for an amorphous GaSb layer in a poly-GaAs/amorphous GaSb heterostructure. Mechanisms limiting the mobility in the GaSb thin films are investigated. It is found that for the GaSb grown directly on GaAs, the mobility is phonon-limited, while the GaSb deposited directly on SiO2 has a Coulomb scattering limited mobility, and the poly-GaAs/amorphous GaSb heterostructure on SiO2 displays a mobility which is consistent with variable-range-hopping. GaSb films grown at low temperatures demonstrate a far greater potential for implementation in p-channel devices than for implementation in ICs.