Junxia Shi

Influence of acceptor-like traps in the buffer on current collapse and leakage of E-mode AlGaN/GaN MISHFETs

Current collapse has been observed in transient simulations even with highly efficient surface passivation by HfO2. In this work, current collapse and buffer-leakage mechanisms in E-mode (normally off) AlGaN/GaN metal-insulator-semiconductor heterostructure field-effect transistors (MISHFETs) have been investigated by physics-based simulations. The influences on both current collapse and off-state leakage from the energy levels of the acceptor-like traps in the buffer and also the density of the traps have been systematically studied. It is shown that traps with higher energy levels will induce less current collapse and less buffer leakage. It is demonstrated that current collapse is closely related with the change of the densities of ionized acceptor-like traps before and after the stress. Results further indicate that with increasing buffer trap density, the off-state leakage significantly decreases while current collapse increases. Finally, a non-uniform distribution of the acceptor-like traps in the buffer has been proposed to balance the trade-off between current collapse and buffer leakage.

Low-temperature thermal transport and thermopower of monolayer transition metal dichalcogenide semiconductors

We study the low temperature thermal conductivity of single-layer transition metal dichalcogenides (TMDCs). In the low temperature regime where heat is carried primarily through transport of electrons, thermal conductivity is linked to electrical conductivity through the Wiedemann-Franz law (WFL). Using a k.p Hamiltonian that describes the [Formula: see text] and [Formula: see text] valley edges, we compute the zero-frequency electric (Drude) conductivity using the Kubo formula to obtain a numerical estimate for the thermal conductivity. The impurity scattering determined transit time of electrons which enters the Drude expression is evaluated within the self-consistent Born approximation. The analytic expressions derived show that low temperature thermal conductivity (1) is determined by the band gap at the valley edges in monolayer TMDCs and (2) in presence of disorder which can give rise to the variable range hopping regime, there is a distinct reduction. Additionally, we compute the Mott thermopower and demonstrate that under a high frequency light beam, a valley-resolved thermopower can be obtained. A closing summary reviews the implications of results followed by a brief discussion on applicability of the WFL and its breakdown in context of the presented calculations.

Effects of passivation on breakdown voltage and leakage current of normally-off InAlN/GaN MISHFETs—a simulation study

Effects of passivation on breakdown characteristics of normally-off InAlN/GaN MISHFETs have been investigated via physics based simulations. The product of passivation layer permittivity and thickness is found to dramatically affect breakdown voltage, due to alleviation of peak electric field at the gate edge on the drain side, leading to a much smoother field distribution in the channel. A proposed structure with a 985 nm TiO2 top and 15 nm HfO2 bottom passivation stack exhibits a breakdown at ~750 V with leakage current ~4 μA mm−1, showing ~15× increase in breakdown voltage compared to the structure without the TiO2 top layer (50 V).

Spin-dependent magneto-thermopower of narrow-gap lead chalcogenide quantum wells

A semi-classical analysis of magneto-thermopower behaviour, namely, the Seebeck and Nernst effect (NE) in quantum wells of IV-VI lead salts with significant extrinsic Rashba spin-orbit coupling (RSOC) is performed in this report. In addition to the spin-dependent Seebeck effect that has been observed before, we also theoretically predict a similar spin-delineated behavior for its magneto-thermal analog, the spin-dependent NE. The choice of lead salts follows from a two-fold advantage they offer, in part, to their superior thermoelectric properties, especially PbTe, while their low band gaps and high spin-orbit coupling make them ideal candidates to study RSOC governed effects in nanostructures. The calculations show a larger longitudinal magneto-thermopower for the spin-up electrons while the transverse components are nearly identical. In contrast, for a magnetic field free case, the related power factor calculations reveal a significantly higher contribution from the spin-down ensemble and suffer a reduction with an increase in the electron density. We also discuss qualitatively the limitations of the semi-classical approach for the extreme case of a high magnetic field and allude to the observed thermopower behaviour when the quantum Hall regime is operational. Finally, techniques to modulate the thermopower are briefly outlined.

Rashba-driven anomalous Nernst conductivity of lead chalcogenide films

The presence of a finite Berry curvature

The electrothermal conductance and heat capacity of black phosphorus

\left(\Omega\left(k\right)\right)

Optically adjustable valley Hall current in single-layer transition metal dichalcogenides

leads to anomalous thermal effects. In this letter, we compute the coefficients for the anomalous Nernst effect

Incorporation of Al or Hf in atomic layer deposition TiO2 for ternary dielectric gate insulation of InAlN/GaN and AlGaN/GaN metal-insulator-semiconductor-heterojunction structure

\left(ANE\right)

Investigating compositional effects of atomic layer deposition ternary dielectric Ti-Al-O on metal-insulator-semiconductor heterojunction capacitor structure for gate insulation of InAlN/GaN and AlGaN/GaN

and its spin analogue, the spin Nernst effect

Influence of delta doping on intersubband transition and absorption in AlGaN/GaN step quantum wells for terahertz applications

\left(SNE\right)