W. D. Hu

Two-dimensional transient simulations of drain lag and current collapse in GaN-based high-electron-mobility transistors

The intrinsic mechanisms of drain lag and current collapse in GaN-based high-electron-mobility transistors are studied by using two-dimensional numerical simulations. Simulated drain lag characteristics are in good agreement with reported experimental data. The dynamic pictures of trapping of hot electrons under drain-pulse voltages are discussed in detail. Hot-electron buffer-trapping effect plays an instrumental role in the current collapse mechanism. Polarization-induced interface charges have significant effect on the hot-electron buffer trapping and the current collapse can be weakened by increasing the interface charges. The trapped charges can accumulate at the drain-side gate edge, where the electric field significantly changes and gate-to-drain-voltage-dependent strain is induced, causing a notable current collapse. The simulation results show that the drain voltage range, beyond 5 V, is already in the field of the well-developed hot electron regime. The hot electrons can occupy a great number of...

Self-heating simulation of GaN-based metal-oxide-semiconductor high-electron-mobility transistors including hot electron and quantum effects

Undoped GaN-based metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs) with atomic-layer-deposited Al2O3 gate dielectrics are fabricated with gate lengths from 1 up to 40μm. Using a two-dimensional numerical simulator, we report the results of self-heating simulations of the GaN-based MOS-HEMTs, including hot electron and quantum effects. The simulated electrical characteristics are in good agreement with reported experimental data. The effect of the gate and source/drain extension lengths on both the output performance and self-heating is discussed in detail, allowing for device optimization. The dissipated Joule electric power causes the self-heating effects, which lead to negative differential output conductance. Our results demonstrate that the hot electrons make a negligible contribution to the negative differential output conductance in our long channel MOS-HEMTs. In order to investigate their joint interactions to the MOS-HEMT’s operation, the different static interface trap and...

Parameter determination from resistance-voltage curve for long-wavelength HgCdTe photodiode

A data-processing approach has been developed to obtain device parameters from resistance-voltage (R-V) curves measured on long-wavelength HgCdTe n-on-p photodiodes. The physical model used for R-V curve fitting includes the dark current mechanisms induced by diffusion, generation recombination, trap-assisted tunneling, and band-to-band tunneling. Moreover, the series resistance effect is also taken into account. Six parameters, which include the dopant density Nd in the n region, the ratio of mobility to lifetime of electrons μn∕τn in the p region, the effective lifetime τ0 in the depletion region, the relative energy position of trap level Et∕Eg and its density Nt in the depletion region, and the series resistance Rs, can be extracted from measured R-V curves. The fitting procedure has been presented in detail and the error ranges of the extracted parameters have been discussed. By fitting to the R-V characteristics of three long-wavelength devices with different Cd compositions, the applicability of ou...

Demonstration and dynamic analysis of trapping of hot electrons at gate edge model for current collapse and gate lag in GaN-based high-electron-mobility transistor including self-heating effect

Using a two-dimensional simulator, the authors report on demonstration of trapping of hot electrons at gate edge model in GaN-based high-electron-mobility transistors. Dynamic picture of hot electrons under gate pulse voltage is discussed in detail. Trapped charges may accumulate under punch-off gate voltage at gate edge drain side, where the electric field significantly changes and gate-voltage-dependent strain is induced. Significant band barrier is formed at the gate edges causing a notable current collapse. Self-heating effect is one of the reasons for current collapse and gate lag.

The role of ultrathin AlN barrier in the reduction in the hot electron and self-heating effects for GaN-based double-heterojunction high electron mobility transistors

We propose an AlN/GaN/InGaN/GaN double-heterojunction high electron mobility transistor (DH-HEMT) structure with a 4 nm thin AlN barrier layer. The performance of the DH-HEMT device is investigated by using two-dimensional numerical simulation. The conduction band profile is obtained by using the Poisson’s equation and Fermi–Dirac statistics in combination with the polarization charges. Due to large conduction-band offset of the AlN/GaN interface and strong polarization of AlN, the minor channel at GaN/InGaN interface can be eliminated. Further, the hot electron and self-heating effects on the transport properties of this DH-HEMT are investigated by using hydrodynamic model. In comparison with the AlGaN barrier DH-HEMT and conventional HEMT, this kind of DH-HEMT can effectively reduce the hot electron effect under high voltage. The reason is that the maximum field strength is far below the critical value for the existence of the hot electron effect in the AlGaN barrier DH-HEMTs and conventional HEMTs with...

An experimental acoustic cloak for generating virtual images

A two-dimensional acoustic cloak is realized to generate virtual image in air. The cloak is designed to make the backscattering characteristics of a quadrangular prism the same as that of a plate based on transformation acoustics. The required anisotropic parameters of the cloak shell are obtained by acoustic metamaterial made of perforated plates, which are easy to fabricate and unitize. The measurements of the backward and near-backward scattering fields confirm the validity of the proposed cloak. Experimental results show the possibility to hide the shape of an object by changing the reflection properties by using transformation acoustics.

The role of localized junction leakage in the temperature-dependent laser-beam-induced current spectra for HgCdTe infrared focal plane array photodiodes

We have performed the study on the dependence of laser beam induced current (LBIC) spectra on the temperature for the vacancy-doped molecular beam epitaxy grown Hg1−xCdxTe (x = 0.31) photodiodes by both experiment and numerical simulations. It is found that the measured LBIC signal has different distributions for different temperature extents. The LBIC profile tends to be more asymmetric with increasing temperature below 170 K. But the LBIC profile becomes more symmetric with increasing temperature above 170 K. Based on a localized leakage model, it is indicated that the localized junction leakage can lead to asymmetric LBIC signal, in good agreement with the experimental data. The reason is that the trap-assisted tunneling current is the dominant leakage current at the cryogenic temperature below 170 K while the diffusion current component becomes dominant above the temperature of 170 K. The results are helpful for us to better clarify the mechanism of the dependence of LBIC spectra on temperature for th...

Development of an acoustic filter for parametric loudspeaker using phononic crystals.

The spurious signal generated as a result of nonlinearity at the receiving system affects the measurement of the difference-frequency sound in the parametric loudspeaker, especially in the nearfield or near the beam axis. In this paper, an acoustic filter is designed using phononic crystals and its theoretical simulations are carried out by quasi-one- and two-dimensional models with Comsol Multiphysics. According to the simulated transmission loss (TL), an acoustic filter is prototyped consisting of 5×7 aluminum alloy cylinders and its performance is verified experimentally. There is good agreement with the simulation result for TL. After applying our proposed filter in the axial measurement of the parametric loudspeaker, a clear frequency dependence from parametric array effect is detected, which exhibits a good match with the well-known theory described by the Gaussian-beam expansion technique. During the directivity measurement for the parametric loudspeaker, the proposed filter has also proved to be effective and is only needed for small angles.

Effects of absorption layer characteristic on spectral photoresponse of mid-wavelength InSb photodiodes

We report on 2D numerical simulations of photoresponse characteristic for mid-wavelength InSb infrared photodiodes. Effects of thickness of absorption layer on the photoresponse have been investigated for both front-side and back-side illuminated devices. Optimal thickness of absorption layers for different diffusion lengths are extracted theoretically. An empirical formula is proposed to predict a reasonable optimal thickness of absorption layer.

Joint FDTD-Optical/FEM-Electrical Numerical Simulation of Reflection-Type Subwavelength-Microstructure InSb Infrared Focal-Plane Arrays

The design of a reflection-type subwavelength microstructure has been numerically investigated to concentrate incident light onto pixels for improved photoresponse of InSb infrared focal-plane arrays. Compared with traditional microlenses placed on top of the detector substrate, this reflection-type microstructure is better suited for extremely small pixel pitches. The structure is simulated using a joint numerical method combining the finite-difference time-domain method based on Maxwell’s curl equations and the finite-element method based on the Poisson and continuity equations. The results show that this advanced design could effectively improve device response without sacrificing crosstalk. The optimal structure parameters are obtained theoretically, with response increase of approximately 100%.